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Rational development of the solvent loaded, inductively coupled argon plasma Weir, Douglas Glenn John 1994

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RATIONAL DEVELOPMENT OF THE SOLVENT LOADED, INDUCTIVELY COUPLED ARGON PLASMA  By DOUGLAS GLENN JOHN WEIR Bachelor of Science, University of Alberta, 1986  A THESIS SUBMITfED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY  in THE FACULTY OF GRADUATE STUDIES Department of Chemistry  We accept this thesis as conforming to the required standard  THE UMVERSITY OF BRITISH COLUMBIA September, 1993 © Douglas Glenn John Weir, 1993  _______  requirements for an advanced In presenting this thesis in partial fulfilment of the that the Library shall make it degree at the University of British Columbia, I agree that permission for extensive freely available for reference and study. I further agree be granted by the head of my copying of this thesis for scholarly purposes may It is understood that copying or department or by his or her representatives. be allowed without my written publication of this thesis for financial gain shall not permission.  (Signature)  Department of The University of British Columbia Vancouver, Canada Date  DE-6 (2/88)  tL  ABSTRACT  In routine trace metal analyis, the inductively coupled argon plasma (ICAP) converts analyte into atoms and atomic ions, then excites them to emit intense, characteristic line spectra. But none of the spectrochemical techniques based on the ICAP are free from interference effects. This dissertation focuses on only one source of interference effects: the solvent plasma load associated with injecting sample solution into the discharge as an aerosol mist. This dissertation reveals several physical phenomena that are responsible for both the spectral and nonspectral interference effects associated with solvent plasma loading, physical phenomena that are spatially, temporally and parametrically complex. In order to address that complexity, a strategy was proposed for characterizing and developing electrical gas discharges for spectrochemical analysis. The strategy consisted of three steps which are not strictly sequential. Step one: systematically survey the parametric, spatial and temporal complexity of analyte and background emission (Chapters 3-6); Step two: drawing guidelines from step one, characterize the physical properties of the discharge (Chapters 7-9); Step three: Improve the design and methodology of the spectrochemical method based on the insight gained in the first two steps. The first two steps proved to be very effective, but the third step remains untested.  (1  Table of Contents Page Abstract  .  Table of Contents  ii iii  ListofTables List  of Figures  Glossary of Symbols and Abbreviations  xix  Acknowledgements  xxii  Openning Quotation  xxiii  Chapter 1: Thesis Introduction and Summary  1  1.1 Context and scope  1  1.2 General objective  2  1.3 An overview of the solvent load problem, from sample introduction to signal detection.. 4 1.4 Relevant properties of the sample aerosol  9  1.5 The interface between the sample aerosol and the discharge  10  1.6 Relevant properties of the inductively coupled argon plasma  11  1.7 Thesis objectives and summary  17  1.8 References  20  ‘U  Chapter 2: The Experimental Setup  22  2.1 Sample introduction  22  2.2 Torch  29  2.3 Power Supply  29  2.4 Ignition procedure  33  2.5 Light collection optics  33  2.6 References  36  Chapter 3: Spectral Survey and Observations  37  3.1 Introduction  37  3.2  42  Spectral survey  3.3 Photographs  48  3.4  51  Detailed observations  3.5 Conclusions  68  3.6 References  69  Chapter 4: The Parametric Complexity of a Chloroform Loaded Inductively Coupled Argon Plasma 70 4.1 Introduction  70  4.2 Experimental section  96  4.3 Results  98  iv  4.4 Chapter summary  .  124  4.5 Conclusions  125  4.6 References  126  Chapter 5: The Spatial Complexity of the Solvent Loaded Inductively Coupled Argon Plasma 128 5.1 Introduction  128  5.2 Experimental  138  5.3 Results and discussion  142  5.4 Discussion  178  5.5 Conclusions  179  5.6 References  180  Chapter 6: The Temporal Complexity of the Solvent Loaded Inductively Coupled Argon Plasma182 6.1 Introduction  182  6.2 Experimental  194  6.3 Results  197  6.4 Conclusions  214  6.5 References  215  Chapter 7: A Parametric Survey of Electron Density in the Solvent Loaded Inductively Coupled Argon Plasma 219 7.1 Introduction  219  V  7.2 Guidelines offered by previous chapters for investigating the physical characteristics of the discharge 220 7.3 An accurate physical description of the ICAP: Plasma parameters  223  7.4 Determining electron number densities from the absolute intensity of a single Ar I line... 237 7.5 Experimental  249  7.6 Results  251  7.8 Discussion  264  7.9 Conclusions  266  7.10 References  267  Chapter 8: The Response of Electron Density to Solvent Plasma Load and Inner Argon Flow Rate in the Induction Region of the Inductively Coupled Argon Plasma 269 8.1 Introduction  269  8.2 Experimental  273  8.3 Results  282  8.4 Discussion  294  8.5 Conclusions  300  8.6 References  301  Chapter 9: The Effect of Solvent Load and Inner Argon Flow Rate on the Atomic State Distribution Function of Iron in an Inductively Coupled Argon Plasma 305 9.1 Introduction  305  9.2 Experimental  311  vi  9.3 Results.319 9.4 Conclusions  .  350  9.5 Reference  351  Chapter 10: Concluding Remarks and Future Directions  354  vi’  List of Tables Pag Table 2.1: Summary of the ranges of solvent plasma load investigated  27  Table 2.2: Summary of the experimental setup  29  Table 9.2.1: Wavelength, excitation energy, statistical weight of the upper state, atomic transition probabilities, and error in the transition probability for Fe II lines used in this work  316  Table 9.2.2: Wavelength, excitation energy, statistical weight of the upper state, atomic transition probabilities, and error in the transition probability for Fe I lines used in this work  317  vu’  List of Figures Page Figure 1.1: Introductory overview of the solvent loaded inductively coupled argon plasma, showing the confinement tube, the argon flow rates, and the major components of the  discharge  3  Figure 1.2: The path taken by sample aerosol from nebulizer to plasma  7  Figure 1.3: The flowfield in the inductively coupled argon plasma  14  Figure 2.1: A schematic overview of the experiment  23  Figure 2.2: (a) Calibration plot of the chloroform plasma load versus the temperature of the aerosol desolvating condenser; (b) Schematic flow chart of the continuous weighing method used to calibrate the solvent plasma load against condenser temperature  25  Figure 2.3: The flow of power in a solvent loaded ICAP  32  Figure 2.4: Sample maps of the light collection efficiency for the optical trains used in this work  35  Figure 3.2.1: Spectral survey of the visible emission from the inductively coupled argon plasma loaded with methanol  43  Figure 3.2.2: Spectral survey of the visible emission from the inductively coupled plasma loaded with water  44  Figure 3.2.3: Spectral survey of the visible emission from the inductively coupled argon plasma loaded with chloroform  45  Figure 3.2.4: The relative luminosity curve or relative eye sensitivity of a standard observer adjusted to bright lighting  46  ix  Figure 3.3.1: Photographs of a chloroform loaded ICAP  49  Figure 3.3.2: Photographs of  50  Figure 3.4.1: Visually observed emission structures of a solvent loaded ICAP  52  Figure 3.4.2: Representative observations of an ICAP discharge  55  Figure 3.4.3: Representative observations for an ICAP loaded with methanol  59  Figure 3.4.4: Representative observations for an ICAP loaded with chloroform  62  Figure 3.4.5: Observations of how the shape of the inner plume varied as the chloroform plasma load was increased  64  Figure 3.4.6: Representative observations for an ICAP loaded with an intermediate level of chloroform on an inner argon stream at varying flow rates  66  Figure 4.1.1: The spatial averaging intrinsic to line of sight measurements  94  Figure 4.3.1: Axially resolved profiles of Mg II emission (279.55 nm) from an inductively coupled argon plasma loaded with chloroform  100  Figure 4.3.2: Axially resolved profiles of Mg I emission (285.21 nm) from an inductively coupled argon plasma loaded with chloroform  103  Figure 4.3.3: Axially resolved profiles of atomic carbon emission (248 nm) from an inductively coupled argon plasma loaded with chloroform aerosol  106  Figure 4.3.4: Axially resolved profiles of diatomic carbon emission (516 nm) from an inductively coupled argon plasma loaded with chloroform aerosol  110  Figure 4.3.5: Axially resolved profiles of cyanogen radical emission (388 nm) from an inductively coupled argon plasma loaded with chloroform aerosol  113  Figure 4.3.6: Axially resolved profiles of the ratio of Mg II emission (279.55 nm) to Mg I emission (285.2 mm) from an inductively coupled argon plasma loaded with chloroform  121  ....  x  Figure 5.1.1: Four possible optical configurations for capturing monochromatic images of a spectrochemical source  132  Figure 5.3.1: The reference frame and contour intervals for the spatially resolved intensity maps  143  Figure 5.3.2: An overview of spatially resolved maps of analyte and background emission from the solvent loaded inductively coupled argon plasma  145  Figure 5.3.3: Maessen and Kreuning’s spatially resolved profiles of emission from solvent pyrolysis products  149  Figure 5.3.4: Maessen and Kreuning’s spatially resolved profiles of emission from solvent pyrolysis products  150  Figure 5.3.5: The characteristic structure of emission plumes for (a) soft, (b) intermediate, and (c) hard line emission from the inductively coupled argon plasma  151  Figure 5.3.6: Isocontour maps of CN (388.34 nm) emission intensity for a chloroform loaded ICAP  154  Figure 5.3.7: Isocontour maps of C 2 (516.56 nm) emission intensity for a ineta-xylene loaded ICAP  157  Figure 5.3.8: Isocontour maps of C I (247.61 nm) emission intensity for a chloroform loaded ICAP  159  Figure 5.3.9: Isocontour maps of C I (247.61 nm) emission intensity within the induction region  162  Figure 5.3.10: The response of a hard line plume (Mg II 279.55 nm) to chloroform plasma load  164  Figure 5.3.11: The response of a soft line plume (Mg I 285.21 nm) to chloroform plasma load  167  Figure 5.3.12: The response of the magnesium line intensity ratio (Mg II 279.55 nm / Mg I 285.21 nm)) to chloroform plasma load  169  xi  Figure 6.1.1: The 3D varicose wave that falls off the argon jet into a ring vortex  191  Figure 6.1.2: Vortex shedding from the inductively coupled argon plasma  192  Figure 6.3.1: The effect of chloroform plasma load on the 0 the Mn II line at 257.610 nm  198  Figure 6.3.2: The effect of chloroform plasma load on the 0 CN emission at 388.340 nm  -  -  500 Hz noise spectra for  500 Hz noise spectra for 199  Figure 6.3.3: The effect of methanol plasma load on the 0 500 Hz noise spectra for the Mn II line at 257.610 nm  200  Figure 6.3.4: The effect of methanol plasma load on the 0 500 Hz noise spectra for CN emission at 388.340 nm  201  Figure 6.3.5: The effect of solvent plasma load on the vortex shedding frequency  204  Figure 6.3.6: The transient signal for the Ca I line at 422.673 nm  209  Figure 6.3.7: Noise spectra for the Ca I line at 422.673 nm  211  Figure 6.3.8: Low frequency noise spectra for C2 emission at 5 16.520 nm  213  -  -  Figure 7.3.1 The departure of experimental atomic state distribution functions from the Saha distribution. (a) depicts the case of an overpopulation of the low lying atom levels balanced by and underpopulation of the upper ion levels. (b) depicts the case of an underpopulation of the low lying atom levels balanced by and overpopulation of the upper ion levels  231  Figure 7.4.1 A schematic illustration of determining the electron density and electron temperature in the inductively coupled argon plasma from the absolute intensity of a single argon line  238  Figure  7.4.2 The effect of departures from local thermal equilibrium on the determination of electron density and electron temperature in the inductively coupled argon plasma. Note that the departure from LTE has been exaggerated for clarity  xii  242  Figure 7.4.3 Error in the absolute line intensity method for realistic departures from local thermal equilibrium  244  Figure 7.4.4 Variation of four key quantities of the absolute line method over electron temperatures typically encountered in the inductively coupled argon plasma  246  Figure 7.4.5 Electron density versus the absolute intensity of a single argon line (687.127 nm). The solid line was calculated assuming complete LTE. The dashed lines assume partial LTE and set the bounds of realistic departures from LTE. For the determination of electron density from the absolute intensity of the Ar I 687.127 nm line, this Figure shows that the error owing to departures from LTE is bounded by ± 30%  248  Figure 7.5.1 The emission spectrum of the inductively coupled argon plasma in the vicinity of the 687.129 nm line used in this work  250  Figure 7.6.1(a): The response of electron density to r.f. power and methanol load, revealed by isocontour maps of electron density e (1015 cm) 3  253  Figure 7.6.1(b): The response of electron density to r.f. power and methanol load, revealed by isocontour maps of electron density e (1015 cm ) 3  254  Figure 7.6.1(c): The response of electron density to r.f. power and methanol load, revealed by isocontour maps of electron density e (1015 cm ) 3  255  Figure 7.6.2(a): The response of electron density to r.f. power and chloroform load, revealed by isocontour maps of electron density e (1015 cm ) 3  257  Figure 7.6.2(b): The response of electron density to r.f. power and chloroform load, revealed by isocontour maps of electron density e (1015 cm ) 3  258  Figure 7.6.2(b): The response of electron density to r.f. power and chloroform load, revealed by isocontour maps of electron density e (1015 cm ) 3  259  Figure 7.6.3(a): The response of electron density to r.f. power and water load, revealed by isocontour maps of electron density e (1015 cm ) 3  261  Figure 7.6.3(b): The response of electron density to r.f. power and water load, revealed by isocontour maps of electron density e (1015 cm ) 3  262  xlii  0.61 11mm; (b) 0.3 mg/s and 0.81 Llmin; (c) 10.0 mg/s and 0.61 11mm; (d) 10.0 mg/s and 0.81 1/mm. The r.f. power was 1.25 kW. 290 Figure 8.3.5 The response of radially resolved electron density profiles to inner argon flow rate and water plasma load. The r.f. power was 1.25 kW  292  Figure 8.3.6 The response of radially resolved electron density profiles to inner argon flow rate, chloroform plasma load and xylene plasma load. The r.f. power was 1.25 kW  293  Figure 8Figure 7.6.3(c): The response of electron density to r.f. power and water load, revealed by isocontour maps of electron density e (1015 cm) 3  263  Figure 8.2.1 Electron density as a function of electron temperature in an argon plasma, according to the Saha equation. The solid curve assumes local thermal equilibrium. For the dashed curves, the argon ground state was assumed to be overpopulated by a factor of 10 or underpopulated by a factor of 0.1 with respect to local thermal equilibrium  274  Figure 8.2.2 Cubic spline interpolated, theoretical profiles for the Hp line for a range of electron densities  275  Figure 8.2.3 Electron density versus the theoretical full width at half maximum of the H line. The widths were determined from cubic spline interpolated, theoretical profiles  277  Figure 8.2.4 Log plot of electron density versus the theoretical full width at half maximum of the Hp line. The widths were determined from cubic spline interpolated, theoretical profiles  278  Figure 8.2.5 Comparison of a normalized, cubic spline interpolated, theoretical Hp line profile with a closely matching(by visual inspection) experimental proffle. Evident in the Figure is spectral interference that could corrupt line width determinations  279  Figure 8.2.6 The effect of aperture size on radially resolved profiles of electron density.... 281 Figure 8.3.1 The response of radially resolved electron density profiles to inner argon flow rate for an inductively coupled argon plasma loaded with 0.3 mg/s methanol. The r.f. power was 1.25 kW  284  xiv  Figure 8.3.2 The response of radially resolved electron density profiles to inner argon flow rate for an inductively coupled argon plasma loaded with 1.0 mg/s methanol. The r.f. power was 1.25 kW  286  Figure 8.3.3 A summary of the response of the plasma region of the discharge to methanol plasma load. The isocontours in this figure represent electron density qualitatively; (a) low methanol load, (b) high methanol load  288  Figure 8.3.4 The decay of electron density profiles with axial distance in a discharge loaded with 0.3 or 10.0 mg/s methanol at an inner argon flow rate of 0.61 or 0.81 11mm: (a) 0.3 mg/s and.3.7 A summary of the response of the plasma region of the discharge to chloroform or xylene plasma load. The isocontours in this figure represent electron density qualitatively; (a) low solvent load, (b) high solvent load  294  Figure 9.1.1 The LTE framework. Within this framework, the departure of experimentally determined atomic state distribution functions (ASDFs) from local thermal equilirium (LTE)may be assessed. The solid line represents the theoretical ASDF at LTE, or the Saha distribution. The dashed curves represent the predictions fa collisionalradiative (CR) rate model. The open circles represent the experimental ASDF  309  Figure 9.2.1 The map of light collection efficiency in the XZ plane) for the optical train used to collect iron line emission (see Figure 2.1—the XY plane is the horizontal plane defined by the lateral direction across the discharge and the viewing direction or line of sight. In this map, the collection efficiency has been integrated over a distance of 2.0 mm in the Y direction (vertical, parallel to the discharge axis)  312  Figure 9.2.2 (a) foreground spectrum, (b) background spectrum, (c) detector response function, and (d) the corrected spectrum (foreground background) of iron lie emission for the linear photodiode array detector window at 370 nm, at an observation height of 15 mm -  above the induction coil, for an ICAP loaded with 6 mg/s of chloroform, and an r.f. power of 1.25 kW  314  Figure 9.2.2 (a) foreground spectrum, (b) background spectrum, (c) detector response function, and (d) the corrected spectrum (foreground background) of iron lie emission for the linear photodiode array detector window at 265 nm, at an observation height of 15 mm above the induction coil, for an ICAP loaded with 6 mg/s of chloroform, and an r.f. power of 1.25 kW  315  -  xv  Figure 9.3.1 Radial profiles of normalized Fe I emission intensity for lines covering a range of excitation energies  321  Figure 9.3.2 Radial profiles of Fe I emission (373.7 13 nm) at 15 mm above the induction coil, for a range of distances between the tip of the inner boundary and the observation zone  323  Figure 9.3.3 Curvature in line if sight and radially resolved Fe I and Fe II Boltzmann plots. Values of ln(nlg) represented by the open circles were taken from a radial position (in the radially resolved plots) of r = 0.0 mm or a lateral position (in the line of sight plots) of x 0.00 mm; the crosses, r = 0.60 mm or x = 0.60 mm; the filled circles, r 1.8 mm or x =  1.8 mm  325  Figure 9.3.4 Radial and lateral profiles of (a) Fe I excitation temperature, (b) curvature of the Boltzmann plot, and (c) temperature uncertainty for a water loaded ICAP, 15 mm above the induction coil; water loads: A = 0.10, B flow rate = 0.81 1/mm; r.f. power = 1.25 kW  =  0.15, C  =  0.20 mgls; inner argon 327  Figure 9.3.5 Fe I excitation temperature at 15 mm above the induction coil plotted against the axial displacement f the dissociation front from upstream from that viewing height  330  Figure 9.3.6 Radial profiles of Fe II emission (273.955 nm),normalized to unt intensity at 15 mm aboe the induction coil, from an ICAP loaded with (a) chloroform, (b) methanol, (c) water, for ranges of solvent load; inner argon flow rates: 0.61, 0.81, 1.01 1/mm; r.f. power = 1.25 kW. except 1.00 kW for the dotted curve in frame (c)  332  Figure 9.3.7 Radially resolved profiles of tangential excitation temperatures and Boltzmann plot curvature. The inductively coupled argon plasma was loaded with 7.4 mg/s of chloroform, the r.f. power was 1.25 kW and the inner argon flow rate was 0.81 1/mm. The Fe I tangential temperatures were evaluated at excitation energies of 3.2, 4.7 and 6.2 eV with respect to the atomic ground state. The Fell tangential temperatures were evaluated at excitation energies of 12.7, 14.4 and 16.0 eV with respect to the atomic ground state  336  Figure 9.3.8 Radially resolved profiles of tangential excitation temperatures and Boltzmann plot curvature. The inductively coupled argon plasma was loaded with 1.0 mg/s of methanol, the r.f. power was 1.25 kW and the inner argon flow rate was 0.81  xvi  1/mm. The Fe I tangential temperatures were evaluated at excitation energies of 3.2, 4.7 and 6.2 eV with respect to the atomic ground state. The Fe II tangential temperatures were evaluated at excitation energies of 12.7, 14.4 and 16.0 eV with respect to the atomic ground state  337  Figure 9.3.9 Profiles of argon ionization temperature at z = 15 mm above the induction coil, an r.f. power = 1.25 kW and for loading by various solvents, at different solvent plasma loads and inner argon flow rates. If one assumes that the argon plasma is close to local thermal equilibrium (or that the non equilibrium parameter for the argon atomic ground state b 1 in equatuation 9.2 is close to 1.0), then these profiles may be regarded as proffles of electron temperature  340  Figure 9.3.10 The error intrinsic to electron temperatures obtained from equation 9.2 and accurate electron densities. The error results from the argon ASDF departing from local thermal equilibrium. Several realistic values of b 1 were inserted into equation 9.2. The axes extend over the range of electron temperatures and electron densities likely to be incountered in the inductively coupled argon plasma. For an example of how to guage the error in the electron temperature, the b 1 = 0.1 curve (partial LTE) lies below the solid b 1= 1.0 curve. Consequently, the electron temperature would be overestimated by 800 K if one assumed that the plasma was in LTE  342  Figure 9.3.11  Experimentally determined state densities for Fe I and Fe II (the experimental ASDF) within the LTE framework. The bold, solid line defines the Saha distribution—for an electron temperature calculated from an accurate electron density and equation 9.2. The other solid lines assume that the electron temperature has been overestimated (b 1 < 1.0) because the argon plasma gas is recombining. Also shown are the predictions of a collisional radiative (CR) model (dashed curve)  344  Figure 9.3.12(a) Experimentally determined state densities for Fe I and Fe II (the experimental ASDF) within the LTE framework an r.f. power of 1.25 kW and chloroform load  346  Figure 9.3.12(b) Experimentally determined state densities for Fe I and Fe II (the experimental ASDF) within the LTE framework for an r.f. power of 1.25 kW and water load  347  xvii  Figure 9.3.12(c) Experimentally determined state densities for Fe I and Fe II (the experimental ASDF) within the LTE framework for an r.f. power of 1.25 kW and high methanol  load  348  Figure 9.3.12(d) Experimentally determined state densities for Fe I and Fe II (the experimental ASDF) within the LTE framework for an r.f. power of 1.25 kW and low methanol load.  349  xviii  GLOSSARY OF SYMBOLS AND ABBREVIATIONS Physical Quantities A,  atomic transition probability of q —> p optical transition, 10 8 s 1  b  nonequilibrium parameter; ratio of the actual, or experimental atomic state density to the atomic state density for local thermal equilibrium  121  nonequilibrium parameter for the atomic ground state  b  nonequilibrium parameter for the ith excited state  e  electron charge, 1.602 19  E,  ionization potential  E, Eq  excitation energy of state p, of state q  g  gain of photomultiplier;  X ‘ 9  C  degeneracy or statistical weight of atomic state statistical weight of atomic ground state, excited state q, excited state p, and the atomic ion ground state (singly ionized atom) h  Planck constant, 6.626 18 x  ‘qp  absolute intensity of q — p optical transition, W cm3  j  s  energy difference between atomic state p and the ionization limit,  q 1  energy difference between atomic state q and the ionization limit k  Boltzmann constant, 1.38066  X  me  electron mass, 9.10953 x 10  31  n  atomic state density;  10  j  kg  number of photoelectrons per photon for a photomultiplier fli ,  ,  flq n ,  state density for the atomic ground state, atomic excited state p, atomic excited state q, and the ion ground state.  xix  ‘7  .  hi, T1p  q’ 17  jf, 17  i  state density per statistical weight. state density per statistical weight for the atomic ground state, atomic excited state p, atomic excited state q, the ion ground state, and ionization limit.  j(fl  (p) 5 p  B() 1 ,  LSE value of 17 for atomic state p. 17 for the atomic ion ground state, LBE value of  i  for atomic state p.  e 1  free electron density of the plasma  vqp  frequency of light for the optical transition q —* p  P  atmospheric pressure, iO Pa  S  shot noise signal  Te, Tg , , 107 Texc T  electron temperature, atomic gas temperature, Saha ionization temperature and Boltzmann excitation temperature  2AAS  full width at half maximum of the Doppler and Stark broadened line profile  Lexpt A2 2 A ,  experimental and instrumental full width at half maximum electrical conductivity, Ohms 1 cm1  Qi  coolisional cross section of electrons with plasma species i, cm 2  Abbreviations ASDF  atomic state distribution function  CR  collisional radiative  ICAP  inductively coupled argon plasma  ICAP-AES  inductively coupled argon plasma atomic emission spectrometry  LBE  local Boltzmann equilibrium  xx  LIE  .  local isothermal equilibrium  LSE  local Saha equilibrium  LTE  local thermal equilibrium  Mg ratio  ratio of the intensity of the Mg 11(279.55 nm) ion line to the intensity of the Mg 1(285.21 nm) atomic line  pLTE  partial local thermal equilibrium  PMT  photo multiplier tube  RSD  relative standard deviation  RSDB  relative standard deviation of the background signal  SBR  signal to background ratio  SNR  signal to noise ratio  xxi  ACKNOWLEDGEMENTS I FIRST WISH TO EXPRESS MY GRATITUDE AND SINCERE LOVE TO MY FATHER, DR. DONALD ROBERT WEIR, A HYDROMETALLURGIST AN]) A PRACTICAL MAN OF SCIENCE, TO MY MOTHER, MRS. ANN MARIE WEIR, A DEDICATED HIGH SCHOOL TEACHER, AND TO MY GRANDMOTHER, MRS. BARBARA WEIR, A CHAMPION CURLING SKIPPER. IF IT WERE NOT FOR THEIR LOVE, SUPPORT AND KThJI) WISDOM, I WOULD NEVER HAVE COMPLETED THIS ADVENTURE. NEXT, I WOULD LIKE TO THANK TWO CLOSE FRIENDS WHO SHARED MY APPRECIATION FOR THE LIFE AND ACCOMPLISHMENTS OF RICHARD P. FEYNMAN—B. OLAV ANDERSON AND CHARLES W. LEBLANC. SPECIAL THANKS ARE DUE TO THE MAN WHO FAiTHFULLY PROOF READ MUCH OF THIS WORK, WHO YOU WOULD DEFINITELY WANT TO HAVE IN THE TRENCHES WITH YOU—GUY KENNETH CLENTSMITH. THANKS ALSO TO MYLUAGANAM, CHANDRAKUMAR, IVAN, TIM, ADRIAN, ALEXIS, LYLE, PETE, EDGAR, BERNARD, MARTIN, NARLY, MAHEEN, BEN, MARK, ROB, LAURELLE, CHRISTOPHER, KEN, DAMON, LEE, CHRISTIAN, CAROLINE, CHRISTIANE SCHACHT VON LEVERKUSEN, ROCIO, DEBBIE, SYLVIA, SUSAN, AND ALL THE CHEMISTRY DEPARTMENT SECRETARIES, MACHINE SHOP AND ELECTROMCS SHOP STAFF, LARGELY FOR KEEPING ME SANE. MANY THANKS TO MY RESEARCH SUPERVISOR, DR. MICHEAL W. BLADES, FOR KEEPING THINGS RUNNING SMOOTHLY, KEEPING THE BUREAUCRATS OFF MY BACK AND FOR LENDING ADVICE, YET ALLOWING ME TO WORK INDEPENDENTLY. FOR SIMILAR REASONS, I WOULD LIKE TO THANK DR ADRIAN P. WADE. FINALLY, I AM INDEBTED TO THE PATIENCE AND UNDERSTANDING OF MY SON AND BEST FRIEND, CAMERON KIMBALL LAING, AND HIS MOM, ELIZABETH. THIS DISSERTATION IS DEDICATED TO THE MEMORY OF MY GRANDPARENTS, ANNA MARIA GASZLER, VINCENT GASZLER AND DONALD HOWARD WEIR.  xxii  OPENNING QUOTATION  Whereas I believed myself born for the common good, and reckoned the care of the common weal to be among those duties that are of public right, open to all alike, even as the waters and the air, I therefore asked myself what could most advantage mankind, and for the performance of what tasks I seemed to be shaped by nature. But when I searched, I found no work so meritorious as the discovery and the development of the arts and inventions that tend to civilize the life of man. Above all, if any man could succeed— not merely in bringing to light one particular invention, however useful—but kindling in .  .  nature a luminary which would, at its first rising, shed some light on the present limits and borders of human discoveries, and afterwards, as it rose still higher, would reveal and bring into clear view every nook and cranny of darkness, it seemed to me that such a discoverer would deserve to be called the true Extender of the Kingdom of Man over the universe, the Champion of human liberty, and the Exterminator of the necessities that now keep men in bondage. Moreover, I found in my own nature a special adaptation for the contemplation of truth. For I had a mind at once versatile for that most important object—I mean the recognition of similitudes—and at the same time sufficiently steady and concentrated for the observation of subtle shades of difference. I possessed a passion for research, a power of suspending judgement with patience, of meditating with pleasure, of assenting with caution, of correcting false impressions with readiness, and of arranging my thoughts with scrupulous pains. I had no hankering after novelty, no blind admiration for antiquity. Imposture in every shape I utterly detested. For all these reasons I considered that my nature and disposition had, as it were, a kind of kinship and connection with truth.  FRANCIS BACON  xxiii  Chapter 1 Thesis Introduction and Summary 1.1. Context and Scope This thesis contributes to a particularly active field of research in analytical chemistry— the development of spectrochemical methods for trace metal analysis. In this field, workers generally have two principal aims: 1., to invent new spectrochemical methods for trace metal analysis and 2., to develop existing methods rationally. In order to invent new methods, they may begin by devising new ways to convert a sample into a volume of atoms or atomic ions. This is usually accomplished with a spectrochemical source, such as a chemical flame or an electrical discharge. Next, the volume of atoms in the spectrochemical source is probed for atomic emission, fluorescence, absorbance or atomic ion density. The resulting spectrometric signal must finally be decoded in order to obtain the trace metal composition of the original sample. In order to do that, a new methodology may have to be devised. Of course, when a new spectrochemical method is devised this way, the prototype usually performs poorly. Nevertheless its analytical performance, precision, accuracy and detection limits, can usually be improved by tailoring the spectrometer to suit the spectrochemical source, or the source to suit the sample. Indeed, even established methods have room for improvement. A good example is atomic emission speetrometry with an inductively coupled argon plasma as a spectrochemical source. In summary, the task of workers in this field is not only to devise new spectrochemical methods, but to rationally develop spectrochemical methods that are already established as well. Boumans argued that in order to develop a spectrochemical method rationally, the experimentalist must follow four steps [1]. First, the parametric response of the method must be established. That reveals where the optimum performance lies in the operating parameter space. Moreover, the performance at non-optimal parameters reveals how the method may suffer  Introduction  2  interferences. Second, the physical properties of the method must be characterized. In order to do that one must resort to spectroscopy in order to characterize the physical properties of the spectrochemical source—but not always. Sometimes the interface between the source and the sample introduction system must be examined. Other times, the interface between the source and the spectrometer requires scrutiny. Ideally, the experimentalist should characterize every step in the method, from sample introduction to signal detection. Having characterized the method satisfactorily, the third step is to explain the analytical performance and the parametric response in terms of the physical properties. That gives one a rational basis for the fourth step—to develop, refine and improve the method. One can do that by tailoring the source, optics and spectrometer, or prescribing a better protocol for routine analysis, all with the insight into how the physical properties affect the analytical performance. 1.2. General Objective The general objective of this thesis was to rationally develop a relatively mature, yet by no means flawless spectrochemical method: Inductively Coupled Argon Plasma  -  Atomic  Emission Spectrometry (ICAP-AES). At the heart of this spectrochemical method lies an electrodeless discharge, the inductively coupled argon plasma (ICAP). This source is illustrated in Figure 1.1. It is more accurate to call it a discharge rather than a plasma because it consists of both plasma and boundary regions. At the core of the discharge lies a toroidal plasma region, aptly described as a low temperature (< 10000 K), weakly ionized (< 2 %) thermal plasma at atmospheric pressure. Its outside diameter is roughly 15 mm, its inside diameter 4 mm, and its length 15 mm. Because the plasma gas retains much of its energy as it flows out of the toroidal induction region into the room air, a conical region of plasma extends beyond the exit of the torch. This region may be referred to as the plasma decay region. Here, steep thermal gradients between the induction region and the axial channel decay as the plasma gas flows downstream. The gradients decay by transport processes such as heat conduction, escape of radiation and ambipolar diffusion. On the outside of the jet, however, cold room air entrained into the plasma  3  Introduction  downstream boundary region analyte plume pisrna tail cone or decay region  I  cm  0  0 0 0  induction coil O  0 torch wall inner argon flow,  injector tube  0.6 to 1.0 I 1mm Outer, coolant argon flow, 10.0 to 12.0 1/mm intermediate argon flow, 0 to 1.0 11mm  Figure 1.1. Introductory overview of the solvent loaded inductively coupled argon plasma, showing the confinement tube, the argon flow rates, and the major components of the discharge.  Introduction  4  jet extinguishes the plasma before it has a chance to decay. In fact, air entrainment gives the plasma its characteristic conical shape, a shape reminiscent of the potential core of a jet flowing into quiescent room air. In this case, one may say that thermal gradients actually grow steeper at the outer boundaries the decay region as a result of heat conduction and radiation, and that these gradients cause transport by diffusion. (In fact, this development of thermal gradients rather than decay was emphasized by the external examiner of this dissertation, P.B. Farnsworth, so strictly speaking, decay region is a misnomer for what is really a conical tail region.) In any case, the conical decay region generally has an apex that is 20 to 25 mm downstream from a 15 mm diameter base. The entire plasma, cone and toroid together, is surrounded by a boundary region consisting of two distinct components, one upstream and one downstream. The upstream boundary region (shown in solid black) resides within the torch and wraps around the base of the plasma. This component is often made clearly visible by intense emission from diatomic carbon, a solvent pyrolysis product. In contrast, the downstream boundary region (shown in gray) differs considerably from the upstream region. It may be described as a tail flame. Specific regard was paid to the effects that solvent plasma load had on both the discharge and the analytical performance of ICAP AES. Solvent plasma load may be defined -  simply as the amount of solvent delivered to the discharge per unit time [2],  Solvent Plasma Load,  QSPL  amount of solvent delivered umt time .  .  It may be conveniently expressed in units of miffigrams per second, mg/s, or micro moles per second, jimolls. In spite of its simple definition, solvent plasma load complicates trace metal analysis by ICAP-AES quite considerably.  Nevertheless, solvent load is intrinsic to  conventional sample introduction practices because analysts rely on the favorable sampling statistics of homogeneous solutions. It is clearly beneficial to take a sample from a solvent extract or an acid digest, and feed it directly into a spectrochemical instrument.  Introduction  The general thesis objective was to rationally develop the ICAP for spectrochemical applications that cannot avoid solvent plasma load. In order to define that objective more clearly, we will now examine the problem of solvent plasma load in greater detail. 1.3. An Overview of the Solvent Load Problem, from Sample Introduction to Signal Detection Briefly, a nebulizer (shown in Figure 1.2.) converts sample solution into an fine mist of aerosol droplets [3], [4]. At the tip of the nebulizer, the mist is known as the primary aerosol. After nebulization, the primary aerosol is swept through a spray chamber on an argon stream. As the aerosol travels through the spray chamber, its small droplets partially evaporate while the large ones collide with the spray chamber walls. In this way, the spray chamber acts as a cutoff filter for droplet sizes. It removes most of the droplets larger than a limiting diameter, usually lOj.tm to 25 JIm, and allows the rest of the droplets to partially evaporate. The resulting secondary aerosol leaving the spray chamber may then be swept through a desolvating device such as the heater and condenser assembly shown in Figure 1.2. This device can remove excess solvent so that the aerosol stream does not overload the ICAP with solvent mass later on. Beyond the desolvator, more argon or molecular gases may be added through a device similar to the one shown. A number of other technologies may be incorporated to further modify the aerosol stream before it finally reaches the torch, the general idea being to tailor the aerosol such that it neither destabilizes the ICAP, creates excessive noise, nor interferes with the spectrochemical analysis. Nevertheless, many of the problems resulting from solvent plasma load can be traced back to the aerosol modification step. Certainly there is much room for improvement here.  5  Introduction  Once the aerosol finally reaches the torch, it is known as the tertiary aerosol. The tertiary aerosol generally consists of argon, solvent vapor, a large number of aerosol droplets less than 25im in diameter, and perhaps even some desolvated particles of sample material. The tertiary aerosol travels into the injector tube of the torch, and at the end of the injector tube, a nozzle constricts the gas flow and injects the aerosol stream into the ICAP, as shown in Figure 1.2. The aerosol stream next travels into the hollow base of the toroidal induction region, at the upstream end of the discharge, and then follows the axial channel through the centre of the toroidal induction region. In one capacity, the toroidal induction region acts as a cylindrical oven. Ideally, it rapidly heats the aerosol stream flowing through it, desolvates the aerosol droplets and then vaporizes the desolvated particles of sample material. Further downstream, several processes transport mass, energy and electrical charge from the outer toroidal plasma into the axial channel, thus imparting energy to the sample material. Eventually, the aerosol stream coalesces with the plasma somewhere downstream. Beyond that point, energetic species in the plasma collide inelastically with the analyte atoms, thereby ionizing and exciting them.  6  Introduction  7  Point where Coalesces with Plasma, —  0 I S I  44  Induction Region  I pr  ‘ICAP Torch  I I  I I a.  —c.Tertiary Aerosol Injector Tube Condenser Neb ulizer  Secondary Aerosol  Sample ‘Uptake  Figure 1.2 The path taken by the sample aerosol from nebulizer to plasma.  Introduction Electronically excited analyte atoms and atomic ions then emit characteristic line emission amenable to atomic emission spectrometry. Alternatively, the plasma may serve as an absorption volume for atomic absorption spectrometry (ICP-AAS) or as an excitation volume for atomic fluorescence spectrometry (ICP-AFS). Perhaps the most powerful method involves skimming atomic analyte ions into a vacuum chamber for mass spectrometry (ICP-MS). Whichever method is used, the analytical information must be extracted from the small region downstream from where the aerosol has coalesced with the plasma and upstream from where the analyte ultimately flows out of the plasma and mixes with room air. From this description, one might expect the ICAP to perform satisfactorily as a speetrochemical source. Presumably, the toroidal induction region imparts enough energy into the aerosol stream to completely atomize, excite and ionize the analyte. Moreover, one would expect the flicker noise in the background signal, which determines detection limits for ICAP AES, to be low for a discharge with a presumably stable flow field. One might even expect good precision for replicate determinations, because the ICAP can be operated steadily and continuously, in contrast to d.c. arcs and graphite furnaces, which have short burn times and finite heating cycles. One might further expect the chemical interferences to be minimal because the plasma presumably atomizes everything injected into it. For the same reason, one might expect spectral interference from molecular band emission to be negligible. In short, one might have inordinately high expectations for simultaneous, multielement, trace metal analysis when one uses the ICAP as a spectrochemical source. But in most practical situations, the samples one injects into the ICAP complicate matters considerably. When the sample aerosols injected into the discharge are suspended in slurries, dissolved in volatile solvents, have complex or unknown matrices, or contain large concentrations of concomitant solutes, then interference effects—both spectral and non spectral—are encountered. For further details, Olesik has provided a comprehensive overview of the interference effects afflicting both ICP-AES and ICP-MS [5j.  8  Introduction  9  In particular, solvent plasma loading may cause spectral and non spectral interferences and further degrade the analytical performance by introducing noise. This becomes clear when we examine the analyte and background signals. First consider the background signal. Solvent plasma loading may drastically increase the intensity of both atomic and molecular background emission [6], [7], [2]. Solvent pyrolysis products, principally C2 and CN in the boundary regions of the discharge and atomic carbon in the plasma region, are the sources of background emission. Their complex spectra may overlap with analyte lines, and thus interfere with background subtraction. Moreover, intense background signals can degrade the detection limits for ICAP-AES. On the other hand, non spectral interferences may result from the effect solvent plasma load has on the analyte signal. Obviously, solvent plasma load may lower the amount of energy available to the analyte. It is likely that the power required to desolvate aerosol droplets, atomize the solvent molecules and then excite the solvent pyrolysis products is supplied at the expense of the power available to vaporize, atomize and excite the analyte. 10 to 100 W are typically required to dissociate solvent molecules, while the other processes require far less (see Figure 2.3). True, these powers are insignificant in comparison to the total power dissipated in the discharge (500 to 1750 W), but they are quite significant in comparison to the small percentage of total power available to the sample (<10%). Alternatively, solvent plasma load may cause non spectral interferences by increasing the amount of power available to the analyte. One way it can do this is by altering the geometry of the discharge. In general, any molecular material entrained from the aerosol channel into the coolant gas will drastically alter the temperature profile over the toroidal induction region. For example, the plasma may shrink and grow hotter if solvent contaminates the coolant argon flowing into the plasma, because the molecular material increases the thermal conductivity of the plasma gas. Essentially, the gas of higher thermal conductivity cools the boundary regions. This lowers the electrical conductivity of the plasma boundary. Consequently, the volume occupied by the plasma shrinks while its power density increases in order to maintain the  Introduction  10  overall power balance. As a result, the plasma power may actually concentrate towards the axis of the discharge. Then solvent loading would actually increase the power available to the analyte. Alternatively, solvent material could simply increase the thermal conductivity of the plasma gas, irrespective of the discharge geometry, and hence increase the rate of transporting energy to the analyte. Of course, all of these arguments are purely speculative at the moment, but they show that the non spectral interference effects associated with solvent plasma load are complex, and possibly unpredictable. Apart from causing spectral and non spectral interferences, solvent plasma loading may also introduce noise. One can expect signal noise when the physical characteristics of the plasma fluctuate in response to the vaporization of incompletely desolvated droplets, or when the overall solvent plasma load drifts. In short, solvent plasma loading introduces noise to both the analyte and background components of the analytical signal. Summary of the Solvent Load Problem The problem of solvent loading is complex, poorly understood and warrants investigation because it adds noise and leads to both spectral and non spectral interference effects, thus degrading the accuracy, precision and detection limits of any spectrochemical method that uses the ICAP for a spectrochemical source. 1.4. Relevant Properties of the Sample Aerosol Four physical properties of the sample aerosol have received a great deal of attention: 1. the solvent plasma load, 2. the distribution of solvent mass between the vapour and droplet phases of the aerosol, 3. the size distribution of the aerosol droplets, and 4. the physical properties and chemical composition of the solvent. Maessen et al. [2] described reliable methods for determining both the total solvent plasma load and the distribution of solvent between vapour and droplets phases. In fact, this thesis relies on much of their pioneering work. Canals et al. [8] provided insight into the droplet size distributions of solvent aerosols. Their findings indicate that the influence of desolvating droplets in the discharge may not be  Introduction  11  significant when the ICAP is loaded with organic solvents. On the other hand, Farnsworth [9] and Olesik [10] have demonstrated that droplet effects are significant in ICAPs loaded with aqueous aerosols. Olesilc et aL [10], [11] demonstrated that although the overwhelming majority of droplets in an aqueous aerosol are very small, a few large droplets—statistically few yet significant in mass—may survive the traverse through the toroidal induction region and on up to the analytical viewing zone of the discharge. That brings us to the interface between the plasma  gas and the sample aerosol. 1.5. The Interface between the Sample Aerosol and the Discharge Several workers have looked beyond the properties of the sample aerosol and into the interface between the sample aerosol and the plasma gas. In particular, [12], [111, [13], [10], [14] have studied the interface between the plasma gas and incompletely desolvated water droplets. It was found that desolvating droplets not only survive in the plasma, they create regions of localized cooling about 1.5 mm in diameter as they travel downstream. This was a startling revelation indeed, because it implies that the temperature profile of the ICAP fluctuates as droplet disturbances flow by. It also calls into question the conclusions of many previous investigations which assumed that the solvent loaded ICAP had a steady temperature profile. There is another perspective largely ignored by the droplet investigators. They assumed that the water loading was confined to the aerosol channel, a reasonable assumption if water enters the ICAP predominantly as droplets rather than vapour. Droplets will follow the argon  stream along the axial channel. But for many organic solvents the vapour phase predominates. In contrast to droplets, vapour mass can diffuse across streamlines and away from the axial channel. Consequently, the distribution of solvent over the argon stream is an important solvent load parameter, at least for volatile organic solvents. Browner [7] and Maessen [15] recognized this parameter in their work with organic solvents. Browner investigated it experimentally by varying the auxiliary argon flow rate [7]. The effects of the distribution of solvent vapour over the argon stream turn out to be critical to the solvent loading process. Further details may be found in subsequent chapters.  Introduction  12  Whereas the interface between aerosol and plasma gas has received concerted attention from experimentalists only recently, the plasma gas has received experimental scrutiny for decades. 1.6. Relevant Properties of the Inductively Coupled Argon Plasma In characterizing the ICAP, the physical properties of interest in this work are the flow dynamics, the thermal state of the plasma gas, the transport processes in the discharge, and the collisional radiative processes in the discharge. flow dynamics The flow dynamics in the ICAP and similar discharges have been investigated with high speed photography [16], [17], anemometer probes (pitot tubes) [18], [19], [20], particle tracking [18], and analysis of temporally resolved emission [9]. However, most of the flow field remains inaccessible to experiment for both fundamental and non fundamental reasons. Invasive probes disrupt the flow stream, high temperatures of the plasma melt the probes and vaporize tracking particles, and the intense emission complicates laser Doppler anemometry. Fortunately, computer simulations offer complete access to the flow field, as Patankar points out [211. The simulations results relevant to solvent loading are the flow structure they predict within the confinement tube, where solvent distributes over the argon stream. Beyond the torch exit, however, the complex flow field has so far defied simulation. Nevertheless, insight into the flow field beyond the confinement tube may be found in the literature on axisymmetric jets and flames [22], [23]. Both of these flow systems resemble the tail flame of the ICAP in many respects. That insight, other experimental results, and the revelations of computer simulation are summarized in Figure 1.3. The recirculation eddy at the base of the discharge can mix solvent vapour from the inner aerosol stream into the outer coolant stream. Note that this eddy is not necessarily a turbulent phenomenon. Downstream from the eddy, the flow field develops a relatively flat velocity profile, except for a central maximum. Particle tracking (of aerosol droplets) reveals  Introduction  13  that the central flow velocity is approximately 25 mIs. Moreover, Reynolds numbers  <<  2000  validate the assumption that the flow field here is laminar rather than turbulent. The Reynolds number is defined as  Re  Lpv / 0 u,where L is the characteristic length of the structure  confining the flow, v 0 is the centerline velocity, p is the density of the fluid, and u  ,  its  viscosity. In the confinement tube of the ICAP, approximate values for these are 0.Olm, 10 mlsec, 0.2 kg/rn 3 and 2 x 10kg/msec, respectively [24], so 100 Re <<2000. The laminar flow field has both an axial and tangential component, the latter imparted by the tangential gas inlet for the coolant flow. The tangential component, or swirl, helps to stabilize the discharge. Simulations reveal that the swirl also concentrates the power density towards the axis [24]. This happens because the centrifugal moment of the swirl holds the bulk of the coolant stream against the confinement tube. As a result the outer boundary of the induction region is kept cool, and both the electrical conductivity and power dissipation are kept low beyond a certain radius, so the plasma is confined to a smaller radius than if swirl were absent. All of these flow field characteristics help us understand how solvent material can be transported through the discharge by convection. Beyond the exit of the torch, the flow field becomes far more complex. When the plasma jet flows out of the torch into the quiescent room air, the flow field is no longer bounded by the torch wall, but extends beyond into the argon stream. Where the flow field crosses from the argon jet into the air, there is a surface of discontinuity, or sudden jump between flowing argon and the air at rest (For clarity, we will ignore the fact that the surrounding air is actually drawn into the argon jet). Varicose instabilities form at this cylindrical surface of discontinuity between the argon jet and the quiescent room air. As these instabilities develop, they  14  Introduction  Shedding Ring Vortex  • Entrainment  Development into a Unidirectional Flowfielcl, with both Axial and Tangential Velocity Components  Recirculation Eddy at the Base of the Discharge  Tangential Argon Inlet Port  Figure 1.3 The flowfield in the inductively coupled argon plasma.  Introduction  15  modulate the diameter of the argon jet. Instabilities of this sort are familiar to anyone who has seen the jumping orange flame of a Bunsen burner. As the varicose pulsations propagate downstream, they roll up into ring vortices. Winge et al. provide high speed movies of these structures. Experimental evidence indicates that the varicose pulsations penetrate to the very axis of the discharge. Chapter 6 provides further details.  The thermal state of the Plasma Gas We can largely understand the analytical performance of the ICAP in terms of a physical description if we know the thermal state of the plasma and how it is modulated by dynamic process such as vortex shedding and aerosol vaporization. We have already discussed these dynamic processes, and more will be said about them in Chapter 6. As for the thermal sate, it turns out that the thermal state of the ICAP approaches local thermal equilibrium. In local thermal equilibrium, the kinetics of the plasma electrons may be described by the Maxwell velocity distribution. Moreover, the atomic state distribution functions for bound electrons may be described by the Saha and Boltzmann distributions. All of this is discussed in greater detail in the introductory sections of Chapters 7 and 9. The reader is referred to those sections for further discussion of the thermal state of the plasma, and how it departs from local thermal equilibrium. Two classes of processes determine the thermal state of the plasma—collisional radiative processes and transport processes. We now turn to them.  Introduction  16  collisional radiative balances the Saha Balance The thermal state of the plasma is dominated by the kinetics of the plasma electrons. Through inelastic collisions, free electrons rapidly ionize atoms and recombine with atomic ions. In doing so, they rapidly redistribute the atomic species over successive ionization stages. Consider, for example, the balance between collisional ionization and the reverse process of three body recombination, between an excited atom and ground state ion. This balance between two collisional processes may be written X+e+(I)  S  )X+e+e,  (1.1)  where X, is an atom of element X in excited state p, e an electron, X an ion in the ground state, and I, the energy required to ionize state p. If this balance prevails, then it can bring the ion ground state towards local Saha equilibrium (LSE) with the excited states of the atom. In LSE, the atomic state distribution function may be described by the Saha distribution. More precisely, the density of state p in LSE or  1 S (p),  is determined by the electron density  electron temperature Te and the ion ground state density  e  the  jjf  ,  I, n( h 2 exp—, I 2 22rmekTe j kTe  ‘i (p)=ij(l)—-j  where the state density i7 is defined as the level density per statistical weight, other symbols retain their usual significance.  (1.2)  ij  g  and the  Introduction  17  the Boltzmann balance Now consider balances involving a single ionization stage such as the neutral atom of an atomic species in the plasma. When free electrons collide with atoms and atomic ions, they rapidly excite and de-excite the bound states through collisional excitation and de-excitation: Xq +e+(Epq)—B-->Xp+e,  (1.3)  Here, X is an atom in state q, which lies at a lower energy than state p’ and Epq is the difference in excitation energy between the two states  ( E = E Eq ). If free electrons —  redistribute the bound states rapidly enough, then the atomic state distribution function will approach local Boltzmann equilibrium (LBE). In this thermal state, the excitation temperature Tex equals the electron kinetic temperature Te, and the atomic state distribution function may  be described by the Boltzmann distribution, jB(p)=j(q)exp  -Epq  (1.4)  e  where B (p) is the density of state p in LBE, and ij(q) is the density of state q. The Saha and Boltzmann balances are proper balances in which the forward collisional process is balanced by the reverse process and both processes are dominated by the kinetics of the plasma electrons. These balances tend to bring the atomic state distribution function towards local thermal equilibrium by bringing the ASDF into LTE. In contrast, improper balances are not dominated by electron kinetics. In fact they are controlled by energetic atoms, ions and photons. Moreover, the forward processes are not balanced by the corresponding reverse process and they cause the atomic state distribution function to depart from local thermal equilibrium. Perhaps the most important improper balance is radiative de-excitation.  introduction  18  radiative de-excitation  x  RDX+h  X  ‘  q  (1.5)  X + 3 hv  Here, atoms and ions in upper states p and r spontaneously emit a photon. As a result, they are de-excited from upper states p and r to lower states q and s, respectively. It has been suggested that this improper balance provides the dominant pathway for atomic species to depart from LBE. transport processes Transport processes can upset the Saha and Boltzmann balances outlined above, and provide a path for further departures from local thermal equilibrium. Chapters 7 and 8 discuss transport processes in greater detail. Fey provides a lucid summary of the transport processes which predominate in the ICAP—energy input, particle transport, escape of radiation and heat conduction [25]. Cambel provides a more general introduction to transport phenomena in plasmas in Chapter 7 of his text on plasma dynamics [26]. Dresvin’s text is also worth consulting for lucid descriptions of plasma phenomena [181. Two transport properties of the plasma gas are particularly relevant to this work— electrical conductivity and thermal conductivity. They appear to govern how the induction region of the plasma responds to solvent loading. The reader is referred to Chapter 8 for further details.  Introduction  19  1.7. Thesis Objectives and Summary When all of the relevant properties of the plasma are put together—the flow dynamics, the thermal state, sundry dynamic processes, collisional-radiative processes and the transport processes—one is left with an incomprehensibly complex physical system. Indeed, Feynman remarked that only a few simple physical processes are required to create an incomprehensibly complex system. In fact, the challenge facing experimenters today is not to discover the fundamental mechanisms, because they are already known for the most part, but to understand the complexity of their interaction. The same may be said of the solvent loaded ICAP. We already know the fundamental mechanisms at work in the discharge, but we do not quite understand the complexity with which they interact. We cannot even be sure which ones predominate, at least not for a wide range of operating conditions. So the basic challenge facing us is to understand the complexity of the inductively coupled argon plasma, the complexity of its temporal behavior, its spatial structure, and in this thesis work, the complexity with which it responds to loading by various solvents. In order to face that challenge, a systematic strategy was eventually devised for investigating the solvent loaded, inductively coupled plasma. The first step of the strategy was to survey the complexity in physical space, parameter space and time. The idea was that the preliminary survey of the complexity would direct and facilitate investigations of the physical properties of the discharge. Once the physical properties had been characterized, at least the ones relevant to the problem of solvent loading, then we would have the most rational perspective from which to improve the design and methodology of analysis techniques which use the solvent loaded ICAP as a spectrochemical source. The objective of this thesis was simply to apply this strategy to a very specific ask—to develop the inductively coupled argon plasma for applications in which solvent loading imposes an obstacle to trace metal analysis. In fact, the next eight chapters follow the strategy step by step. Chapter 3 reports detailed observations of how the ICAP responds to loading by water, chloroform and methanol. In this way, it provides a preliminary indication of the complexity of the discharge, as far as the  Introduction  20  eye can discern. It even anticipates a few of the physical properties that will be revealed in the succeeding chapters. Chapter four then surveys the complexity with which the analytical signal responds to solvent plasma load. Chapter 5 and 6 explore the spatial and temporal complexity of the discharge, and together with Chapters 3 and 4, offer valuable guidelines for exploring its physical properties. Chapters 7 through 9 then explore the physical properties. Chapter 7 discusses the rationale behind exploring the electron densities of the plasma gas, then reports the results of an extensive electron density survey. The electron densities in this survey were determined from the absolute intensity of a highly excited argon line, and provide much insight into how the physical properties of the discharge responds to solvent plasma load. Chapter 8 presents more accurate electron densities determined from the Stark broadening of a hydrogen line. The spatially resolved profiles of electron density presented in this chapter reveal how the induction region responds to solvent loading, and sort out many of the contradictory results reported in the literature. But Chapter 8 still leaves ignorant of how the bound states of the analyte atoms are excited. That question is addressed in Chapter 9, which rigorously examines how the atomic state distribution function for iron responds to solvent plasma load. Finally, Chapter 10 summarizes all of the conclusions, assesses the effectiveness of the strategy originally proposed, and suggests directions for future work in this area. 1.8 References 1.  Boumans, P.W.J.M., SpectrochimicaActa, Part B, 1991. 46B(617): p. 711-739.  2. p.3.  Maessen, F.J.M.J., G. Kreuning, and I. Balke, Spectrochimica Acta, Part B, 1986. 41B:  3.  Sharp, B.L., Journal ofAnalytical Atomic Spectrometry, 1988. 3: p. 613-652.  4.  Browner, R.F. and A.W. Boom, Analytical Chemistry, 1984. 56(7): p. 787A-798A.  5.  Olesik, J.W., Analytical Chemistry, 1991. 63(1): p. 12A-21A.  Pan, C., G. Zhu, and R.F. Browner, Journal ofAnalytical Atomic Spectrometry, 1990. 5: 6. p. 537. 7. Pan, G., G. Zhu, and R.F. Browner, Journal ofAnalyticalAtoinic Spectrometry, 1992. 7: p. 1231-1237. 8.  Canals, A. and V. Hernandis, Journal ofAnalytical Atomic Spectrometry, 1990. 5: p. 61-  Introduction  21  9.  Cicerone, M.T. and P.B. Farnsworth, Spectrochimica Acta, Part B, 1989. 44B: p. 897.  10.  Olesik, J.W., L.J. Smith, and E.J. Williamsen, 1 sheet. Anal. Chem., 1989. 61: p. 2002.  11.  Olesik, J.W. and J.C. Fister, SpectrochimicaActaPartB, 1991. 46B(6/7): p.851-868.  12.  Olesik, J.W. and J.C.F. Ill, SpectrochimicaActa, Part B, 1991. 46B: p. 869-883.  13.  Olesik, J.W. and S.-J. Den, SpectrochimicaActa, PartB, 1990. 45B(7): p.731-752.  14. Horlick, G. and F. Qin. in Federation ofAnalytical Chemistry and Spectroscopy Societies Meeting XVII. 1990. Cleveland, OH: .  15. 384.  Maessen, F.J.M.J. and G. Kreuning, Spectrochimica Acta, Part B, 1989. 44B(4): p. 387-  16.  Winge, R.K., et aL, Journal ofAnalytical Atomic Spectrometry, 1988. 3: p. 849-855.  17. Winge, R.K., J.S. Cram, and R.S. Houk, Journal ofAnalytical Atomic Spectrometry, 1991. 6: p. 601 604. -  18. Donskoi, A.V., V.M. Goldfarb, and V.S. Klubnikin, Physics and Technology ofLowTemperature Plasmas. English Edition ed. 1977, The Iowa State University Press. 471. 19.  Barnes, R.M. and R.G. Schleicher, Spectrochimica Acta, Part B, 1981. 36B: p. 81-101.  20.  Barnes, R.M. and J.L. Genna, Spectrochimica Acta, Part B, 1981. 36B: p. 299-323.  21. Hill.  Patankar, S.V., Numerical Heat Transfer and Fluid Flow. 1980, New York: McGraw-  22.  Becker, H.A. and T.A. Massaro, Journal of Fluid Mechanics, 1968. 31: p. 435-448.  23. Dahm, W.J.A., C.E. Frieller, and G. Tryggvason, Journal of Fluid Mechanics, 1992. 241: p. 37 1-402. 24. Benoy, D.A., Modelling of ThermalArgon Plasmas. Ph.D. Dissertation 1993, Technische Universiteit Eindhoven: 25. Fey, F.H.A.G., Excitation Balances and Transport in an inductively coupled Plasma. Ph.D. Dissertation 1993, Technische Universiteit Eindhoven: 26. Cambel, A.B., Plasma Physics and Magnetofluidmechanics. McGraw Hill Series in Missile Technology, ed. H.S. Seifert. 1963, New York: McGraw Hill Book Company, Inc. 304.  Chapter 2 The Experimental Setup Figure 2.1 depicts the overall experimental setup used to study the effects of solvent plasma load and other parameters of the inductively coupled argon plasma. The setup essentially consisted of a sample introduction system (a-k), inductively coupled argon plasma (1), light collection optics (o,p), and a grating spectrometer (q—x).  2.1 Sample Introduction Briefly, a peristaltic pump (b) fed the nebulizer (e) with test solution (a) through line (d). The nebulizer generated an aerosol stream on argon which flowed through the spray chamber (g). The aerosol stream was then directed through tube (i) into a thermoelectrically cooled, desolvating condenser (j). The peristaltic pump drained the condenser through line (h) and the spray chamber through line (f) into the waste flask (c). Narrow bore, Intramedic ® tubing was used for all of these lines in order to minimize the overall dead volume. Because other avenues of solvent loss were insignificant (e.g. leakage and evaporation), the weight difference between the test solution and the waste solution determined the amount of solvent mass delivered to the plasma—the solvent plasma load. Moreover, the solvent plasma load could be calibrated accurately and reproducibly against the condenser temperature. The aerosol gas flows were similarly accurate and reproducible. In most experiments, the primary aerosol gas flow was set at 0.61 11mm by the high pressure, cross flow nebulizer, while any extra gas fed through adapter (k) was controlled accurately and precisely by a mass flow controller. The extra gas, or adgas, was argon in most experiments, but gases such as oxygen, nitrogen, and hydrogen could also be added. In fact, a safe, convenient way to add oxygen to a combustible aerosol stream was to add oxygen diluted in argon through the adgas adaptor (k). In short, the solvent plasma load, inner gas flow rate and inner gas composition could be controlled reproducibly and accurately and the major sources of irreproducibifity in the experimental setup were eliminated.  22  -  -  Figure 2.1. A schematic overview of the experiment. Shown are the sample introduction system (a k), the inductively coupled plasma torch (1), and the grating spectrometer (q x). The torch is shown inside its Faraday shield or torch box. The shorter box mounted alongside houses the impedence matching network. Both boxes are mounted on a translation stage. The stage was driven by a computer controlled stepper motor (z), and was capable of translating the entire torch, matching network and suspended condenser (j) in the X direction, or across the optical axis (in the Z direction). The r.f. power supply, detector hardware, data-acquisition hardware and computer are not shown. See the text for further details.  x  (q)  (p)  The Experimental Setup  24  nebulizers A MAK (Meddings, Anderson and Kaiser) nebulizer and MAK spray chamber were used for most experiments. The MAK high pressure, cross flow nebulizer [1] not only proved robust to rough handling, but delivered extremely stable and reproducible argon streams. For the MAK nebulizer used in this work, the inner argon flow rate was 0.61 ± 0.01 11mm, where the standard error was principally due to the technique used to calibrate the flow rates (flow rates could be reliably calibrated by timing the passage of soap bubbles through a burette, downstream from the nebulizer). The experimental work also incorporated Meinhard concentric nebulizers with a Scott, double pass spray chamber. Work with these nebulizers was restricted to water loading studies.  solvent plasma load: calibration and control For all of the organic solvents investigated, the solvent plasma load was calibrated against the condenser temperature using the method depicted schematically in Figure 2.2.b., the continuous weighing method devised by Maessen et al. [2] In this work, the method was modified slightly by equipping the sample and waste flasks with rubber septa and hypodemiic needles. Essentially, the solvent plasma load was determined by monitoring the decrease in solvent mass. Maessen et al. [2] provide further details, including the appropriate intervals for sampling the decrease in solvent mass, and the determination of the error in the solvent plasma load. Figure 2.2.a depicts a sample calibration plot for chloroform plasma load, showing a cubic polynomial fit through the data by least squares regression. In general, the dependence of solvent load on condenser temperature could be satisfactorily fit with a cubic polynomial. A sigmoidal function would not be suitable because the solvent plasma load actually began to increase at very low condenser temperatures. Maessen et aL [21 explained this phenomenon by pointing out that at very low temperatures the solvent vapour condenses on the aerosol droplets rather than the condenser wall. Regardless of the true functional form of the solvent load curves, the experimental work did not rely on least squares interpolations. Instead, the solvent load was calibrated for a set  25  Experimental  E 0 -J  a:  0 LI.. 0 a: 0 -J  I  C.)  -20  -10  0  10  20  CONDENSER TEMPERATURE, C  condenser drain  Figure 2.2. (a) Calibration plot of chloroform plasma load versus the temperature of the aerosol  desolvating condenser. (b) Schematic flow chart of the continuous weighing method used to calibrate the solvent plasma load against condenser temperature.  The Experimental Setup  26  of condenser temperatures, and those condenser temperatures were used to set the solvent plasma load. The continuous weighing method was not suitable for measuring water plasma load because large water droplets tended to cling to the condenser and spray chamber walls. Rather than adding surfactant to make the water drain as freely as the organic solvents, the water plasma load was determined by trapping the aerosol at the exit of the condenser. A cold trap condensed the vapour while glass wool trapped the droplets. Table 2.1 summarizes the maximum and minimum solvent plasma loads used in the experimental work. Note that the table expresses solvent load in units of mass, moles of solvent molecules, moles of dissociation product atoms, and total bond dissociation energy, all per unit time. These units are somewhat helpful in interpreting the experimental results. The error in the tabulated values should be taken as ± 5%. While the continuous weighing method was capable of determining the solvent plasma load within a standard error of± 2%, a conservative value of± 5% allows for long term drift.  further aerosol modification: adgas In order to vary the total inner argon flow rate, extra argon was added downstream from the condenser through an adgas adapter. In this way, the total inner argon flow rate could be varied without perturbing the solvent load or the analyte transport efficiency through the nebulizer. The extra gas surrounded the primary aerosol stream as an annular sheath. Observations of laser light (red helium neon) scattered off the aerosol confirmed that the annular sheath did not mix with the central aerosol stream, in agreement with the low Reynolds number for the flow stream. As a result, the aerosol test species were probably concentrated towards the center of the aerosol stream, while freely diffusing solvent vapour was probably distributed more evenly. In some experiments, a sheath of oxygen diluted in argon was added to the aerosol stream. It turned out that adding oxygen this way was both convenient and safe, in contrast to experiments conducted by earlier workers. Interestingly, when oxygen diluted in argon was added in equimolar proportion  27  Table 2.1. Summary of the ranges of solvent plasma load investigated.  Bond Chlorine Dissociation Load, Load,  Mass Load,  Molar Load  Carbon Hydrogen Oxygen Load, Load, Load,  Q SPL  Q SPL  Q CPL  Q HPL  Q OPL  Q C1PL  Q dissoc  mg/s  pmo1/s  jimol/s  jimol/s  Jimolls  !ImolJs  Watts  0.10  5.6  11.2  5.6  0.30  16.7  33.4  16.7  0.20  1.9  15.2  19  0.45  4.2  33.6  42  0.30  5.0  15.0  40.0  5.0  1.0  16.7  50.1  134.0  16.7  0.20  6.3  6.3  25.2  6.3  1.3  40.6  40.6  162.4  40.6  carbon  1.3  8.4  8.4  tetrachloride  6.3  40.9  40.9  chloroform  3.0  25.0  25.0  25.0  10.0  83.3  83.3  83.3  Solvent  water  m-xylene  i-propanol  methanol  —  —  —  —  5.2 15.5  —  14.8 32.8  —  —  —  22.0 73.5  —  13.0 83.6  33.6  11.0  164.0  53.2  100.0  35.0  250.0  117.0  The Experimental Setup  28  to the carbon plasma load of xylene or chloroform—solvents that do not contain oxygen—this resulted in a discharge similar to one loaded by methanol. For brevity, the results of those oxygen addition experiments are not presented in this document.  observations of the aerosol stream: the distribution of solvent plasma load between aerosol droplets and vapour The size distributions of the aerosol droplets and the transport efficiency of the test analytes were not determined in this work. However, observations of the aerosol stream indicated that the transport efficiency through the condenser was close to 100%, except at very low condenser temperatures, and that large droplets were not abundant in desolvated aerosols. Observations of the aerosol stream within the condenser indicated a high transport efficiency through the condenser: The aerosol stream was clearly stratified and separated from the condenser wall. It appeared as though the aerosol had imparted a static charge on the condenser tube. One could suppose that charged droplets had initially collided with the wall and stuck onto it. Then the charged wall repelled all the following droplets of like charge. Hence, electrostatic repulsion kept the droplets from colliding with the condenser wall, so the transport efficiency through the condenser was kept high. Another set of observations gave some indication of the overall trends of the droplet size distribution. When the aerosol was sufficiently desolvated, the visibility through the aerosol at the exit of the condenser increased, presumably because the droplets had vaporized. In this case, the mass of the vapour component of solvent plasma load undoubtedly predominated over droplet component. Scattered laser light (from a red helium neon laser) helped in these observations, except when the fluorescence from red test analytes (such as iron acetylacetonate) swamped out the scattered light. In general, the observations indicated that the vapour component predominated over droplets for most desolvated aerosols. Even so, a rigorous analysis of the droplet sizes should be conducted, but is beyond the scope of this thesis.  29 Table 2.2.  Summary of Experimental Setup  ICP Unit  Plasma Therm 1CP2500  Power supply Impedance matcher Incident power Reflected power Induction Coil Torch  Plasma Therm HFP2500F Plasma Therm AMN-PS-1 1.00 to 1.50 kW 0 to 50W 3 turn coil,1 in. ID, 1/16.in. spacing between turns, 1/8 in. OD copper tubing, 14C to 15C cooling water U.B.C. Low Flow, copied from MAK design  Argon Flow Rates Outer Intermediate Inner  Translation stage  10.0 1/mm (needle valve control,rotameter reading) 0.5 1/mm (needle valve control,rotameter reading) 0.61 to 1.01 1/mm (0.61 1/mm nebulizer flow rate plus 0.00 to 0.40 1/mm adgas controlled by mass flow controller) stepping motor driven (Superior Electric Slo-Syn type M062-TDO3), variable scan distance N*0.0125mm lateral steps  Sample Introduction System—suspended from translation stage Nebulizer  high pressure, cross-flow(MAK)  Spray Chamber  double pass (MAK)  Desolvator  variable temperature condenser (U-shaped, 1.3 m pyrex tube, 11 mm ID, cooled by 6 thermoelectric coolers (Melcor model CP1.4-127-06L) backed by water cooled plates  Adgas fitting  inner tube (aerosol) 4mm ID,outer tube (adgas) llmmID  Sample Uptake rate  1.0 mL/min (controlled by Gilson Minipuls 2 peristaltic pump equiped with 2mm ID Isoversinic pump tubing—Mandel Scientific)  Solvent load caffibration  Organic solvents: continuous weighing method Water: glass wool filter followed by cold trap  30  Table 2.2. (continued) Imaging lens  Oriel model 41775 fused silica, piano-convex; centre thickness = 6.9 mm, edge thickness = 2.0 mm; diameter = 50.8 mm; radius of curvature= 68±7 mm; focal length (589nm)=l5Omm; back focal length 145.2 mm  Spectrometer Monochromator  Detectors  im Czerny-Turner (Schoeffel-McPherson 2061) 120 x 140mm, 1200 g/mm holographic grating, 0.833 nm/mm reciprocal linear dispersion (Schoeffel-McPherson Model AH-3264) Photodiode Array Detectors Spatial Domain: Reticon RL4096/20; 4096 pixels,7pm wide on 15 tm centres; Spectral Domain: Reticon RS 2048; 2048 pixels, 12.tm wide on 25 tm centres both cooled to -15 C by thermoelectric coolers (Melcor CP14-71-1OL) backed by water cooled plates; dry nitrogen purge prevented frosting Photomultiplier Tube Hamamatsu R955; Kiethley model 427 current amplifier; Kepco ABC 1500 high voltage dc power supply  frradiance Standard  General Electric tungsten iodine QL-10 lamp  Data Acquisition Board  RC Electronics ISC-16 board, sampling rate up to 1 MHz for single channel operation  Solvents  BDH analytical grade chloroform, carbon tetrachloride, iso-propanol, meta-xylene, methanol  Test Analytes  Organic solvent solutions: 2,4 pentanedionates of iron, magnesium, calcium and manganese, Thiokol Chemicals; note that the magnesium and calcium pentanedionates were only slightly soluable in chloroform, xylene and carbon tetrachloride. Aqueous solvent solutions: ammonium ferrous sulfate; (the water was boiled, then sparged with nitrogen to deoxygenate it and prevent oxidation of Fe 2 to Fe j 3  Test Solution Concentrations  lion solutions for Fe I and Fell measurements (Chapter 9) were made up to 1000 .tg/ml (0.0182 M). Other solutions were 5pg/ml (all concentrations refer to the analyte metal).  The Experimental Setup  31  flush time Several characteristics of the sample introduction system make it unsuitable for routine trace metal analysis. Perhaps the most important drawback is the flush time, or the time required to completely flush all traces of a sample from the spray chamber and condenser. This time exceeded two minutes, while flush times must be less than one minute for economical routine analysis. It would not be difficult, however, to tailor the system for practical applications. However, no attempt to do so was made in this work.  2.2 Torch The aerosol stream was fmally directed into the ICP torch. This work relied on torches of the MAK, high efficiency design. Dimensions for this design are given elsewhere [3].  2.3 Power Supply Details of the r.f. power supply, impedance matching network and induction coil are summarized in Table 2.2. Briefly, the r.f. power at 27.12 MHz was operated between 0.75 and 1.75 kW, with most experiments conducted at 1.00, 1.25 and 1.50 kW. Figure 2.3 reveals the flow of power to the ICAP discharge. The reader is referred to the discussion by Ripson and de Galan [4] for further details. Note that much of the power dissipated in the plasma, say 550 Watts, is spent heating the outer coolant stream. As a result, much of the plasma energy is carried away by convection. This may be understood if one considers that the power dissipated in the induction region of the discharge (see Figs. 1.1 and 1.2) is rapidly transported as heat, outward to the outer sheath of the coolant stream and through the torch wall by thermal conduction. Additionally, a smaller fraction of the total power, say 100 Watts, is lost from the induction as ultraviolet, visible and infrared radiation. Ultimately, if one balances all of the avenues of power consumption as Ripson and de Galan did, one finds that at an r.f. power of 1.25 kW, only 300 W is available to heat, vaporize, dissociate, and excite the sample material, including the analyte and the solvent. Note that the carrier gas must also be heated, excited and ionized to some extent. Of that 300 W available, then, perhaps 100 W is required to heat the carrier argon, and another 100 W may be lost  32 Experimental  <1 W anaMe line emission  L  ? 2 100 W air entrainment, emission from C  C  100 ‘A’ solvent dissociation  C  100 W inner argon  300W available to sample 100W radiation from argon plasma  550 W coolant argon and heat conduction through torch wall  300 W power loss to coil and matching network  1250 W generator power  Figure 2.3. The flow of power in a solvent loaded ICAP (adapted from reference [6]). Most of the power dissipated in the discharge is carried away by convection with the coolant flow or conducted out through the torch wall. A minor fraction is left to dissociate the solvent. The solvent bond dissociation load is appreciable in comparison with the power available to the sample.  The Experimental Setup  33  radiatively by the brightly emitting solvent pyrolysis products. That leaves 100 W to dissociate the solvent material. Looking back at Table 2.1, one finds that the bond dissociation load (by far the most significant component of the solvent load, in terms of energy consumption) is comparable to the power available. In that case, it is conceivable that the sample could be incompletely pyrolyzed. Clearly, Table 2.1 shows some values of bond dissociation load exceed the power available to the sample. It is useful to bear this in mind in the following chapters.  2.4 Ignition Procedure Maessen et aL [2] described an elaborate ignition procedure for ICAPs loaded with organic solvents. Their procedure was designed to prevent carbon soot from forming on the walls of their torch during ignition. The plasma is ignited by first applying r.f. power to the three turn induction coil around outer tube of the torch. Next, the high voltage output from a Tesla coil is used to create an initial discharge in the argon stream. This initiates the ICAP by allowing the induction coil to couple power into a conductive gas. Finally, the ICAP is sustained by the r.f. induction current, independently form the Tesla coil. However, problems arise during the initial ignition because the initially unstable ICAP often touches the torch wall. If solvent material were present in the argon stream, then pyrolysis products could form soot on the torch wall during ignition. Any small amount of soot acts as an anchor or nucleation site for further soot build, which eventually blocks the argon flow. The answer is to prevent any soot buildup at all during plasma ignition. Maessen’s procedure involved cleaning out all of the solvent material from the sample introduction system. In the work presented in this dissertation, it was found that the ICAP could be easily ignited by simply turning up the adgas to extremely high flow rates, thereby diluting the aerosol stream with argon, and preventing any solvent material from interacting with the plasma during ignition.  The Experimental Setup  34  2.5 Light Collection Optics Specifications of the lens used in this work are provided in Table 2.2. In general, it was fitted with a vertical, 30 mm x 5 mm aperture to limit the acceptance angle in the lateral direction yet pressure the light throughput in the vertical direction. Figure 2.4(a—d) depicts some typical light collection maps for the optical train. These maps were calculated with an exact ray tracing procedure developed by Farnsworth et al.  [51. In this work, the light collection efficiency was  calculated for the entire optical train, or from source to detector. Figure 2.4(a) depicts a map for unit magnification and no aperture fitted to the lens. Clearly, the light collection envelope is smeared out and would be unsuitable for collecting intensity profiles destined for Abel inversion. Figure 2.4(a) depicts a map for unit magnification and a 30 mm x 5 mm aperture, a configuration clearly suitable for Abel inversion.  The same applies to Figure 2.4(c)—a map for 0.5X  magnification (2 to 1 imaging) with a 30 mm x 10 mm aperture. In maps (a) through (c), the light collection efficiency has been integrated over a 2 mm vertical distance. Figure 2.4(d) shows the light collection efficiency integrated along the line of sight. This map reveals that the vertical resolution is limited to ±0.3 mm, which was the vertical dimension of a diode batch on the vertical array detector (w) in Figure 2.1. Note that these maps do not account for reflectance loss off the lens surfaces. Also note that chromatic aberration was corrected by adjusting the image and object distances according to the Lens Maker’s equation (see Chapter 5). Further experimental details are either summarized in Table 2.2 or provided later in this thesis. The interested reader is urged to consult the introduction to Chapter 5, where several spectroscopic imaging configurations are discussed.  r C  CD ,-: r  ‘.-  C  z CD  •.  CD  ABSOLUTE EFFiCIENCY,  C -.  3Q  iO  c,  <‘ ABSOLUTE EFFICIENCY,  4.  5  2  4  6  6  io  70  ,  c;r Ui C CCD —  *CD 0 CD  I—.  CD  CD 0  44—’.-, .  EFFiCIENCY,  10  io  Ui CD  4-’. -4  C 4_  -  z  E.  -  4-’.  0  CD  -4  o 0 o CD CD I.— C CD CD Ct -4. —  ,—‘  9 jviuauuadx’  The Experimental Setup 2.6 References 1.  2.  3. 4. 5.  Anderson, H., H. Kaiser, and B. Meddings. Proceedin gs of the Winter Conference on Plasma Chemistry. 1980. San Juan: Heydon, London 1981. Maessen, F.J.M.J., G. Kreuning, and J. Bailce, Spec trochimica Acta, Part B, 1986. 41: . 3 p. Rezaaiyaan, R., etal.,AppliedSpectroscopy, 1982.36: p. 627-631. Ripson, P.A.M. and L.de Galan, Spectrochimica Acta , Part B, 1983. 38: p. 707-726. Farnsworth, P.B., B.W. Smith, and N. Omenetto, Spec trochimica Acta, Part B, 1990. 45(10): p. 1151-1166.  36  Chapter 3 Spectral Survey and Observations  3.1 INTRODUCTION  3.1.1 Objectives Ever since Greenfield  [1] first used the ICP for spectrochemical analysis, a growing  number of research groups have contributed their work to an ever expanding body of ICP-AES and ICP-MS literature. However, out of all of the spectroscopic and mass spectrometric studies, few authors have reported what their ICP discharge actually looked like when viewed through welder’s glass. Fewer still have described how their discharge altered in appearance when they varied its operating parameters. This general failure to report visual observations strikes one as absurd when one considers the excellent case for reporting them in detail. That case is stated in this chapter in an attempt to convince the reader that visual observations are an essential part of developing and characterizing any spectrochemical plasma. In our preliminary investigations of the solvent loaded ICAP, visual observations held several advantages over spectroscopic measurements.  Not only did observations yield  information more rapidly, they opened access to a far greater range of the operational parameter space. For example, they opened access into regions of the parameter space where the discharge became unstable, far too unstable to measure ‘steady state’ spectroscopic quantities with the available hardware, yet stable enough to view its behavior through welder’s glass. Moreover, the entire discharge could be observed at once, including regions within the load coil (observed by looking down into the torch). By contrast, these regions\ would have been hidden by the load coil  Observations 38 when viewed side on, say for spectroscopic measurements. (In order to take advantage of the cylindrical symmetry of the discharge for calculating radially resolved intensities, spectroscopic measurements must be made side on.) Moreover, the three dimensional structure of several visible emission features could be perceived by viewing them from any angle. In short, visual observations opened access to a greater range of the parameter space than that available to spectroscopic measurements, a greater range which could be surveyed more rapidly. In addition, by looking at the discharge through welder’s glass, one could judge the temporal stability of the discharge or discern spatiotemporal behavior with time contants greater than lOOms. One could also check that the discharge was in focus for the spectroscopic measurements by checking that the spatial resolution of the spectroscopic measurements were consistant with the appearance of the discharge. Lastly, observations could often confirm that the spectroscopic measurements were realistic. Although there were several limitations intrinsic to visual observations, including the limited spectral response of the human retina, observations revealed much about the physical properties of the discharge. For example, one of the most conspicuous emission features of an ICAP loaded with organic solvent fell well within visible range of wavelengths--emission from diatomic carbon. The parametric behavior of this emission feature proved informative indeed. It revealed where sample atomization approached completion as well as indicating where the boundary of the atomic plasma resided. This and all of the advantages of visual observation cited above made it indispensible to our investigation. One fmal reward of noting and reporting obsevations is worth mention: Carefully reported observations provide researchers with a spatial reference frame for comparing their ICPs with those from other laboratories. Koirtyohann demonstrated that a visually observable, definite emission zone within the structure of the discharge could be reliably used as a spatial reference point for interlaboratory comparisons [2], [3]. The objective of this chapter was to take full advantage of visual observations in order to extract as much information on the behavior and physical characteristics of the solvent loaded ICAP as possible. In order to achieve these objectives, literature reports of ICAP observations  Observations 39 were surveyed. Also surveyed were the backround emission spectra over the visible region in order to identify the spectral components under observation (as well as to present a survey of spectral features, to uncover the ones which may be useful and most amenable for spectroscopic measurements). Finally, the detailed observations made during this thesis work were compiled. The reports of these observations have been carefully illustrated by photographs and detailed drawings.  3.1.2 Literature Survey  Boumans et al. [4] described general appearance of an ICAP loaded with methyl isobutyl ketone. Their sketches roughly portray the spatial relation between the boundary regions of the discharge where molecular emission predominates, and the plasma region where atomic emission predominates. Moreover, the response of these structures to the variation in operating parameters is described. Overall, the observations reported by these authors are consistent with those reported in this chapter. Mixed Gas and Molecular Gas ICPs Much literature reports visual observations of mixed gas ICPs, and ICPs sustained by molecular gases such as nitrogen and oxygen. This literature is pertinent to the analogous solvent load problem; It offers physical explanations for the response of the ICP to variation of gass composition and flow rate. In special cases, these physical explanations may also explain the response to solvent loading. Lastly, the literature on mixed gas ICPs is pertinent to solvent loaded ICAPs because it supplies a terminology for describing what the ICP discharge looks like. Truit and Robinson introduced a terminology for describing mixed gas ICPs [5], which indeed proves convenient for discussing solvent loaded ICAPs. They described their mixed gas ICP as consisting of three zones: the brilliant blue white, opaque core; the bright white, transparent secondary or transition region (weaker in intensity than the core); and the faint blue, transparent tailfiame. Their core (also known by other investigators as the energy loading region) resided within the load coils and assumed the shape of an annular cylinder or toroid. This region  Observations 40 appeared opaque because its emission was so intense that only a very bright object could be percieved behind it. The transitition region appeared dimmer than the core, hence appeared transparent—objects behind it could be readily percieved. (This region is also known by other authors as the decay region, under the assumption that energy dissipation from the plasma outweighs energy loading there.) Finally, the tailfiame capped the secondary region, and was not strictly part of the plasma at all. It was actually a boundary region where air was entrained into the plasma gas resulting in molecular emission and weak atomic emission. The tailfiame was faint, transparent violet. Truit and Robinson also conducted a spectroscopic study of an ICP into which they had introduced organic compounds [6]. They supplied a typical emission spectrum from such an ICP, but they did not go into extensive detail in reporting their observations beyond describing the overall structure of the discharge. Montaser, Fassel and Zalewski supplied a photographic record of their observations of an argon  -  nitrogen ICP supplied with various proportions of the two gases, sustained at several  different powers [7]. They compared the effect of introducing nitrogen as a coolant (outer flow) with the effect of introducing it through the aerosol injector tube (inner flow). When nitrogen was introduced through the injector tube, the central channel increased in diameter, while the peripheral diameter remained constant, so that the plasma formed a thinner annulus of constant outer diameter. Introducing yttrium revealed the analyte distribution within such a discharge (blue emission from ions, red from the diatomic oxide): The analyte distribution did not expand in diameter along with the central channel. This implied that nitrogen introduced through the injector tube had degraded the interaction between the sample and the plasma. On the other hand, when nitrogen was introduced as the outer flow, the discharge assumed the shape of an inverted cone: its diameter was constricted within the load coil, but expanded radially further downstream. They explained this observation in terms of a thermal pinch, originally suggested by Thorpe [8] and explained in detail by Greenfield et al. [9]. Apparently, this thermal pinch enhanced the interaction between the analyte and the plasma.  Observations 41 Briefly, the thermal pinch effect depends on the thermal and electrical conductivity of the plasma gas and their effects on energy loading into and energy dissipation out of the plasma. When a plasma gas containing molecular species (including solvents and diatomic gases) reaches a sufficient temperature (at a given pressure) the bulk of molecular constituents dissociate, and the enthalpy of their dissociation not only cools the plasma gas, but increases its thermal conductivity (the thermal conductivity of nitrogen increases from approximately 10 times that of argon to 36 times that of argon when the temperature increases from 5000K to 7000K). This increase in thermal conductivity accellerates heat conduction away from the plasma, especially across the steep thermal gradients at its boundary. This accellerated heat loss rapidly cools the peripheral regions of the plasma volume. As the peripheral regions cool down, they lose their electrical conductivity, causing the plasma volume to shrink But the overall power loading into the plasma tends to remain constant. In order to keep its total power loading constant as it shrinks in volume, the plasma reacts by increasing its power density. This process eventually results in a more compact, hotter plasma, able to maintain a stable balance between energy dissapation and energy loading. The contraction is known as the thermal pinch effect. As mentioned above, the thermal pinch effect manifested itself in two ways in mixed gas ICPs. In one way the plasma cantracted into an inverted cone when the molecular gas was introduced via the coolant or outer flow. In the other way the central channel expanded, resulting in a thinner plasma annulus when the molecular gas was introduced via the carrier stream or the inner flow. As a digression, it is interesting to note that Montaser et al. ‘s photograph of an argon ICP loaded with nitrogen through the injector tube appeared very similar to an argon ICP (or ICAP) loaded with methanol, or very similar to an ICAP loaded with other organic solvents to which oxygen had been added to the carrier gas in stoichiometric proportions to form carbon monoxide (see chapter 10). Greenfield et al. have provided the most careful analysis of visual observations for mixed gas ICPs [1], [9]. Here, we will only note that they projected an image of the discharge onto graph paper in order to trace the boundaries of plasma region. From such geometric records, they  Observations 42 produced a table of plasma dimensions for different flow rates of molecular gas added to the discharge.  3.2 Spectral Survey The line of sight emission from a solvent loaded ICAP was surveyed over visible wavelengths, for several observation heights, and for loading by three different solvents: water, methanol and chloroform. The results of this survey are presented in Figures 3.2.1, 3.2.2, and 3.2.3. In each figure, all of the spectra (each corresponding to a different height above the load coil) were scanned simultaneously by a linear photodiode array mounted vertically, to sample emission from a range of observation heights. The resulting emission survey made it possible to identify the conspicuous emission features visible to the observer’s eye, and provided a survey of how these emission features depended on the observation height. (The emission survey also revealed which emission signals would be most amenable to spectrocopic measurement by revealing their relative intensity, their signal to background ratio, and their freedom from spectral interference.)  Observations  O.4  0.4j  E L”____ 400  440  480  520  560  600  640  680  Wavelength, nanometers  Figure 3.2.1.Spectral survey of the visible emission from the inductively coupled argon plasma loaded with methanol. The spectra were recorded simultaneously at the different heights, 6 mm, 9 mm, 12 mm, 15 mm, 18 mm, and 21 mm above the load coil, as described in the experimental section.  720  44  Observations 0.6-  o.4*Ji 0.20.0-  I  I  I  I  0.60.4-  IlBmmI  0.2-  JI  0.00.60.40.20.00.60.4 0.2-  JLmJJ  0.00.60.40.20.00.60.40.20.0400  440  480  520  560  600  640  Wavelength, nanometers  Figure 3.2.2. Spectral survey of the visible emission from the inductively coupled argon plasma loaded with water.  680  720  45  Observations 0.6-  I  0.4-  121mm1  0.20.0-  .  I  .  L  I  .  .  I  I  0.60.40.2-  —  0.0-  0.6— 0.40.2-  0.60.40.20.00.60.40.2-  LLtrt I  I  I  I  I  I  I  I  •  I  I  IL  I  U LLJJ JhzI  I  I  •  I  •  I  •  I  I  •  •  I  •  I  •  I  4jFjj  0.00.60.40.20.0400  440  480  520  560  600  640  Wavelength, nanometers  Figure 3.2.3 Spectral survey of the visible emission from the inductively coupled argon plasma loaded with chloroform.  680  720  —.  -“  • Cl)  CD  C,,  CD  CD  —  rCD  C  C) CD  ‘  CD  CCD  C  -  —.  CD  3  D C  C  DC  CD CD D CD  C  C  C  C  C  C C  Relalive Sensitivity C  C  —  —  •4•L•oocc  C —  C  CD  CD  0  C  CD  CD  CD  CD  I  Observations 47  A comparison of the spectral survey, in Figures 4.2.1 through 4.2.3, with the photopic (bright light adapted) sensitivity of the eye, shown in Figure 4.2.4, reveals which emission features are accessible to observation. The light adapted eye is most sensitive at 555-nm, in the yellow-green region. Its sensitivity drops below 1% of this maximum value towards the violet wavelength of 430-nm and towards the red wavelength of 690-nm [10], [11]. If we let 1% of maximum sensitivity define the limits of the visible region, then comparison of Figures 3.2.1 through 3.2.3 with 3.2.4 reveals that green emission, from diatomic carbon, and white emission, from atomic argon line and continuum emission, should be visible. However, the photopic sensitivity only drops off asymptotically at the violet and red extremes, down to 0.01% at 370-nm and 760-nm [12]. Consequently, very intense emission, say from the CN bandhead at 388 nm, is observable within these 0.01% limits. In short, the most conspicuous visible emission features are 1) band emission, from diatomic carbon and cyanyl radicals in boundary regions of the discharge, 2) line emission from atomic carbon and argon and 3) the ubiquitous continuum emission from the atomic plasma. Also conspicuous is incandescent emission from soot particles, but this is not evident in the spectral survey. Of the three solvents surveyed, chloroform loading resulted in the most intense diatomic carbon emission, while both chloroform and methanol loading resulted in intense cyanyl radical emission. By contrast, diatomic carbon emission from the methanol loaded ICAP was weak, presumably because carbon monoxide formation competed with diatomic carbon formation. In this case, it is likely that the concentration of carbon monoxide predominated over diatomic carbon because of the higher bond energy and greater stability of the carbon monoxide molecule. Of course, niether cyanyl radical, diatomic carbon or atomic carbon emission were observed for water loading. In general, these emission features displayed four distinct trends for the dependence of intensity on observation height: Atomic line emission from carbon and argon and the plasma continuum emission decreased monotonically with observation height. This makes sense because their emission originated from the atomic plasma volume. Their decrease in intensity  Observations 48 with height is consistent with the decay and extinction of the plasma as it flowed out of the induction region into cold air downstream. In contrast, emission from the cyanogen radical increased with observation height, consistent with its formation by air entrainment into the plasma gas (molecular nitrogen from the air combining with atomic carbon from the plasma). The emission from diatomic carbon displayed more complex behavior. It began at a low intensity, at low observation heights, then proceded through a maximum further downstream, ultimately to dissappear almost completely beyond a certain height. Because diatomic carbon emission originated from the interface between the aerosol channel and the atomic plasma, its axial trend may be reconciled with the structure of the upstream boundary region. This region was a hollow cylinder capped by a bullet of intense green emission. Once the solvent material had flowed past the top of the green bullet and then into the atomic plasma, the intensity of diatomic carbon emission dropped to insignificant levels. Finally, the analyte emission began weak at low positions, then increased as energy is transfered in towards the axis from the toroidal plasma. When the analyte ultimately flowed out through the top of the plasma volume, the analyte emission decayed to zero because the analyte was no longer supplied with sufficient excitation energy.  3.3 Photographs Figure 3.3.1 compares an ICAP loaded with two different settings of chloroform load. The image on the left depicts the lower load while the one on the right depicts a higher load. Note that the inner plume of diatomic carbon emission extends farther downstream with increasing load. In contrast, the methanol loaded ICAP depicted in Figure 3.3.2 appears to rise up with increasing load. It will be useful to refer back to both of these figures while reading the following chapters.  Figure 3.3.1 Photographs of a chloroform loaded inductively coupled argon plasma. The chloroform load increases from left to right.  Figure 3.3.2 Photographs of a methanol loaded inductively coupled argon plasma. The methanol load increases from left to right.  Observations 51 3.4 Detailed Observations Figure 3.4.1 depicts a medium power ICAP operating under conditions of moderate chloroform load. This figure illustrates all of the components of the ICAP emission structure one would likely encounter when observing an ICAP loaded with any of the solvents investigated in this work. This and all of the following figures in this section depict the spatial structure of the visible emission from the discharge as it would appear in cross section, so that the actual plasma may be regarded as a body of revolution about the cylindrical axis shown. The black area in each figure represents the region of intense emission from diatomic carbon (which appears brilliant green through the observation port). In some figures, horizontal bars instead of solid black represent the observed diatomic carbon emission, indicating that it was conspicuously weak or diffuse rather than intense. Whether intense, weak, or diffuse, the diatomic carbon emission usually occupied the spatial structure similar to the one shown in Figure 3.3.1, which may be conveniently regarded as an outer cup joined concentrically at its upstream end (or base) with a hollow, inner plume, which culminates in a bullet shaped tip as shown. Enveloped within the region of diatomic carbon emission sits the region of intense continuum emission and argon line emission. This region will be hereafter refered to as the atomic plasma, for convenience of discussion. The atomic plasma consists of three discernable components: the plasma core, the bright secondary plasma, and the dim secondary plasma, from all of which one observes continuum emission and atomic argon line emission to the exclusion of molecular emission. These regions typically form an annulus or toroid within the load coils which appears to coalesce into a cone further downstream, as shown in the figure. Although these three regions of the atomic plasma are not strictly distinct, as they may all blend into one gradual transition, it is useful to invoke them for the convenience of discussion.  52  Observations  LEGEND ATOMIC PLASMA PLASMA CORE  -  j BRIGHT SECONDARY PLASMA DIM SECONDARY PLASMA BOUNDARY REGION ENTRAINMENT REGION,  %1 OR TAILFLAME  DIATOM IC CARBON EMISSION REGION INCANDESCENT RADIATION  0 0 0  Figure 3.4.1. Visually Observed Emission Structures of a Solvent Loaded ICAP  Observations 53 A cross section through the plasma core is represented by the two oval regions of lightest grey sitting side by side within the confmement tube of the torch. Enveloping the plasma core is the bright secondary plasma, which in turn is enveloped or bounded by the dim secondary plasma. Distortions of these two secondary emission regions of the atomic plasma, in response to variation of the operating parameters and the solvent loading, are perhaps the most important observations to note in this chapter; They qualitatively indicate responses in the physical properties of the ICAP, responses which determine how energy is transferred to the analyte, and hence responses critical to the analytical performance of ICAP-AES. One further feature of the atomic plasma worth introducing here is the channel along the axis, through the centre of the toroid. This will be refered to as the axial channel, and does not necessarily coincide with the aerosol channel or distribution of analyte injected into the discharge. Once again, the term, axial channel, has been introduced for convenience of discussion. The region enveloping the downstream cone of the atomic plasma represents the entrainment region or tailfiame of the discharge. Weak violet emission from the air entrainment region, from species such as CN, was clearly visible from this region. One additional emission structure may be observed under certain operating conditions: a hollow cone of incandescent emission, possibly consisting of glowing carbonaceous soot particles, is often found nested within the hollow axial plume of diatomic carbon emission. The solid curve nested underneath the discharge represents the hollow cone of incandescent radiation. This emission feature was usually observed for high levels of loading by solvents with excess carbon relative to their oxygen content. Significantly, the bright orange cone was never observed for methanol loading or for ethanol/water mixtures, nor was buildup of carbonaceous soot on the torch wall ever a problem for these solvents. In both cases, carbon and oxygen were present in stoichiometric proportions. On the other hand, build up of carbonaceous soot accompanied the appearance of the hollow incandescent cone for relative high loading of solvents such as xylene, chloroform and hexane. The following figures illustrate how the emission structure depicted in Figure 3.4.1 varied with solvent, solvent load, inner argon flow rate and forward power. The behavior depicted in  Observations 54 these figures includes the response of the plasma volume (owing to the thermal pinch effect), vertical translation and vertical contraction of the plasma, the spatial structure of the diatomic carbon emission plume (including nested cone structures) and the behavior of the normal analytical zone (at the apex of the plasma decay region).  Solvent Comparison In order to describe the behavior and emission structure of an ICAP loaded with any of the solvents investigated in this thesis work, it is sufficient to consider the distinctive behavior and emission structure that resulted from loading by only three of them: water, methanol and chloroform. Representative observations for ICAPs loaded by these three solvents are depicted in Figures 3.3.2(b—d); Figure 3.3.2(a) depicts representative observations for a pure argon ICAP flowing into air as a basis of comparison. In all four discharges, the atomic plasma assumed the geometry of a torus which coellesced downstream into a cone. Within this general geometry, one may distinguish the atomic plasma regions of the four figures by the extent that their central channel has dilated, how  far up through the load coil the plasma has translated, and how much the downstream portion of the secondary plasma has bloomed open.  t  tn In  Figure 3.4.2  a.  Representative observations of an inductively coupled argon plasma (ICAP) discharge a. without solvent load; and loaded with b. water, c. methanol, and d. chloroform.  c.  Ui  I  C  Observations 56 In response to loading by water, the atomic plasma region did not appear to have dilated or to have translated downstream to any great extent when compared with the pure argon ICAP (without solvent loading). However, upon close inspection, one may note that the central channel became less difuse and that the atomic plasma appeared to translated upwards approximately 0.5 mm to 1.5 mm. The same minor changes with respect to the pure argon ICAP may be noted in response to chlorform loading. In contrast, the response of the plasma region to methanol loading was more pronounced. In this case, the atomic plasma clearly translated downstream, far enough, in fact (under the moderate conditions listed in table 3.3.1), for its base (or upstream edge) to reside the middle and upper turns of the load coil. Also in response methanol loading, the central channel of the ICAP appeared to dilate, strongly indicating that methanol loading resulted in a thermal pinch effect. Beyond the characteristics of the atomic plasma region, no further characteristics were found to distinguish a pure argon ICAP from a water loaded ICAP. In contrast, both the methanol and chloroform loaded ICAPs exhibited brilliant plasma boundary regions owing to emission from solvent pyrolysis products. The chloroform loaded ICAP displayed brilliant green emission from a sharply defined spatial structure described previously as an outer annulus joined at the upstream end to a hollow, inner plume. For an indication of how sharply defined this emission structure was, the cylindrical wall of the inner plume was often less than 0.5mm thick, while the boundaries appeared perfectly sharp. (Incidently, the small barbs of diatomic carbon emission on the downstream end of the outer cup are not artifacts of the figure, but were observed reproducibly, and were clearly visible. They probably indicate the presence of a back eddy in the outer gas stream, just beyond the exit of the torch. In contrast, the methanol loaded ICAP displayed relatively dim green emission from a diffusely defined spatial structure.  The  boundaries of this diffuse structure gradually faded over a distance of approximately one millimeter. However, under operating conditions of high methanol loading or low power, the methanol loaded ICAP exhibited a brilliant green, sharply defined structure similar to that of the chloroform loaded ICAP. This sharply defmed structure invariably nested within the diffuse  Observations 57 structure. The methanol and chloroform loaded ICAPs also differed from the pure argon and water loaded ICAPs by displaying weak violet emission from their tailfiames. In general, the appearance of the discharge depended on the relative amounts of oxygen and carbon in the aerosol stream. For example, xylene, propanol, and hexane loaded ICAPs were similar in appearance to a chloroform loaded ICAP. On the other hand, an ICAP loaded with an ethanol water mixture was similar in appearance to an ICAP loaded with methanol. Interestingly, an ICAP loaded with xylene, but with oxygen added to the aerosol stream in equimolar proportions to the solvent carbon,.was also similar in appearance to an ICAP loaded with methanol. Because of this general dependence on the relative amounts of carbon and oxygen load, the following discussion will be confined to chloroform, methanol, and water loaded ICAPs, and their response to solvent load, power, and gas flow rates. The appearence of an ICAP loaded with any other solvent, solvent mixture, or combination of solvent loading and oxygen addition, may be regarded as a hybrid or intermediate of the methanol and chloroform loaded ICAPs.  Response to Water Plasma Load In comparison to its response to other solvents, the ICAP appeared to respond very slightly to water loading (For this reason and because no molecular emission revealing the location of the plasma boundary region was visible either, the response of the ICAP to water loading has not been illustrated here). In fact, the atomic plasma region appeared to translate downstream through the load coil by only 1mm when the water load was increased from its minimum attainable level (0.15mg/si) to its maximum attainable level (0.3Omgis). Whether or not this was a downstream translation or a contraction of the atomic plasma along the direction of flow owing to the thermal pinch effect was not clear from observations alone. Perhaps the plasma was swept downstream or lifted off the tulip of the torch by expanding water vapor and vaporizing droplets near the base of the plasma[13]. Whatever the reason, the central channel became perceptibly darker and more clearly defined in response to water loading. On the whole the ICAP appeared to be relatively insensitive to water loading, an insensitivity which may be  Observations 58 explained by the characteristically low mass loading of water compared with other solvents when nebulized by conventional pneumatic nebulizers. Typically, the maximum water load that pneumatic nebulizers are capable of delivering to the ICAP is less than 0.35mg/s. One should note that ultrasonic nebulizers are usually capable of delivering much greater water loads to an ICAP than pneumatic nebulizers. However, they must be fitted with some sort of desolvation device such as a condenser in order to desolvate the aerosol and reduce the water load before the aerosol stream reaches the plasma. Otherwise, one might anticipate that the plasma could be swept too far through the load coil and become unstable. The extreme water load would then likely extinguish the discharge.  Response to Methanol Solvent Load A far greater range of methanol loading was accessible to observation. How the ICAP responded to methanol loading is illustrated in Figures 3.4.3(a) 3.4.3(b) and 3.4.3(c). They depict typical observations for an ICAP loaded with the minimum obtainable, intermediate and maximum levels of methanol loading. The most obvious responses were the way the diatomic carbon plume extended along the central channel as the methanol load increased, and the way the atomic plasma apparently translated. In addition to its downstream translation, the atomic plasma contracted in the direction of flow with increasing methanol load, a contraction which may have resulted from the thermal pinch effect.  0 0 0  Figure 3.4.3  a.  1H Representative observations for an ICAP loaded with methanol at a.minimum obtainable, b. intermediate, and c. maximum tolerable solvent load.  0  r 0 C.  Ui  I  0  Observations 60 The obvious translation and contraction are accompanied by a more subtle response: The secondary plasma appears to bloom open in response to an increase in the methanol load. In Figure 3.4.3(a), the bright secondary plasma (light grey) retains the characteristic shape of a toroid capped by cone.  Then as the methanol load increases from the lowest attainable to the  intermediate level, the cone of the bright secondary plasma is almost completely penetrated, leaving only a thin arch near its apex, as shown in Figure 3.4.3(b). In response to higher methanol loading, the apex of the bright secondary plasma blooms open. The dim secondary plasma appears to bloom open in a similar manner, but a step behind the bright secondary plasma. This blooming holds profound implications for the analytical performance of ICAP-AES: It reveals that methanol loading drastically alters how much energy flows into the central channel from the energy dissapation region or plasma core, energy required to desolvate, vapourize, atomize, ionize and excite the analyte. At maximum methanol load, the discharge may be regarded as having had bloomed open. It effectively retracted from the analyte. As a result, the plasma interacted incompletely with the analyte, if at all. At the other extreme of minimum methanol loading, one would expect the plasma to have interacted or have supplied energy to analyte quite effectively. Directly linked with the blooming of the secondary atomic plasma is the behavior of the diatomic carbon emission: As the secondary plasma bloomed open, the green, C 2 plume extended downstream. In general, the cup and plume extended further downstream with increasing solvent load.  Observations 61 Response to chloroform solvent load Figures 3.4.4(a), 3.4.4(b), and 3.4.4(c) illustrate typical observations of how the ICAP responded to the variation of chloroform loading. In a manner similar to methanol loading, chloroform loading also caused the the diatomic carbon emission to extend further downstream but several other characteristics distinguished the response of the ICAP to chloroform loading. In contrast to methanol loading, chloroform loading did not cause the atomic plasma to translate very far downstream, and no obvious thermal pinch effect was observed. Moreover, the diatomic carbon emission structure was always appeared sharp. Nested within the sharply defined central plume, a bright orange, hollow cone of incandescent soot particles formed at high chloroform loads, something never observed for methanol loading. This hollow orange cone became brighter with greater chloroform loading and appeared to nest closely within the central plume of diatomic carbon emission, as shown by the continuous solid curve in Figure 3.4.4(c) Because the spatial structure of diatomic carbon emission was so sharply defined in a chloroform loaded ICAP relative to the same structure in a methanol loaded ICAP, several subtle spatial responses to chloroform loading may be noted, and are depicted in Figure 3.4.5. The most remarkable response is the way the central plume changed shape as it extended up through the central channel of the discharge. At minimum chloroform loading, the hollow inner plume can be regarded as a cone.  At intermediate solvent loads, the base of the plume remained  approximately conical, but the tip of the plume extended downstream to form a cylindrical annulus capped by a bullet shaped region. At maximum chloroform loading, the top of the cylinder dilated to give the plume a bulbous end. Accompanying this extension and expansion were changes in the thickness of the wall of the hollow plume. As the plume extended downstream, the wall of its leadind edge appeared to grow thicker, i.e. thicker in the apparent direction of gas flow, whereas the thickness of the walls in the radial direction appeared to remain constant.  0 0 0  Figure 3.4.4  a.  H  b.  WI  C.  H  Representative observations for an ICAP loaded with chloroform at a. minimum obtainable, b. intermediate, and c. maximum tolerable solvent load.  I  0 0  I  0  r 0  I  Observations 63 It is likely that the steepness of thermal gradients across the wall of the plume determined its observable thickness; Diatomic carbon is probably only stable over a relatively narrow temperature range, or a small distance along a steep thermal gradient. At temperatures above its range of stability, diatomic carbon tends to dissociate, or become unstable relative to atomic carbon, while at temperatures below its range of stability, it tends to associate into polyatomic carbon containing species. It follows that the cross section through the diatomic carbon plume may be regarded as an isothermal contour in space, thinner when the thermal gradient crossing it is steeper. This explains why the wall of the plume varied in thickness—the side walls were thinner than the tip because the temperature gradient was steeper there. In Figure 3.4.5(a), the triangular cross section of the plume may be understood as a dissociation front receding radially towards the discharge axis. The aspect ratio of the cone is determined by the gas flow velocity and the competitive rates of heat consumption by the enthalpy of dissociation and radial heat conduction towards the axis from the toroidal core. In Figure 3.4.5(a), heat conduction overtakes heat consumption before the gas flows out of the torch. In Figure, 3.4.5(b), they are nearly balanced. Figure 3.4.5(c) is more difficult to explain: It appears as though heat consumption exceeds heat conduction, so that the solvent material is not completely pyrolyzed until very far downstream.  (b)  (c)  Figure 3.4.5 Observations of hjow the shape of the inner plume varied as the chloroform plasma load was increased. (a) The shape of the inner plume at the lowest chloroform load, (b) at an intermediate load, and (c) at the highest load.  (a)  4. .  I  0  Observations 65 The outer or peripheral cup of diatomic carbon emission also displayed behavior determined by thermal gradients, gas flow patterns and heat conduction: It was thickest around the base of the discharge and thinnest between the plasma torus and the torch wall, presumably for reasons similar to those determining the thickness of the walls of the central plume. However, it is questionable whether the outer cup can be regarded as a dissociation front similar to the inner plume. If it were, then solvent material would have to be swept around the base of the plasma, and enter through the outer periphery. Another, more realistic possibility, is that solvent material was folded into the outer argon stream by a recirculation eddy at the upstream edge of the discharge (in the wake of the intermediate tube). In that case, solvent material could have been either swept around the base of the discharge or folded into the plasma, then transported to the periphery by diffusion, (by either path reaching the region of the outer cup and forming diatomic carbon, via dissociation of solvent molecules or via association of carbon atoms diffusing out of the atomic plasma.) In short, the behavior of the outer cup indicated how solvent material had been distributed within the discharge: The observed response of a chloroform loaded ICAP to variation of the inner argon flow rate provided further insight into how solvent material might be distributed in the discharge. These observations are illustrated by Figures 3.4.6(a—c). All three frames in this figure depict an ICAP loaded with an intermediate amount of chloroform, but at low, moderate and high inner argon flow rates (the inner argon flow rate was adjusted indedendently of the solvent load by adding adgas to the aerosol stream immediately before it was sent into the torch—see Chapter 2). As shown in the figure, the length that the plume extended downstream was inversely related to the length that the outer cup extended downstream.  0 0 0  Figure 3.4.6  a.  H  ie  0 b.  r 0 c.  Representative observations for an ICAP loaded with an intermediate level of chloroform on an inner argon stream at a flow rate of a. 0.6, b. 0.8, and c. 1.0 liters I minute.  U  r  U  I.  0  C’ C’  Observations 67 Effect of Inner Argon Flow Rate and Power The atomic plasma also responded to variation of the inner argon flow rate in a conspicuous manner. As the inner argon flow rate was increased, the atomic plasma appeared to move upstream, or sit down within the intermediate tube, presumably because higher inner argon flow rates prevented the recirculation eddy at the base of the discharge from folding solvent material into the outer argon steam, thus preventing the downstream translation and thermal pinch effect. The observed response of the solvent loaded ICAP to variation of power may be stated quite simply: At lower powers, the discharge responded to all of the other parameters as described above, only more sensitively. By contrast, the discharge was more robust at higher powers; At powers approaching 2.0 kiloWatts, the discharge became insensitive to variation of any other parameter, including solvent plasma load.  Extention of visually observed responses to other solvents It was stated earlier that the observed response of the ICAP to loading by any solvent investigated in this work could be conveniently described as similar to a chloroform loaded ICAP, similar to a methanol loaded ICAP, or resembling a hybrid of the two, depending on the relative content of carbon and oxygen in the solvent. If the carbon to oxygen content of the solvent approached 1:1, then its appearance was similar to that of a methanol loaded ICAP. If the carbon content greatly exceded the oxygen content, then its appearance was similar to a chloroform loaded ICAP. This generalization may be extended further, to solvent mixtures and to oxygen addition to the aerosol stream. For example, the appearance of an ethanol loaded ICAP may be described as a hybrid of chloroform and methanol loaded ICAPs, but loading by an equimolar mixture of ethanol with water results in a discharge which is virtually indistinguishable from a methanol loaded ICAP. The same is true for a ICAP loaded with xylene when oxygen has been added to the aerosol stream in equimolar proportion to the amount of carbon.  Observations 68  3.5 Conclusions The detailed observations reported here reveal that any further investigation must take distortions and translations of the macroscopic structure of the discharge into consideration. They also reveal that the appearance of the discharge depends on the relative proportions of oxygen and carbon in the aerosol stream, irrespective of the chemical form that the oxygen and carbon are introduced. Beyond that, a number of physical phenomena are evident in the observations. Thes include a thermal pinch and convective distribution of solvent material over the argon stream. Moreover, incandescent radiation was observed from a conical shell nested within a dissociation front, indicating that solvent pyrolysis proceeded via macroscopic soot particles. Overall, the observations reported in this chapter provide a valuable survey of the parametric behavior of the solvent loaded ICAP.  Observations 69 3.6. References  1.  S. Greenfield and H.M. McGeachin, Analytica ChimicaActa 74: p. 225-245 (1975).  2.  J.S. Koirtyohann, Jones, C.P. Jester and D.A. Yates, Spectrochimica Part B 36: p. 49-59 (1981).  3.  Koirtyohann, S.R., C. Baber, and M. Franklin, Spectrochimica Acta B 31: p. 589-587 (1976).  4.  Boumans, P.W.J.M. and M.C. Lux-Steiner, SpectrochimicaActa B 37(2): p. 97-126 (1982).  5.  Truitt, D. and J.W. Robinson, Analytica Chimica Acta 49: p. 401 (1970).  6.  Truitt, D. and J.W. Robinson, Analytica ChimicaActa 51: p. 6 1-67 (1970).  7.  Montaser, A., V.A. Fassel, and J. Zalewski, Applied Spectroscopy 35: p. 292 (1981).  8.  Thorpe, NASA Contractor Report 1143, 1968..  9.  Greenfield, S. and H.M. McGeachin, Aiwlytica Chimica Acta 100: p. 101-119 (1978).  10.  Halliday, D. and R. Resnick, Fundamentals ofPhysics. 2 ed. 1981, New York: John Wiley and Sons, Inc. 947.  11.  Sears, F.W., Optics. 3 ed. 1958, Reading: Addison-Wesley. 386.  12.  Kaufman, L., Chapter 3: Sensitivity to Light, in Sight and Mind:An Introduction to Visual Perception. 1974, Oxford University Press: New York.  13.  B.L. Caughlin and M.W. Blades, Spectrochimica Acta, Part B 42(1/2): p. 363 (1987).  Chapter 4 The Parametric Complexity of a Chloroform Loaded Inductively Coupled Argon Plasma  4.1. TNURODUCTION ACCORDING TO FEYNMAN [1], the elements of the scientific method are observation,  reasoning and experiment. This is perhaps the most accurate definition because of its generality. Using more specific terms, Box and Hunter [2] have described the scientific method as an iterative learning process of first testing a hypothesis against experimental results, followed by modifying the hypothesis (be it a conjecture, model, or theory), then retesting the hypothesis, and so on, until the problem is understood. Boumans [3] clearly applied the scientific method from both perspectives when he formulated a rational approach for developing spectrochemical sources. The first step of his approach prescribes careful observation in order to establish the parametric response of the method. His second step prescribes experimental investigation, in order to characterize physical properties of the spectrochemical source. His third step prescribes reasoning and hypothesis formulation in order to explain the analytical performance of the source. His final step is the rational development of the spectrochemical method. In other words, the fmal step is to refine the method after gaining an understanding of the physical characteristics of the source and how the physical characteristics determine the analytical performance. Following Feynman’s, Boumans’, and Box and Hunters’ lead, the rational strategy proposed by this thesis adds something more. It surveys the complexity of the discharge, a complexity that otherwise  The Parametric Complexity  71  defies the formulation of clear hypotheses. Rather than simply establishing the parametric response in the first step, this thesis prescribes an extensive survey of the parametric, spatial and temporal complexity. Then the rational approach can proceed with physical characterization and so forth, as per Boumans’, Feynman and Box and Hunter. This chapter surveys the parametric complexity of an ICAP loaded with chloroform only. The next two chapters will explore the spatial and temporal complexity and extend the investigation to other solvents. Then Chapters 7 through 9 will explore the physical characteristics. And although the rational strategy proposed here provides the glue for binding all the thesis together, Feynman’s description of the scientific method—observation, reasoning and experiment— should be kept in mind throughout. The survey of the parametric complexity presented here confirms and extends what is already known about the analytical performance of ICAP-AES, particularly in the area of aerosol desolvation; it confirms the findings of Maessen et al. [4], that the analytical performance of ICAP-AES can be substantially improved by desolvating the sample aerosol before it reaches the plasma. The results further reveal that relatively low forward powers (750 W lower than the lowest power used by Maessen et al.) can be used to both sustain a stable plasma and achieve acceptable analytical performance, provided the aerosol has been sufficiently desolvated before reaching the discharge. The survey also reveals important physical relations amongst emitting species in the discharge. For example, they reveal a relationship between excitation of the analyte and solvent dissociation in the boundary region of the discharge. Briefly, the boundary between cold aerosol and hot plasma gas is revealed by intense diatomic carbon emission. This boundary resides upstream from the onset of analyte emission, so the solvent molecules dissociate before the analyte atoms are excited, assuming that upstream events precede downstream events. Consequently, species such as diatomic carbon play a minimal role in analyte excitation, because they are separated from analyte excitation in both space and time.  The Parametric Complexity  72  Another insight revealed in this work is the connection between air entrainment and the axial maximum of analyte ion line intensity. This connection has not been explicitly identified before, and it may explain several anomalous findings reported in the literature. 4.1.1 Scope and Objectives There were two objectives for the work presented in this chapter. The first one was to survey the parametric complexity of the analytical method, ICAP-AES. The second was to survey the parametric complexity of the discharge alone. The overall survey should provide a rational basis for investigating the physical characteristics of the ICAP by revealing which regions of the parameter space are worth investigating. The parametric survey should also provide a clear trail back to the analytical performance from the physical characteristics are known. Such a trail would aid immensely in explaining the analytical performance in terms of the newly discovered physical properties. Ideally, both a survey of the parametric complexity and a survey of the physical characteristics would extend over the same parameter space. This would allow one to span the gap between the insight held by the analyst (who measures net line intensities and interference coefficients), and the insight held by the experimentalist (who measures physical properties). Boumans pointed out that this gap remains an obstacle in ICAP-AES [3]. Moreover, it is the most frustrating barrier that blocks the path to refining and developing the method any further. Of course, one may argue that such extensive surveys would be superfluous because only the conditions of optimum analytical performance warrant interest [5]. One could find those optimal conditions and then confine further investigations to them. Such an argument would be reasonable if there were only one unambiguous set of optimal operating conditions. In practice, however, several different optima may be established for spectrometric methods based on the ICAP. The different optima depend upon the optimization criteria and the method of detection.  For example, optimum conditions for low detection limits are often inconsistent with those for minimizing matrix interferences. Optimum conditions for the detection of ion line signals often  The Parametric Complexity  73  differ from those for the detection of atom line signals. Furthermore, optimum conditions for mass spectrometry are almost always inconsistent with those for atomic emission spectrometry. Clearly, the optimum conditions for all of the above examples could only be contained within an extensive survey. Moreover, the experimentalist can study interference effects more easily for non optimal operating conditions because interference effects are exaggerated away from the optimum. 4.1.2 Literature Review of the Parametric Response ofICAP AES A comprehensive review of the ICAP-AES literature is beyond the scope of this thesis. This literature review is limited to the available parametric insight into the solvent plasma load problem. Boumans has reviewed the literature concerning the parametric response ICAP-AES up to 1984. His review may be found in Ref. [6]. Important parametric investigations into the analytical performance of ICAP-AES are covered, from 1969 to 1984. Boumans also lists the parametric trends for ICAP-AES, and proposes a procedure for finding the best compromise of parameter settings for multielement analysis (a compromise because not all of the analytical lines have the same parametric optimum). Overall, he clearly spells out how the analytical performance for ICAP-AES may be optimized. From the outset, however, Boumans laments that “...data are scattered in the literature...” Here, the details of Boumans’ review relevant to solvent plasma loading will be recapitulated, then brought up to date. According to Boumans, one must explore forward power; observation height; the sample uptake rate of the nebulizer; and the outer, intermediate and inner argon flow rates in order to find optimal compromise conditions for multielement analysis. In addition to these parameters, Maessen et al. found that solvent plasma load must also be explored [4], [7], [8].  The Parametric Complexity  74  Out of all these parameters, the analytical performance is not critically dependent upon the outer argon flow rate. Consequently, this may be set to the lowest, most economic level necessary to sustain a stable plasma for long periods. For ICAPs loaded with aqueous solvent aerosols, or desolvated organic solvent aerosols, only 10.0 1/mm is required (for a high efficiency torch, such as the one used in this work). In contrast, Boumans and Lux-Steiner found in early studies that ICAPs loaded with organic solvents require 15.0 to 20.0 1/mm  ,  presumably to  prevent carbonaceous soot form building up on the inner surface of the torch [5]. But in this early work, effective measures to desolvate the aerosol were not taken. In fact, solvent plasma load did not receive much attention in early work at all. Largely ignoring solvent plasma load, early investigations (1966 to 1978) focused on only four parameters: forward power, viewing height, nebulizer gas flow rate, and the excitation and ionization energies of the analyte. For example, Boulos et al. [9] investigated how the net line intensity of several prominent emission lines responded to variation of the nebulizer flow rate. Their investigation covered a range of viewing heights. They found that the emission lines could be classified according to excitation and ionization potentials. They also attributed parametric behavior to macroscopic phenomena, by suggesting that the analyte followed different paths through the plasma toroid for different nebulizer flow rates. Overall, they concluded that the inner argon flow rate is a critical operating parameter. Horlick et al.. later surveyed a grand total of four parameters —line energetics, viewing height, inner argon flow rate, and r.f. power [10]. They facilitated their emission measurements by mounting an array detector vertically in the exit focal plane of their grating spectrometer, an arrangement that allowed them to sample an entire range of viewing heights (an entire axial profile) simultaneously. That neatly took care of one parametric dimension and allowed them to explore the others more extensively, which they did. They demonstrated that emission lines from the ICAP could be unambiguously classified as hard or soft lines according to their excitation energy and parametric response. The classification depends on how the axial profile of )  the emission line responds to inner flow rate and power. Briefly, they classified the emission line  The Parametric Complexity  75  as hard if the maximum intensity of the axial profile did not move higher or lower above the induction coil in response to variation of the operating parameters. On the other hand, they classified the line as soft if the axial profile was sensitive to variation of the operating parameters. In general, they found that ionic lines and atom lines with relatively large excitation potentials, say greater than 6 eV, are hard. On the other hand, atom lines with excitation potentials significantly lower than 6 eV were soft. Atomic lines with intermediate excitation potentials displayed intermediate behavior. This early work established the parametric response for conventional ICAP-AES in which the sample is introduced as an aqueous aerosol. But solvent plasma load was not taken into consideration, because it was found that water plasma load does not vary appreciably from about 0.3 mg/s for most nebulizers. More recently, Maessen et at. found that solvent plasma load was indeed interesting for other solvents [4], [7], [8]. Maessen et a!. began their investigations by devising the continuous weighing method[4], a method for quantitatively determining and controlling solvent plasma load. Soon after devising this methodology, they discovered how solvent plasma load was determined by two key nebulization parameters—the uptake rate of the sample solution to the nebulizer and the nebulizer gas flow rate [7]. This discovery allowed them to unravel the ambiguous interdependencies between the analyte transport rate, nebulizer gas flow rate, sample uptake rate and solvent plasma load. All these parameters had previously displayed ambiguous interdependencies and had not been previously separated experimentally. Maessen et al. showed that it made little sense to investigate solvent load effects by varying the sample uptake rate or the nebulizer gas flow rate. They found that controlling solvent load directly was a key step in rationally investigating the effects of solvent load. Equipped with a method for controlling solvent plasma load as a parameter, they investigated the effects of chloroform loading on the analytical performance of ICAP-AES. They reported the parametric response of the signal to background ratio (SBR), the relative standard deviation of the background (RSDB), the background intensity, and the net analyte line intensity.  The Parametric Complexity  76  The net intensity reveals the sensitivity of ICAP-AES. But knowing the sensitivity of a signal is seldom useful unless we also know its precision. Regarding ICAP-AES, we are interested in the net analyte signal when its intensity is very small—very small compared to the background intensity. In this situation, the noise in the background predominates. As a result, the standard deviation of the background determines the detection limits. Often, however, only the background and net analyte intensity are surveyed to the neglect of the standard deviation of the background signal. It is then assumed that the relative standard deviation of the background is constant, so that the standard deviation of the background scales up with the background intensity, as one would expect for source limited noise. It may then be easily demonstrated that the signal to background ratio (SBR) gives a good indication of how the detection limits respond (the higher the SBR, the standard deviation of the background relative to the signal, and the lower the detection limit). Consequently, a parametric survey of the SBR is also worthwhile. With this in mind, Maessen et al. surveyed the net intensity, the background intensity, the signal to background ratio and the relative standard deviation of the background. For the net intensity, they found that emission lines previously classified as hard retained their hardness when the ICAP was loaded with a constant amount of chloroform. But their behavior became more complex when the chloroform loading was varied. The axial maximum for hard lines were sensitive to desolvation. They shifted towards lower axial positions, or to locations further upstream upon desolvation. This shift was accompanied by an overall increase in intensity of the entire axial profile. Such a response is similar to how hard lines respond to wide variations of inner argon flow rate. However, after a certain extent of desolvation, the overall intensity of hard line emission decreased, presumably because the analyte transport fell off at extremely low condenser temperatures. On the other hand, the response of soft lines to desolvation was consistent for typical soft line behavior—it was sensitivity to the variation of any parameter. When they surveyed the response of the background emission intensity, the survey was extended over several wavelength channels in order to probe the response of different spectral background features. Included in the survey were emission from diatomic carbon, CN, the line  The Parametric Complexity  77  wings of atomic carbon, and the background continuum. For comparison, they also surveyed the background emission from an ICAP loaded with 0.5 mg/s of water. Significantly,  the  chloroform loaded ICAP was operated at three relative high forward powers, 1.5, 1.75, and 1.9 kW, while the water loaded one was operated at 1.0 kW. Not surprisingly, they found that the higher power, chloroform loaded ICAP emitted a continuum background ten times more intense than the low power ICAP loaded with water. Presumably, the higher continuum emission observed for the higher power, chloroform loaded ICAP was due to the increase in plasma electron density. This is exactly what one would expect for higher applied r.f. power. (Perhaps lower powers could have been investigated for the chloroform loaded ICAP if the aerosol had been further desolvated.) In addition to more intense continuum emission, the background emission displayed a conspicuous axial structure at the 500-nm channel. At this wavelength, an axial maximum retracted into the torch and decreased in intensity with desolvation. This feature corresponded to the tip of the plume of green diatomic carbon emission commonly observed for organic solvent loaded ICAPs. Evidently, desolvation could significantly improve the analytical performance of solvent loaded ICAPs by reducing the molecular background emission. They surveyed the parametric behavior of the signal to background ratio (SBR) for representative hard and soft lines over forward power, observation height, and chloroform plasma load. Enhancements in the signal to background ratio were found on going from the highest chloroform load to moderate desolvation, especially for the soft line at low observation heights. Beyond a certain extent of desolvation, however, the signal to background ratio became insensitive to chloroform loading power and observation height. This led them to conclude that there was no reason to desolvate the chloroform aerosol beyond the point where the signal to background ratio became insensitive to chloroform loading, if the signal to background ratio were the only criterion for optimizing the analytical performance. But, in order to use the SBR as a criterion for optimizing analytical performance, one must also know the RSDB. Knowing both gives the standard deviation of the background, and how large the signal is in comparison, hence indicates how low the detection limit lies. When  The Parametric Complexity  78  Maessen and Kreuning surveyed the RSDB they found that it was independent of solvent load except a extremely high solvent loading [4]. They also found that when the chloroform aerosol was sufficiently desolvated, that the RSDB fell to values comparable with values for ICAPs loaded with aqueous aerosols. They emphasized, however, the importance of preventing carbonaceous soot from forming on the torch in order to maintain acceptable values of RSDB, and suggested procedures for doing so. The results of their parametric survey of the chloroform loaded ICAP inspired them to investigate the physical properties of the ICAP, under operating conditions involving solvent plasma load by a variety of solvents [8]. Their new objective was to examine the physical properties of the ICAP loaded with known quantities of solvent, and also to examine the effects of plasma loading by several solvents representative of the different classes of solvent used in ICAP-AES applications. The applications of interest included trace metal analysis of solvent extracts, of liquid chromatography eluates, and of samples dissolved in organic solvents. They selected ethanol and methanol to represent the alcohols, n-heptane to represent aliphatic solvents, toluene to represent the aromatic solvents and chloroform to represent the chlorinated solvents. Water was included for comparison. For each of these solvents, only two values of solvent loading were investigated: a solvent plasma load of 15 tmo1Js, and a carbon plasma load of 50 jimolls. They chose 15 pmol/s because 15 pmo1Is conspicuously resulted in the optimal analytical performance for ICAP operation involving both chloroform and water loading. On the other hand, Browner et al. [11] observed that plasma excitation conditions were determined by the amount of carbon loading. This inspired Maessen and Kreuning to compare loading by different solvents, all with their carbon load set to 50 lImo]Js. The choice of these two levels (carbon load  50 jimolJs and solvent load  =  15 pmo1Is)  gave them a somewhat curious parameter space to investigate: for some solvents, the mass loading of 15 iimolls resulted in a carbon load greater than 50 jimolls. For others, the mass loading of 15 pmo1Js resulted in a carbon load less than 50 .tmol/s. At any rate, a two level  The Parametric Complexity investigation in solvent load was conducted as far as possible (the load values, carbon load  79 =  50  pmo1Is and the solvent load = 15 tmo1Js, exceeded the maximum tolerable load or fell short of the minimum obtainable load for a few solvents). The forward power and inner argon flow rates were held constant at the optimal compromise values for multielement analysis. Axial profiles were measured over this parameter space, extending from 5 mm to 30 mm above the load coil in 3 mm increments. They measured axial profiles for several emission signals. The signals included Fe II excitation temperatures, C , CN and C I background emission profiles and analyte 2 emission profiles. When Kreuning and Maessen examined the effect of solvent load on axial profiles of Fe II excitation temperatures, they found that the shape of the profile depended on both the solvent plasma load and the nature of the solvent. Essentially, the behavior of the excitation temperature profiles was not simply solvent specific, but depended on solvent load as welL In general, the excitation temperature decreased when the solvent load was increased. This is just what one might anticipate if one regarded solvent load as a power consuming quantity. The maximum excitation temperature was always found between 10 mm and 20 mm above the load coil, and dropped off rapidly in the tail flame region (above 25 mm). There were few exceptions to this otherwise unremarkable behavior. An interesting revelation became apparent when they compared profiles from the ICAP loaded with different solvents and with the carbon load held at 50 pmo1Is. The spread in excitation temperatures was narrowest for this experiment. The notable outlier was the chloroform profile, which displayed significantly lower temperatures. The authors attributed this to the momentum of the chloroform vapour and the high mass loading of chloroform relative to the other solvents. They suggested that these properties confined the chloroform plasma load to the aerosol channel more than other solvents. In contrast, other solvents may have been distributed about the base of the discharge. As a result, the load on the central channel would have been alleviated. The essential argument was that the nature of the solvent determined the  The Parametric Complexity 80 characteristics of the aerosol stream, which in turn determined how the solvent was distributed about the entrance paths to the plasma. They revealed something even more intriguing when they compared the axially resolved excitation temperature profiles for different solvents, but with the solvent load held at 15 imo1Js this time. They found that the excitation temperature profiles for methanol and ethanol loading were hotter than the profile for 33 tmo1Is of water loading—at all observation heights. Evidently, not all organic solvents cool the axial region of the ICAP with respect to aqueous solvents. Once again, it was demonstrated that both the absolute solvent load and the nature of solvent were critical in determining excitation conditions within the plasma. The authors went on to suggest that the distribution of solvent over liquid droplet and vapour phases determines how the solvent plasma load is distributed about the entrance paths to the plasma. While the droplet phase predominates for water, the vapour phase predominates for methanol. Moreover, methanol vapour may readily diffuse away from the injector gas stream, beyond the exit of the injector tube. Consequently, the methanol can disperse over the argon stream and relieve the load on the aerosol channel. In contrast, small water droplets tend to follow the flow stream of the injector gas, concentrating the water loading on the aerosol channel, resulting in cooler conditions in the channel than for methanol loading. In summary, several parameters determine the spatial distribution of solvent loading within the discharge. They include the forward power, the inner argon flow rate, the distribution of aerosol between droplets and vapour, and the flow patterns and velocities of argon within the torch—not just the absolute solvent plasma load or the nature of the solvent. Revelations of further interest were made when the excitation temperature profiles for water and the alcohols, methanol and ethanol, were compared with the excitation temperature profiles for the solvents that did not contain oxygen, toluene, n-heptane, and chloroform, all at a solvent plasma load of 15 p.molls. Incidentally, this solvent plasma load corresponds to a seven times greater carbon load for n-heptane and toluene than for methanol or chloroform. As one might anticipate, the excitation temperature profiles for toluene and n-heptane loading were  The Parametric Complexity  81  significantly cooler than the profile for methanol. But they were not significantly cooler than the profile for chloroform plasma loading, even though their carbon load was seven times greater. Evidently, the Fell excitation temperature profile for chloroform was anomalous. As for the 50 p.molls of carbon case, Kreuning and Maessen explained the chloroform anomaly by considering the paths the solvent material follows as it enters the base of the discharge. If at one extreme, the solvent was minimally distributed about the base of the discharge, but confined to the central aerosol channel, then the solvent loading effects would be concentrated in the central, aerosol channeL This could happen when the solvent has been injected primarily as droplets, or when the aerosol stream has a high momentum along the axis of the discharge. On the other hand, the recirculation eddy residing in the base of the discharge could mix the solvent vapour of the aerosol stream quite efficiently with the plasma argon. One might expect this opposite situation for an aerosol stream of low momentum, with the mass of solvent largely in the vapour phase. Kreuning and Maessen offered an alternative explanation to account for the difference between the excitation temperature profiles for methanol and chloroform. They suggested that the temperature difference could be attributed to the different pyrolysis products of the two solvents—CC1 and C 2 resulting from chloroform loading, and CO from methanol loading. This idea prompted them to measure both axially and radially resolved profiles of C , CN 2 and C I emission, in order to learn about the distribution of solvent pyrolysis products in the discharge. Both the radially and axially resolved profiles revealed interesting spatial relationships amongst the pyrolysis products. In general, for all solvents, the radially resolved C 2 emission was confined to within 3 mm of the discharge axis, low along the discharge axis (<5 mm above the load coil), and displayed an axial maximum (this contrasts with the observations reported in Chapter 3 and the radial results reported in Chapter 5, in which the C 2 plume was found to be a hollow tube capped by a bullet shaped region). This axially confined plume of C 2 emission was surrounded by an annulus of C I emission; the radially resolved C I emission profile exhibited an off axis, or radial maximum at r  =  3.5 mm and a local minimum at the axis (r  =  0 mm).  The Parametric Complexity 82 Significantly, the axial minimum of C I intensity was close to zero for solvents exhibiting a relative high intensity of C 2 emission at the axis, solvents such as chloroform, toluene and n heptane. On the other hand, for solvents exhibiting a relatively weak intensity of C 2 emission at the axis, the radially resolved C I intensity was quite appreciable at the axis, although these solvents still exhibited an axial minimum of C I emission with respect to the off axis maximum at r  =  3.5 mm. Further up in the discharge, where both the C 2 and C I intensities fell off, CN  emission formed a second annulus which surrounded the C I emission; its radially resolved emission profile exhibited an axial minimum and a radial maximum at r = 5 mm. This result for CN emission was the one Kreuning and Maessen anticipated; CN formed from nitrogen entrainment into the carbon containing plasma (or at least entrainment into its boundary region) from the atmosphere. In summary, Maessen et al. have contributed immensely to our knowledge of the parametric behavior of ICAP-AES for applications in which the effects of solvent load are important. They established that solvent plasma load was a parameter critical to analytical performance: indeed, they found that for certain organic solvents, desolvation (or a reduction in the solvent plasma load) could bring the analytical performance up to par with that of an ICAP loaded with aqueous solvents. They further revealed that emission lines from an ICAP retained their hardness in terms of parametric behavior when loaded with organic solvents; the parametric behavior of emission lines from a ICAP loaded with organic solvents could be classified in the same way as it had been classified previously for ICAPs loaded with aqueous solvents. In addition to this valuable contribution, they discovered several trends and correlations which set the stage for explaining the parametric response of solvent loaded ICAPs in terms of the physical properties of the discharge. The most notable of these was the correlation between the shape of the axial profile of diatomic carbon emission, the shape of the axial profile of line of sight excitation temperatures, and the position of the axial maximum for hard line emission. In spite of their valuable contributions, their work has important shortcomings, at least within the context of establishing the parametric response. First, they did not investigate low  The Parametric Complexity 83 powers. Second, they did not sample the solvent plasma load for more than two levels, except for chloroform. And third, they did not investigate the effect of the inner argon flow rate, although they acknowledged its significance as a parameter and proposed to investigate it. These shortcomings by no means degrade the quality of their work, considering their original objectives, but they must be addressed by anyone attempting to establish the parametric response of the ICAP loaded with organic solvents. Beyond the contributions and shortcomings, their work raised many questions and uncovered the complexity of the solvent load problem. What role do pyrolysis products play? How important are pyrolysis pathways? Do droplets play a significant role in organic solvent loading, and if so, for which solvents? How is solvent material distributed about the entrance pathways of the discharge? Perhaps some of these questions may be answered by extending the investigation over a more extensive parameter space. Perhaps the only way to unravel the complexity of the problem will be to extend the investigation from time averaged, line of sight measurements to spatial and temporal resolution. After completing the work reviewed above, the Maessen group apparently abandoned their investigation into solvent load effects, possibly because of the enormous difficulty posed by conducting spatially resolved measurements of plasma properties and the distribution of plasma species over an adequate range of operating parameters. Even so, several investigators continued to recognize the immediacy of the solvent load problem and continued to investigate phenomena associated with injecting sample solutions into the ICAP as aerosol droplets. Notable amongst the research groups continuing to report their investigations into aerosol plasma interface phenomena are Browner et at. [12], Olesilc et at. [13], Mermet et al. [14], and Huang et al. [15]. Where Maessen’s group left off investigating the effects of solvent load, Browner et a!. resumed their investigations into analyte transport efficiencies and solvent evaporation rates. Moreover, they recognized that the droplet size distribution of the sample aerosol was critical in the solvent loading process. Briefly, they set out to investigate what they called the  The Parametric Complexity  84  solventlplasma interface, by examining both the solvent vapour, aerosol droplet and desolvated aerosol particle components of the overall solvent load. They began by characterizing aerosol droplet size distributions for several solvents nebulized into argon [16]. Their results were quite astounding—they revealed that the long standing empirical formula for predicting droplet size distributions, the Nukiyama Tanasawa equation [17], grossly overestimated the Sauter mean droplet diameter (a measure of the ratio of the total volume to the total surface area of the aerosol) for both aqueous and organic solvent aerosols nebulized under typical ICAP-AES operating conditions. Moreover, they revealed that the mean droplet sizes for several organic solvents were always smaller than those of aqueous solvents contrary to the Nukiyama Tanasawa predictions. They also confirmed earlier reports -  that solvent evaporation rates are critical in determining analyte transport efficiencies. With this insight into the nature of the aerosol stream flowing into the ICAP, they went on to compare the effects that water loading and carbon tetrachioride loading had on ICAP-AES. Unfortunately, they investigated only one inner gas flow rate, just as Maessen et al. had done. In contrast to the work done by Maessen et al., only one forward power and two solvents were investigated, whereas the response measurements were somewhat more elaborate. They reported background and analyte spectra along with axial profiles of diatomic carbon emission, Fe I excitation temperatures and the intensity ratio of Mg Ito Mg II emission lines. In addition, a new quantity, the emission magnitude, was introduced to report emission from background features. The emission magnitudes were simply the integrated axial profiles of C , CN and C I 2 emission and were plotted as a function of the temperature of the condenser used to desolvate the aerosoL These plots revealed that the three carbon containing emitters each displayed a distinct response to solvent loading: when the condenser temperature was raised from -10 C to +20 C, the 2 emission magnitude increased by an order of magnitude, the CN emission magnitude C doubled, and the atomic carbon emission magnitude remained relatively constant. Overall, the emission from C 2 displayed the most sensitive response to desolvation. This lead Browner et al.  The Parametric Complexity  85  to suggest that C 2 emission was a good candidate to use as a working diagnostic for monitoring solvent plasma load [18]. Browner et al. also demonstrated the significance of solvent loading as a source of spectral interference. For example, upon desolvation, the overall intensity of the C 2 axial profile decreased as the position of its axial maximum receded into the torch. This behavior, also observed by Maessen et al. [8], corresponds to the retraction of the C 2 plume with desolvation. In contrast, the Ba II axial profile increased in intensity upon desolvation, while its axial maximum moved upwards with respect to the induction coil. (This fmding was consistent with the behavior observed by Maessen et al. [4]for hard line emission from a chloroform loaded ICAP without desolvation, the C 2 Swan bands with bandheads at 516 nm and 512 nm swamped the Ba II line at 493.4 nm.) When desolvation was employed, the C 2 Swan band disappeared from the spectral background, and the Ba II line stood out clearly, free from spectral interference. Browner et al. hypothesized that the Ball behavior could be explained by the change in the ambient plasma power available to excite the analyte caused by a change in solvent loading. In order to test this hypothesis, they investigated the plasma excitation conditions by measuring an Fe I excitation temperature, and the ionization conditions by measuring the ratio of the intensity of Mg II emission to the intensity of Mg I emission, hereafter referred to as the Mg ratio (Mg ratios were favored because of the absence of molecular background emission features in the Mg II and Mg I wavelength region). When the Mg I to Mg II intensity ratio was determined for an observation height of 20 mm above the load coil, and when the condenser temperature was varied from -10°C to  +  20°C,  the ratio remained close to 25 for water loading, but decreased from approximately 17 down to 2 for carbon tetrachloride loading. This behavior was only representative for the range of observation heights from 15 to 20 mm. Higher up (>25 mm), the authors explained that the variation was less extreme because the maximum possible temperature was much cooler, presumably as a result of the distance from the power loading region. This, however, was dangerous to assume, given that the degree of spatial inhomogeneity and radial stratification of  The Parametric Complexity  86  temperatures at 25 mm are apt to make line of sight Mg intensity ratios inaccurate. Nevertheless, the response of the Mg intensity ratio lead the authors to conclude that the decrease in the ratio could only have resulted from a decrease in the excitation temperature. They also determined Fe I excitation temperatures for the solvent loaded ICAP from line of sight Fe I line intensities, making the usual assumption that the electronic excitation of Fe Tin the plasma followed a Boltzmann distribution. They then compared axial profiles of excitation temperature measured under conditions of water loading to those measured under conditions of carbon tetrachloride loading. These profiles revealed that carbon tetrachloride loading resulted in a lower excitation temperature in the plasma than water loading, at all observation heights, when the aerosol condenser was set to 18°C for both solvents. Moreover, with carbon tetrachloride loading, the axial maximum of excitation temperature resided further downstream, or higher above the load coils, than for water loading. These results suggested that the much greater mass loading by carbon tetrachioride consumed the available excitation power in the plasma. Significantly, the position of the axial maximum for the excitation temperature corresponded to the position of the axial maximum for hard line emission from a water loaded ICAP but not for the carbon tetrachloride loaded ICAP. In this case the hard line maximum resided significantly further downstream (in disagreement with Kreuning et al. ‘s results). Could this be explained by the spatial averaging, and off axis bias intrinsic to line of sight measurements? More recently, Browner et al. [12]. have investigated the role of the auxiliary argon flow rate in the solvent loading process. These results conclusively reveal that the distribution of solvent about the entrance paths of the discharge—partly determined by the auxiliary argon flow rate—are critical to the solvent plasma loading process. In summary, the work conducted by Browner et al. and Maessen et al. firmly established that the nature of the solvent, the absolute quantity of solvent loading, and the physical characteristics of the aerosol stream (including the distribution of solvent between droplet and vapour phases, the momentum of the aerosol stream and the inner argon flow rate) are critical in determining how the ICAP responds to solvent loading. Unfortunately, their investigations  The Parametric Complexity  87  covered only limited regions of the ICAP operational parameter space. Had their investigations extended over a more comprehensive parameter space, and had their determination of the response over this parameter space not been under sampled, then further trends may have been revealed, as postulated above. From an analytical perspective, Ebdon et aL [19] have established that the desolvation of organic solvent aerosols can be employed to optimize the analytical performance of ICAP- AES, although it is not clearly understood why. In short, although many provocative insights have been gained into the solvent load problem, the task remains to fully establish the parametric response of ICAP-AES in the special cases of solvent loading and desolvation. A further task remained to investigate the actual interaction between the aerosol and the plasma—the solvent/plasma interface had to be investigated. This task was initially taken up by Farnsworth et al. [201, then comprehensively pursued by Olesik et al. [21], [22], [23], [13], with some work done by Horlick et al. [24]. These investigators looked at the effect of aerosol droplets which survived the traverse through the toroidal region of the discharge, and resulted in complex spatiotemporal effects further downstream. The reader is cautioned against extending their findings to solvent loading in general, because their work was confined to the investigation of loading by aqueous solvents only. Their work is summarized here because it includes a parametric investigation of desolvating droplet effects, upon which other parametric results reviewed here may hinge. The general scenario of desolvating droplets in the ICAP is as follows: the desolvating droplets represent a minute fraction of the total number of droplets which leave the spray chamber and enter the torch. However, they are the largest droplets in the aerosol, and hence make up a significant proportion of the total volume of the liquid phase of the aerosol—they contain a significant proportion of the analyte transported to the ICAP. When these droplets desolvate as they travel along the axis of the ICAP, they create small regions of local cooling within the plasma. Also within the flowing plasma resides analyte which has been previously atomized. This analyte originated from droplets that were originally much smaller than the ones continuing  The Parametric Complexity  88  to desolvate. As the droplets (and attendant regions of localized cooling) fly through the observation zone, their passage perturbs the otherwise steady state signal from the previously atomized analyte (the precise nature of these perturbations will be described shortly. Briefly, small regions of plasma with radically cooler thermal conditions traverse the observation zone, resulting in spikes in the atom line emission and depressions in the ion line emission). It is important to note that the passing droplets perturb the emission from the analyte that has been previously atomized. By comparison, analyte within the droplets is dark matter. This dark analyte may eventually begin to emit, but further downstream, after the droplet has completely desolvated. Once there, the dark material exists in the form of vaporizing particles, left behind by the progenitive droplets. These vaporizing particles may also perturb the otherwise steady state of the analyte emission, but in a different manner than the desolvating droplets: they give rise to local concentrations of analyte near their surfaces, resulting in spikes in the analyte emission. An in depth overview of the investigations into the droplet effects in the ICAP has been provided by Olesik and Fister [13], [13]. Their work represents the most current in this area. On the sub millisecond time scale, spikes may observed in the time resolved signal for soft line emission while dips may observed in the time resolved signal for hard line emission [21]. Three experiments provided solid evidence that droplet phenomena may be responsible for such fluctuations. These three experiments proved that incompletely desolvated droplets can indeed exist in what was formally thought to be a region of atomic plasma, when ICAPs are loaded with aqueous solvent aerosols generated by conventional pneumatic nebulizers. In a two channel experiment conducted by Olesilc et al. [21], spikes in the sub millisecond, time resolved wave forms for Ba I and Ca I correlated with each other, even when Ba and Ca were introduced through separate nebulizers. The spikes could not have been a result of vaporizing particles or local concentrations of either species, otherwise the spikes from Ba and Ca would not correlate; the only rational explanation was that incompletely desolvated droplets created regions of local cooling, which shifted the ionization balance from ions towards atoms for previously atomized analyte. In that way, transient peaks in the atomic emission for Ba would correlate with those for  The Parametric Complexity  89  Ca. In a second experiment, Olesik et al. [21] found that the emission spikes correlated with the laser light scattering signals. The optical parameters were such that only relatively large aerosol particle—incompletely desolvated droplets rather than desolvated particles—could have resulted in a scattering signaL In yet a third experiment conducted by Horlick et al. [24], the characteristic fluctuations in the analyte signal were found to disappear when the nebulizer spray chamber was heated (thus desolvating all the droplets, so that only solvent vapour and desolvated particles were transported to the ICAP). But when the heated, analyte carrying aerosol stream was combined with an aerosol steam of nebulized water, the fluctuations in the analyte signal returned. All three experiments left little doubt that droplets can and do exist in conventional ICAPs used in routine analytical practice. In order to remove any remaining doubt about the existence of incompletely desolvated droplets within the ICAP, and to probe further into droplet related phenomena, Olesik and Fister [13]conducted an experiment in which they simultaneously recorded two line of sight emission channels and laser light scattering. All signals were resolved on the sub millisecond time scale. This allowed them to search for correlations between time resolved atom line emission, ion line emission, and the scattering signal. They also revealed correlations between the time resolved fluctuations and the form of time averaged, axial emission profiles [23]. This brings us up to date on the literature reports of the parametric response of the ICAP associated with solvent loading. It is difficult to surmise from literature reports what the comprehensive parametric response for ICAP-AES actually is. This difficulty arises primarily because: 1., the investigated parameter space covers an inadequate range; 2., the parameter space has been under sampled; 3., the reference frame used for the spatial coordinates of the discharge varies amongst research groups; and 4., the response signal chosen to investigate the parameter space often provides only limited information. In many cases, only line of sight, net emission intensities of a few analyte lines have been reported—no indication of their noise statistics or the noise statistics of their spectral background components have been reported. Moreover, very little information about the  The Parametric Complexity  90  spatial structure of the discharge has been provided, and visual observations of the discharge have seldom been reported. In spite of these problems, the literature provides valuable guidelines for the choice of the relevant parameters to study, and for choice of the emission signals that enable one to gauge most effectively the response surface over the entire parameter space. Moreover, the literature reports the results of many case studies and parametric investigations, which may not take solvent load into consideration, but still provide a basis of comparison for the results reported here. In addition, many of the literature reports offer physical interpretations for the parametric response, which in spite of being speculative, provide valuable insight. 4.1.3 Surveying the Parametric Response with Axial Profiles In order to establish the parametric response of the analytical performance, 1. the parameter space must be defined and 2. appropriate response signals must be chosen. The parameter space in ICAP-AES is defined by several parameters, including the solvent, the solvent load, the forward power setting, at least three argon flow rates, several nebulizer parameters, the design and dimensions of the nebulizer, the design and dimensions of the torch, the load coil configuration, and the concentration of concomitant solutes in the sample solution. Several of these have more or less been standardized, and only a few are critical in determining the analytical performance and physical characteristics of the discharge. In this study, all of the ICAP-AES operating conditions are held constant except the inner argon flow rate, the forward power, and the solvent load. These three parameters defme the parameter space. Only the results for a chloroform loaded ICAP are presented here—as a case study. The response signal measured over such a parameter space would ideally assess the analytical performance, indicate physical characteristics of the discharge, and be readily determined. More specifically, the criteria for selecting the response signal are the following: 1., the signal must be representative of the important analytical lines used in multielement analysis; 2., the background must be well represented; 3., reliable estimates of the standard deviation of the  The Parametric Complexity 91 net signal and background signal must be obtainable; 4., spectral interference with the signal must be minimal; and 5., rapid measurement of the signal must be possible. It should also be possible to extract physical information from the signal. Unfortunately, no single ICAP emission signal meets all of these criteria; it turns out that several response signals must be measured in order to meet all of these criteria. To this end, the emission intensity of two analyte lines, Mg 11279 nm and Mg I 286 nm, and their background contributions were measured (4 signals). A sufficient number of replicate measurements were made to reliably estimate the standard deviati ons of the both the net signal and the background. The ion line was representative of hard line behavior, whereas the atom line was representative of soft line behavior. These lines are also relatively free from spectral interference from molecular bandheads. Unfortunately, this means that their background contributions are not representative of background emission from solvent dissociation products, which are the most important source of background emissi on for an ICAP loaded with chloroform. In order to represent emission from solvent dissoc iation species, emission from the CN bandhead at 388 nm, the C 2 bandhead at 516 nm and the C I line at 248 nm were measu red. In addition to these three background signals, the ratio of magne sium ion to magnesium atom line intensity was determined in order to indicate the thermal conditions. In total, eight response signals were determined over a three dimensional parameter space. With eight responses to determine over four parameters, it is important to conduct the parametric survey with expediency: 8 x n 4 intensity determinations must be made to cover the parameter space, where n is the number of samples for each parameter (typically 2 or more). An exceptionally expedient method is to measure axial emission profiles using a linear array detector. This enables one to measure intensities simultaneously over a complete range of observation heights. It permits extensive parameter spaces to be sample d statistically and in short order, rapidly providing the investigator with a valuable prelim inary survey—a survey which informs him of the nature of the problem—which he can then probe with acuity, while safe from being misled by the conflicting results of other researchers who may have only considered a limited  The Parametric Complexity  92  range of parameters. So the expediency of measuring the response signals as axial profiles is worth consideration, if not indispensable, at least in the initial stages of an investigation.  4.1.4 Axial Profiles Description of an Axial Emission Profile Analysts and investigators of the ICAP alike usually align the axis of their light collection optics such that they view the discharge side on, and such that the optical axis intersects the discharge axis. When the light collection optics are aligned to collect emission from the discharge in this fashion, the intensity measurements thus obtained are known as line of sight intensities. This distinguishes them from spatially resolved intensities obtained using radial inversion or tomography. (It would be even more precise to call them side on, line of sight intensities. But side on is tacitly understood because the side on geometry is the conventional geometry for viewing the discharge. However, one should note that investigators have obtained promising results by viewing the discharge axially, with the light collection optics aimed down its axis.) To a first approximation, this region of light collection for such an optical configuration may be regarded as a line integral, extending through the discharge, from one side to the other. However, it is more realistic to regard the region of light collection as an envelope, a light collection envelope, over which the efficiency of collecting light varies. When measuring line of sight intensities from the discharge, one may vary the position of the light collection envelope along the discharge axis; equivalently, one may vary the observation height (the reference point for the observation height is usually set to the top of the load coil). When one plots intensity (line of sight) versus the observation height (above the load coil), one obtains an axial profile of the line of sight emission intensity, or an axial emission profile. Axial profiles can be measured in two general ways—laboriously or expediently. Laboriously, the viewing height may be scanned by translating the discharge with respect to the light collection optics. Expediently, an array detector may be mounted vertically in the exit focal plane of a grating spectrometer. Then the detector elements sample different observation heights  The Parametric Complexity 93 and a complete axial profile can be measured at once. In either case, it is important to note that spatial region in the discharge sampled by the detector( or by each detector element) is not simply a line integral through the discharge, but is defined by a light collection envelope. Spatial averaging results from the overlap of this light collection envelope with the emission volume. Moreover, temporal averaging is also intrinsic to axial profiles: in order to obtain acceptably low noise levels, the emission intensity must be integrated for times much longer than the time scale of important dynamic discharge phenomena.  94  The Parametric Complexity  CN Emission  discharge axis collection  —  atomic plasma  2 plume _—C hollow cone of incandescant i radiation wall injector tube  Figure 4.1.1. The spatial averaging intrinsic to line of sight measurements. The light collection envelopes intersecting the discharge collect light over enormous gradients of intensity, temperature and density. Several envelopes stacked vertically collect light for an axial profile.  The Parametric Complexity  95  Spatial and Temporal Integration ofAxial Profiles Figure 4.1.1. shows the plume of analyte emission overlapping with two light collection envelopes. Each light collection envelope corresponds to a different observation height, or a discrete detector element in the exit focal plane of the spectrometer. At each point within the region of overlap between the envelope and the plume, the contribution to the line of sight intensity is simply the product of the collection efficiency and the local intensity. Consequently, the line of sight intensity is spatially integrated over the region of overlap. This spatial integration proves critical in setting the limits for interpreting axial profiles, primarily because it can bias the intensity measurements towards locations away from the discharge axis. This situation becomes evident when one inspects the lower end of the analyte plume in the figure. Clearly, one cannot always assume that line of sight measurements represent conditions along the discharge axis because of the bias towards off axis conditions. It is also clear from Figures 4.1.1 that line of sight intensity information spatially integrates emission over regions in the discharge that span enormous gradients in composition and thermodynamic conditions, so that the information content about the fundamental properties—notably temperature and electron density—is severely limited; the spatial integration intrinsic to axial profiles significantly limits their interpretation. One must also consider temporal integration. Long integration times are routinely used to obtain acceptable signal to noise ratios in both analytical and diagnostic measurements of ICAP emission. These long integration times are far greater than the time scale of important macroscopic phenomena at work in the discharge, masking the vaporization of incompletely desolvated aerosol droplets and the vortex shedding in the tail flame region of the discharge. The experimentalist must realize that he may not study these phenomena directly by measuring axial profiles, at least not in the manner described here. In spite of such severe limitations imposed on the interpretation of axial profiles by spatial and temporal integration, a wealth of information can still be extracted from them, as the following sections attest.  The Parametric Complexity 96  4.2 EXPERIMENTAL SECTION The instrumentation was described in Chapter 2. Several experimental details specific to recording axially resolved profiles are presented here, details involving the imaging optics, the background subtraction procedure, data management and the data collection procedure. For experimental details of very similar experiments, the reader is referred to Horlick [25] ,Maessen [4], Weir [26] and Browner [18]. Briefly, light from the solvent loaded ICAP was focused onto the entrance slit of a one meter Czerny -Turner monochromator with a 2 inch diameter, fused silica, piano convex lens with a focal length of 150 mm at 594 nm. The image to object ratio was 1:2, or 225 mm to 450 mm at 594 nm, so that the magnification of the ICAP’s image on the entrance slit was 0.5. In order to bring spherical aberration of the optical train down to acceptable levels, the flat face of the lens was directed towards the entrance slit and the lens was equipped with a 15 mm diameter aperture. Adjusting the image and object distances, according to the lens maker’s formula for a thick lens and the manufacturer’s specifications for the lens, compensated for the chromatic aberration. The 15 mm aperture also limited the acceptance angle The exit focal plane and the plane of the monochromator was nearly stigmatic with the plane of the entrance slit This meant rays forming a monochromatic image on the plane of the entrance slit would be refocused by the collimator and camera mirrors of the monochromator to form an inverted, monochromatic image on the exit focal plane. Similarly, rays passing through the entrance slit were refocused to form an inverted image of the entrance slit on the exit focal plane. A linear photodiode array placed in the exit focal plane with its pixels aligned along the vertical direction would thus collect an axially resolved profile of plasma emission. Its spectral resolution would be determined by the linear dispersion of the monochromator and the pixel dimension in the horizontal direction, its axial resolution would be determined by the pixel dimension in the vertical direction.  The Parametric Complexity  97  There were two further imaging considerations: the spatial response function of the optics and spectrometer (in the vertical, or Y direction) was measured and found to be acceptably flat. Moreover, the entrance slit was carefully aligned with the 2x reduced image of the ICAP so that the image of plasma’s axis of cylindrical symmetry fell precisely on the slit. Using similar optics, Horlick et al.[25] placed a 512 pixel, self scanning, linear photo diode array in the exit focal plane of their monochromator, orienting it vertically so that it would act as an exit slit while giving axial resolution of 512 points over a 24 mm axial distance along the plasma. This gave them 50 p.m axial resolution. However, in this study 1000 pixels of a 4096 pixel LPDA could be illuminated, covering an axial distance of 30 mm along the axis of the plasma, potentially giving an axial resolution of 30 p.m. But such fine spatial resolution was found to over sample the axial profile excessively, when the most complex profile displayed structural detail that required only sub millimeter resolution. For this reason, the diodes were binned into groups of 20, which essentially meant that the output from diodes within successive batches of 20 along the array was summed together. Binning reduced the effective number of discrete detectors from 1000 to 50, resulting in 0.6 mm axial resolution, which was far more appropriate for this investigation, and resulted in reduced read noise (which was averaged out by binning) and helped improve the data handling capacity—important for rapid and comprehensive statistical determinations of axially resolved emission behavior. Twenty replicates were recorded for each profile, but the statistical results will not be presented here. The averaged profiles were finally smoothed with a digital 5 point smoothing filter [27], [28], [29]. This filter length in no way degraded the vertical resolution of the smoothly varying axial profiles.  The Parametric Complexity  98  4.3 RESULTS The axially resolved results presented in this chapter include emission from analyte species and from atomic and molecular products of solvent dissociation and combustion. For the analyte species, Mg I, Mg II will be considered. How their net emission signals and background contributions vary over the parameter space will demonstrate that desolvation is indeed relevant to seeking optimal analytical performance. In order to complement the analyte emission results, the parametric response of emission from the solvent dissociation products, C I, CN, and C 2 will be considered. These results will demonstrate a.) the diagnostic utility of C 2 and CN emission for optimizing the analytical performance of a chloroform loaded plasma, b.) how the C 2 and CN signals characterize the plasma boundary region whereas C I characterizes the atomic plasma, and c.) how axially resolved emission profiles of these dissociation products can be used in combination with surveys of the background emission spectra to predict spectral interference owing to solvent loading. The results also reveal the nature of non spectral interference effects owing to solvent plasma load. Format of Results  In general, results of the parametric study are presented as shown in Figure 4.3.1: the figure consists of 12 frames, each frame displaying 6 axially resolved profiles. From left to right, the frames are labeled I, II, and III, corresponding to increasing forward powers, 1.00, 1.25, and 1.50 kW. Descending from top to bottom, the frames are labeled A, B, C, and D, corresponding to increasing chloroform loads, 3.2, 4.2, 6.2 and 7.4 mg/s. Within each frame, the six axially resolved profiles are labeled 1 through 6, corresponding to inner argon flow rates which increase from 0.65 1/mm to 0.90 11mm in increments of 0.05 11mm. This format is retained for all of the 12 frame figures in this chapter, and the horizontal axis always represents height relative to the load coil, although the quantities represented by the vertical axes vary.  The Parametric Complexity  99  4.3.1. The Parametric Response ofAnalyte Net Intensity Mg II 279.55 Figure 4.3.1. shows the response of the Net Mg II 279.55 nm signal to variation of viewing height, power, solvent load, and total inner argon flow rate. This figure contains a great deal of information (as do all of the 12-frame figures in this chapter). But the general trends are readily perceived if one notes that all the axial profiles of this figure are similar curves. For all of the Mg II profiles, the net intensity appears to increase from zero at a point within or just above the load coil, then proceed through a maximum somewhere between 5 and 15 mm above the load coil, ultimately to decay back to zero before reaching the axial position of 30 mm above the load coil. Upon closer inspection, the only significant responses of the profile over the entire parameter space appear to be 1. axial translation of the position of maximum intensity, 2. change in the slope of the onset in intensity, and 3. change in the intensity of the maximum. In fact, these three responses appear to be related; in general, if the onset in intensity begins higher above the load coil, then the slope of the onset decreases and the intensity of the maximum decreases. A final noteworthy feature common to all of the profiles is their apparent convergence to a common decay point between about 25 mm and 30 mm above the load coil.  100  The Parametric Complexity 30-  -  IA  25-  -  hA  lilA  IIB  IIIB  \ lB  ‘j’ 25-  /1 7/24 12  20-  lID  2520-  ID  hID  1  1020’30  120’30  Height Above Load Coil (mm) Figure 4.3.1. Axially resolved profiles of Mg II emission (279.55 nm) from an inductively coupled argon plasma loaded with chloroform; Forward Powers: I = 1.00, II = 1.25, 111= 1.50 kiloWatts; Chloroform Loads: A = 3.2, B = 4.2, C = 6.2, D 7.4 milligrams per second; Nebulizer Argon + Sheathgas Flow Rates: 1 = 0.65, 2 = 0.70, 3 = 0.75, 4 = 0.80, 5 = 0.85, 6 = 0.90 litres per minute; The concentration of magnesium in the chloroform was 5 micrograms per millilitre.  The Parametric Complexity  101  Several provocative trends are revealed when individual profiles from among the 12 frames of Figure 4.3.1. are compared. For example, when one compares the profiles labeled 6 (0.90 1/mm inner argon )from frames IA, TB, IC, and ID, (1.0 kW forward power, 1.5, 3.0, 4.5 and 6.0 mg/s chloroform, respectively) one notes that the axial profiles are extremely sensitive to chloroform load at the relatively low power of 1.0 kW and at the high inner argon flow rate of 0.90 1/mm—the intensity of the profile maximum decreases from 6.0 to 1.5 relative units and the onset moves from 5 mm to 12 mm above the load coil, which is accompanied by a shift in the position of the maximum from 12 mm to 17 mm. On the other hand, when one compares the profiles labeled 1 from frames lIlA, IIIB, IIIC, and IIID (1.50 kW forward power, 1.5, 3.0, 4.5 and 6.0 mg/s chloroform, respectively), the profiles of highest power and lowest inner argon flow rate, one notes that the profiles are relatively insensitive to solvent load. Although the onset of intensity is obscured for the profiles in these frames, the position of the maximum shifts less than 2 mm and the intensities of the maxima appear to remain constant, except for a slight decrease in the case of the lowest chloroform load. In the high power, low flow rate case, the profiles display hard line behavior with respect to solvent load—the positions of their maxima are insensitive to a huge increase in solvent loading—whereas in the low power, high flow rate case the profiles display soft line behavior.  The Parametric Complexity  102  Mg 1(285.21 nm) Figure 4.3.2 depicts the response of Mg I 285.21 nm net line intensity to viewing height, chloroform load, r.f. power and inner argon flow rate. There are two conspicuous differences between the Mg II and the response of this soft Mg I line (with an excitation potential  =  4.35  eV). One immediately notes that the intensity appears to increase with increasing solvent load, in contrast to the ion line behavior. The second discrepancy becomes evident when one compares the three columns. The change in intensity with solvent load becomes more pronounced at higher forward powers. A third discrepancy is also evident. The maxima of the Mg I profiles consistently appear at lower axial positions than the corresponding maxima for the Mg II profiles. It appears as though the cooler conditions promote Mg I emission, while hotter conditions promote Mg II emission at the expense of Mg I emission. One might infer that Mg II and Mg I are coupled by a balance which depends on conditions within the discharge. Whether or not this balance is dynamic or determined by some sort of equilibrium is not clear. For the moment, it can only be stated that the balance leans towards Mg I emission (and possibly Mg I density) at relatively cool locations along the discharge axis (averaged space and time), while promoting Mg II emission at relatively hot locations. However, it is clear that the sensitivity of analyte line emission depends on the forward power, solvent load and inner argon flow rate. This is indeed the behavior one might intuitively expect for the solvent loaded inductively coupled argon plasma if one were to interpret its behavior in terms of residence time and power consumption. Indeed, it is tempting to speculate upon the physical processes involving energy and mass transport, the excitation of analyte atoms within the plasma, and upon the thermodynamic conditions within the plasma. But one must proceed with extreme caution when one interprets the behavior of relative, line of sight emission intensities such as these from analyte species within the plasma, firstly because the intensities are temporally and spatially integrated and secondly because they are a function of both the temperature and the concentration of emitting analyte.  103  The Parametric Complexity 3.0-  2.5-  IA  hA  lilA  lB  IIB  TuB  2.01 .5  -  !... 2.5-  HIC  iuc  Ic •1  1111  2.5-  hID  ID  hID  0iti i’0’2’0’30  = = = = = = = = = =  0’2’0’30  Height Above Load Coil (mm)  Figure 4.3.. Axially resolved profiles of Mg I emission (285.21 nm) from an inductively coupled argon plasma loaded with chloroform; Foreward Powers: I 1.00, II 1.25, III 1.50 kiloWatts; Chloroform Loads: A 3.2, B 4.2, C 6.2, D 7.4 milligrams per second; Nebulizer Argon + Sheathgas Flow Rates: 1 0.65, 2 0.70, 3 0.75, 4 0.80, 5 0.85, 6 0.90 litres per minute; The concentration of magnesium in the chloroform was 5 micrograms per millilitre.  The Parametric Complexity 104 4.2.2 The Parametric Response ofBackground Intensity The spectral survey of ICAP background emission presented in chapter 3, which spanned from the near infrared to the near ultraviolet, showed that the most conspicuous background features in common to chloroform and methanol loaded ICAPs were atomic line emission from carbon and argon, and molecular band emission from diatomic carbon and the cyanyl radicaL Relatively weak OH and NO band emission could be detected from ICAPs loaded with water and +, N 2 alcoholic solvents, but emission from other molecular species, including CH, CO. Nil, N 2 and CC1, and from singly ionized argon could not be verified. The parametric behavior of the Ar I, C I, C 2 and CN emission signals are interesting for two reasons: 1., they indicate the level of spectral interference and background intensity; and 2., they reveal where the solvent dissociation front resides with respect to the analytical viewing zone, indicating how robust the plasma is (in a more robust plasma, the onset of solvent dissociation should reside at lower heights). For both reasons, these background signals may serve as working diagnostics of interference effects in ICAP-AES, just as Olesik [30] suggested that OH emission, and Browner [18] suggested that C 2 emission may serve as working diagnostics. Here, axial profiles of C I, C 2 and CN are presented for an ICAP loaded with chloroform over the same parameter space of solvent load, inner argon flow rate and forward power as before. The wavelengths for sampling their emission were chosen simply by the criterion that the intensity maximum selected was freest from interference from other emission features: the 247.86 nm line for C I, The 516.52 nm bandhead for C 2 and the 388.34 nm bandhead for CN. The object of these measurements was simply to obtain a survey of the parametric response of emission from solvent dissociation products. (No supposition pertaining to the relative contributions of different bandheads or lines of a particular species were made).  The Parametric Complexity  105  The Parametric Response of Emission from Atomic Carbon: The C 1247.86 nm Line Figure 4.3.3 shows the parametric response of the line of sight, axially resolved intensity of the atomic carbon line at 247.86 nm from a chloroform loaded ICAP. The C I profiles are presented in the same format as the manganese and magnesium lines, except that the results for only two solvent loads are shown, 3.0 mg/s and 6.0 mg/s. Interpreting these axial profiles is more difficult because the C I signal arises from the atomic plasma, which extends radially well beyond the aerosol channel where the analyte is confmed. At a relatively high power of 1.5 kW, the C I intensity shown in frames HIB and BID increases with chloroform solvent load for all of the profiles at all viewing heights along the profile, with the most pronounced increase at 12 mm above the load coil This is what one would intuitively expect for a robust, relatively high power plasma: an increase in the amount of carbon introduced into a robust plasma results in an increase in the C I intensity, especially at higher viewing heights where the maximum amount of energy has transferred from the r.f. power dissipation region into the aerosol channel In contrast, the presence of the aerosol channel begins to manifest itself at the lowest viewing heights, where the correlation between chloroform load and C I intensity becomes ambiguous, and where a minimum in C I emission may correspond to the relatively cool region of the aerosol channel, where atomization is incomplete.  106  The Parametric Complexity 30-  25-  20-  15-  I  -  4.  0..  25-  ID  20-  15-  I__ 10’2’0’30  Height Above Load Coil (mm) Figure 4.3.3 Axially resolved profiles of atomic carbon emission (248 nm) from an inductively coupled argon plasma loaded with chloroform aerosol; Forward Powers: I = 1.00, II = 1.25, III = 1.50 kiloWatts; Chloroform Plasma Load: B = 4.2, D = 7.4 milligrams per second; Nebulizer + Sheath gas Flow Rates: 1 = 0.65, 2 = 0.70, 3 = 0.75, 4 0.80, 5 = 0.85, 6 = 0.90 liters per minute; Pure chloroform was nebulized for these profiles.  The Parametric Complexity  107  Further insight may be gained by noting the conspicuous correlation between the maxima of the profiles in frames 1118 and I11D with the corresponding Mg II profiles (Figure 4.3.1). This correlation corroborates the notion that hard lines emit most intensely where the maximum amount of energy has transferred into the axial channel. In other words, hard line emission is most intense in the hottest part of the aerosol channel. This assumes that energy continues to transfer into the aerosol channel as the sample material travels downstream until the plasma is extinguished by air entrainment. It also assumes that there is no region along the aerosol channel where the plasma begins to cool down because of energy loss exceeding energy input. In other words, energy is simply supplied to the aerosol channel up to a cut off point, where the plasma is extinguished by air entrainment But this correlation begins to fail for profiles 4,5 and 6 of frame hID. Here one must carefully interpret the spatially averaged results, bearing in mind the extended radial structure of the atomic plasma with respect to the analyte plume. Perhaps the conical structure of the atomic plasma combined with the penetration of the cool region of the aerosol channel masks an otherwise perfect correlation? In order to answer this question, radial resolution is required. In contrast to the behavior of the robust 1.5 kW plasma, frames lB and ID reveal that C I emission from a 1.0 kW plasma exhibits the opposite response to chloroform loading; at 1.0 kW, all of the C I profiles decrease in intensity when the solvent load, hence amount of carbon, is increased, except at heights lower than 10 mm. Presumably the plasma is not robust enough at this power to contain the cool region of the aerosol channel below 5 mm, or within the load coil region. In fact, observations reported in chapter three confirm that the cool region completely penetrates the conical plasma up along its axis, under conditions identical to those of frame ID. Apparently the aerosol material simply blasts straight through the plasma. One further feature of Figure 4.3.3 worth noting is the remarkable similarity between certain profiles in frames lB. 118 and IIIB: the lowest power, low inner argon flow rate profiles labeled 1 and 2 in frame lB closely match the intermediate power, intermediate flow rate profiles labeled 3 and 4 in frame IIB, which in turn match the highest power, high flow rate profiles  The Parametric Complexity  108  labeled 5 and 6 of frame TuB. This similarity reveals that inner argon flow rate and the forward power have competing effects; an increase in forward power increases the C I intensity while an increase in the inner flow rate decreases the C I intensity. Another parametric balance appears to involve power and solvent load, which becomes evident when profiles are compared between diagonally situated frames, from the upper left to the lower right; an increase in forward power increases the C I intensity while an increase in the solvent load decreases the C I intensity. It is interesting to note that the same parametric balances were revealed by the Mg II response. One may surmise from these parametric balances that solvent load and inner argon flow rate are power consuming quantities, but that they do not act independently—the inner argon flow rate determines how the solvent load consumes power and visa versa. In summary, the way C I emission responds to chloroform load, inner argon flow rate and forward power may be understood in the light of three perspectives: 1., how the robustness of the atomic plasma changes over the parameter space; 2., how aerosol material is distributed within the argon flow system; and 3., how far the cool region of the aerosol channel penetrates along the axis of the atomic plasma. The plasma is clearly robust to solvent load at high power— an increase in solvent load increases the carbon signal, indicating fairly constant plasma conditions. But at high power, the plasma remains sensitive to the inner argon flow rate, indicating that the inner argon flow rate may determine how the aerosol material is distributed within the argon flow system and therefore how solvent dissociation consumes power. At low power, the plasma is considerably less robust—an increase in solvent load depresses the C I • intensity, indicating that the plasma conditions have become cooler. At any rate, it is difficult to isolate the effects of power consuming quantities, mass distribution effects and distortion or folding of the atomic plasma from these spatially and temporally averaged, line of sight profiles of C I emission.  The Parametric Complexity  109  The Parametric Response ofEmission from Diatomic Carbon: The C 2 Bandhead at 516 nm In contrast to the C I response, the axially resolved profiles of diatomic carbon emission shown in Figure 4.3.4. reveal a remarkably simple parametric response: it appears that the general shape of the diatomic carbon emission profile remains constant over the parameter space and that only its intensity and axial displacement change. Profile 6 of frame ID reveals the general shape of the axial C 2 emission profile most clearly: it has a single, clearly defined maximum at its upper end which abruptly decays to zero at higher axial positions, but gradually decays to a finite value at lower axial positions. This profile shape may be best understood in light of the observations reported in Chapter three. To recapitulate those observations, the characteristically intense green emission from diatomic carbon formed a cup around the periphery of a plasma loaded with chloroform. This cup wrapped around the plasma’s base to join up with a hollow cylinder of diatomic carbon emission running up along the axis of the plasma toroid. The hollow, axial cylinder eventually coalesced into a bullet shaped tip further downstream, which terminated at axial positions ranging from within the load coils up to 20 mm above them, depending on the operating parameters. This bullet shape tip corresponds to the maxima of the diatomic carbon emission profiles. The hollow, axial cylinder is evident in plot 6 of frame ID; the non zero minimum at 5 mm is the integrated intensity across the hollow plume. The integrated intensity increases with height as the hollow cylinder collapses into a bullet shaped tip, then abruptly decays to zero above the tip. Contributions from the peripheral cup are not apparent because the viewing range is too high—the peripheral cup is confined to the region within the torch because peripheral air entrainment just above the torch rim converts carbon species to CN.  110  The Parametric Complexity 2502001 50-  IA  hA  lilA  1 00500-  -  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  250— 2001 50-  lB  IIB  -  IIIB  1 00-  :  500-  I  250-  I  I  I  I  2001 50—  “IC  1 00— 50— 0—  I  I  I  I  ‘  I  I  I  I  2502001 50  hID  -  ‘Ii 6  1 00-  34  34  500-I 4  I  8  12  •  I  16  4  I  I  I  I  8  12  16  4  •  I  8  •  I  12  Height Above Load Coil (mm) Figure 4.2.4 Axially resolved profiles of diatomic carbon emission (516nm) from an inductively coupled argon plasma loaded with chloroform aerosol; Forward Powers: I = 1.00, II = 1.25, III = 1.50 kiloWatts; Chloroform Plasma Loads: A 3.2, B = 4.2, C = 6.2, D = 7.4 milligrams per second; Nebulizer ÷ Sheath gas Flow Rates: 1 = 0.65, 2 = 0.70, 3 = 0.75, 4 = 0.80, 5 = 0.85, 6 0.90 liters per minute; Pure chloroform was nebulized for these profiles.  •  I  16  The Parametric Complexity  111  The diatomic carbon profiles in Figure 4.3.4 display remarkably simple behavior. Their response to inner argon flow rate and forward power may be described as a linear axial translation while their response to solvent load appears to be a linear scaling of intensity and axial translation with the amount of carbon introduced. Inspection of plots 1 through 6 of frames ID, lID and hID reveals that the maximum intensity of the profiles is insensitive to both the inner argon flow rate and the forward power. Moreover, comparison of the plots in frames IA, TB, IC and ID reveal that the maximum intensity is directly proportional to solvent load. This is surprising because it suggests that the intensity of diatomic carbon emission is independent of plasma conditions, which one would expect to vary considerably over the parameter space, just as the parametric response of Mn II, Mg II and C I indicated. The key to explaining this apparently anomalous behavior of diatomic carbon emission might be that it arises from the boundary region between the aerosol channel and the atomic plasma proper, a point that the observations reported in Chapter 3 suggested. As carbon containing material is transported by diffusion or convection from the aerosol channel, across the boundary region and into the atomic plasma, it must traverse an enormous temperature gradient. Dissociation products ranging from undissociated solvent molecules to completely dissociated atoms may plausibly occupy progressive regions of stability corresponding to progressive ranges of temperature across the temperature gradient, forming a spectrum ranging from molecules to atoms. It is plausible that the diatomic carbon molecule is only stable over a very narrow temperature zone along the boundary gradient, ensuring that the plasma conditions are similar wherever diatomic carbon emission is observed. This would certainly explain the apparent independence of the diatomic carbon emission intensity from plasma conditions and its simple dependence on plasma composition and amount of carbon.  The Parametric Complexity 112 The Parametric Response ofEmissionfrom The Cyanyl Radical: The CN Bandhead at 388.34 nm When CN emission from a ICAP loaded with organic solvent is measured with axial resolution, it becomes clear that the CN emission structure consists of two components. These two component were first reported by Maessen and Kreuning [8], and were further investigated by Browner [18], [12]; their existence is evident in the results presented here. Comparison of the CN profiles in frame ID of Figure 4.3.5 with profiles over the rest of the parameter space reveals the two components quite clearly. The minor component of CN emission is found at low axial positions, and displays a parametric response similar to the C 2 emission profiles presented earlier. On the other hand, the major component of CN emission makes an overwhelming contribution at higher axial positions, and displays a very simple parametric response. It always displays a maximum at approximately 22 mm above the load coil and appears to increase in intensity linearly with solvent load. The minor component originates from the inner boundary region, or the aerosol channel of the discharge, whereas the major component arises from the outer boundary region. The source of nitrogen for the major component is obviously atmospheric nitrogen. Browner established that the source of nitrogen for the minor component is most certainly a trace impurity of nitrogen in the argon. Those fmdings are corroborated by the results presented here. In contrast to the minor CN component, evidence presented in chapter 5 reveals that emission from the major component originates from downstream positions and from beyond the outer periphery of the atomic plasma, or from what may be called the plasma tail flame. It is therefore clear that the major component results from entrainment of atmospheric nitrogen into the plasma and its reaction with carbon containing dissociation products.  113  The Parametric Complexity 40-  IA  30-  IIA  lilA  2010  .  0  r  .  I—  —  40  TB  30  IIB  IIIB  2010  ,—  /  0  N. ‘-1r  r  40  IC  30  74.  6/  20 10  \  ID  30  ,f .-  7  0 5  10  Ijic  IIC  15  20  25  30  p  5  10  15  20  25  30  L:zI 10 15 20 25 30 5  Height Above Load Coil (mm) Figure 4.3.5 Axially resolved profiles of cyanogen radical emission (388nm) from an inductively coupled argon plasma loaded with chloroform aerosol; Forward Powers: I = 1.00, II = 1.25, III = 1.50 kiloWatts; Chloroform Plasma Load: A = 3.2, B = 4.2, C = 6.2, D = 7.4 milligrams per second; Nebulizer + Sheath gas Flow Rates: 1 = 0.65, 2 0.70, 3 = 0.75, 4 = 0.80, 5 = 0.85, 6 = 0.90 liters per minute; Pure chloroform was nebulized for these profiles.  The Parametric Complexity  114  A similar explanation may be offered for the parametric response of the major component as for the minor component, an explanation which also assumes that the CN emission radiates from a boundary region where the thermal conditions are relatively independent of those within the plasma. But in contrast to the steady flow system proposed for the minor component, where material rapidly traverses an enormous temperature gradient, convincing evidence presented in Chapters 5 and 6 and reported in numerous references supports the existence of vortex shedding in the plasma tail flame, resulting in a vortex entrainment mechanism which rapidly extinguishes the plasma by efficiently mixing the plasma gas with air. The net result of this vortex entrainment mechanism is that CN emission depends almost linearly on solvent load, which determines the amount of carbon available for CN formation, and that a maximum in the axially resolved, line of sight, CN emission profile is found at approximately 22 mm above the load coil, at a location just beyond the conical tip of the of the atomic plasma, and close to the point of maximum mixing of plasma carbon with atmospheric nitrogen. Further downstream, the CN intensity decreases as more air is folded into the tail flame by the vortex train and quenches the CN emission. 4.3.3 Magnesium Ion to Atom Ratio Profiles The axially resolved emission profiles presented thus far provide useful insight into the atomization, excitation and ionization processes in the inductively coupled argon plasma loaded with chloroform. But emission intensities depend upon not only on excitation and ionization processes, but also the spatial distribution or density of the analyte species within the plasma. This means that both the physical processes and the spatial distribution of the analyte species must be taken into account, making the interpretation of emission intensities complex, if not fraught with sources of error. However, interpreting intensity data can be significantly simplified by removing its dependence on the spatial distribution of the emitting species. This can be conveniently accomplished by taking the ratio of emission intensity from singly ionized atomic species to the emission intensity from corresponding neutral species. The ion to atom emission intensity ratio thus obtained is more or less independent of the spatial density of the sum of the  The Parametric Complexity  115  ionic and atomic components, and under certain conditions may be regarded as depending on only the excitation and ionization conditions within the plasma. Of course, this reasoning requires that the emitting species exists only as free neutral atoms or free ionized atoms. In other words, none of it can be bound in molecules or trapped in aerosol particles. It also requires that the excitation conditions within the plasma are sufficiently close to local thermal equilibrium (see Chapter 7) for the Saha-Boltzmann distribution to approximate the ionization-excitation distribution of the atomic species in question. Essentially, the analyte must exist as a free atomic gas within the argon plasma and some sort of excitation/ionization equilibrium must exist between the neutral and ionized states of the free atomic analyte. Also, the temperature must be constant along the line of sight. It should be stated parenthetically that experimental evidence shows that this is not true for important regions of the parameter space, primarily because incompletely vaporized droplets trap much of the analyte and create enormous, transient temperature gradients, which drastically pull the plasma away from LTE, and which can only be effectively probed by time resolved measurements. We will assume that ion to atom line intensity ratios are independent of the analyte distribution, and faithfully indicate excitation and ionization conditions in the plasma, over an appropriate range of parameters. It is further assumed that if conditions should depart significantly from the situation where a balance exists only between the atom state and the singly ionized atom, then it will be readily apparent in the results. Selection ofIon and Atom Lines The question remains of what species to select for ion to atom emission intensity ratio measurements. Several compelling reasons favor the measurement of magnesium line intensity ratios. In particular, the selection of the Mg I line at 285.21 nm and either one of the Mg II lines at 280.27 nm or 279.55 nm is favored by several practical advantages and physical attributes. Mermet [31] summarizes many of these: the excitation and ionization mechanisms of Mg in  The Parametric Complexity  116  argon plasma are quite well understood; transition probabilities have been determined accurately for the lines at 285.21, 280.27 and 279.55 nm so that an experimental value of the ion to atom line intensity ratio may be compared confidently with theoretical ones predicted by the Saha and Boltzmann equations (see equation 4.1 below); the excitation potential of the Mg I 285.21 nm line is close to that of the Mg II 280.27 nm and Mg II 279.55 nm lines, so that the effect of the second exponential term in equation 4.1 is negligible, simplifying the dependence of the intensity ratio on the electron density and the ionization temperature; the two Mg II lines have been proven to be sensitive to ICAP excitation conditions, so that a Magnesium ratio incorporating one of them will also be a sensitive measure; the atom and ion lines are spectrally close together, obviating spectral response corrections; many literature reports report magnesium atom to ion line intensity ratios using these lines, forming an established basis of comparison. In addition, for a chloroform loaded plasma, the Mg II and Mg I lines are found in a spectral region relatively far removed from interfering molecular band emission and atomic/ionic line overlap. Finally, the second ionization potential for magnesium (eV) precludes the formation of Mg 2 (Mg ifi lines). Interpreting and Evaluating ofMg Ion to Atom Emission Intense Ratios Once the profiles of the magnesium ion to atom emission intensity ratio have been determined experimentally, they may be interpreted along several avenues. One avenue of interpretation regards the plasma as existing in a thermodynamic state close to local thermal equilibrium. The experimentally determined ion to atom line intensity ratios are compared to theoretical values predicted for the same electron density as the experimental plasma, but at local thermal equilibrium. The theoretical ratios can be calculated using the Saha equation, experimentally determined electron densities, Dalton’s law, the assumption that the plasma pressure is one atmosphere, and from an argon ionization temperature. The theoretical values thus obtained form the basis of comparison for the experimentally determined ones. The further the experimental values deviate from the theoretical values predicted using assumptions based the existence of LTE, the further  The Parametric Complexity 117 the plasma conditions can be regarded to depart from LTE conditions. The departure from LTE can then be accounted for by various collisional and radiative processes. It must be remembered that this avenue of interpretation neglects the effects of transport processes, flow dynamics and sample transformation (such as aerosol droplet vaporization), and requires that the analyte exists only as free atoms and singly or multiply ionized atoms which interact thermally with the neutral argon atoms, the singly ionized argon atoms and the unbound electrons of the plasma. Ample experimental evidence supports the prevalence of these sorts of conditions and the existence of LTE, or at least only a partial departure from LTE in analytical zone of the inductively coupled argon plasma, provided very little or no sample aerosol is being introduced. On the other hand, when sufficient amounts of sample aerosol are introduced for the ICAP to be spectrochemically useful, ample experimental evidence supports the existence of vaporization processes involving aerosol droplets and aerosol particles which drastically affect the emission characteristics of the analyte. Olesik has recently established that ion to atom line emission intensity ratios must be interpreted as the time averaged result of a myriad of droplet induced fluctuations on the sub millisecond time scale. This evidence suggests that ion to atom line intensity ratios ought to be interpreted along an alternative avenue to the one based on partial local thermal equilibrium principles—the ratios ought to be interpreted in terms of dynamic processes.  The Parametric Complexity  118  Mermet has offered a third avenue for interpreting ion to atom line intensity ratios. Essentially, he proposed that experimentally measured ratios of the Mg II and Mg I 285 nm lines must approach 20, the theoretical local thermal equilibrium value, in order for the plasma to be robust. In effect, he regarded the value of the experimental ratio as a measure of the robustness of the plasma. He used the following expression for the theoretical ratio derived from the Saha equation and the Boltzmann function [31]:  = (4.83 x 10y)(iA//)TYz exp(°n Jex[ E)]  (4.1)  where g is the statistical weight, A the transition probability, A. the wavelength, Eexc the excitation energy, E,, the ionization energy, Te the electron temperature, Te the excitation temperature, e the electron density and k the Boltzmann constant. The subscripts i and a refer to the values for the ion and atom lines, respectively. Mermet suggested that the ICAP must be sufficiently robust to completely vaporize all of the aerosol droplets and particles before they enter the analytical viewing zone in order for local thermal equilibrium conditions to prevail. This requires sufficient energy transfer from the plasma to the aerosol sample material, so the forward power must be sufficiently high and the residence time of sample material in the plasma toroid must be sufficiently long. Otherwise, the material may simply flow straight through the plasma toroid and experience minimal vaporization and dissociation. Digressing, it is interesting to note that the optimal analytical performance of the ICAP must strike a compromise between low detection limits and robustness to interference effects caused by constituents of the sample matrix. Boumans and Lux-Steiner have described this  The Parametric Complexity  119  compromise: they reported that the lowest practical operating power in combination with relatively high inner argon flow rates yield the best detection limits for ICAP-AES, primarily because its combination of operating conditions results in a spatial separation of the continuum background emission from the analyte line emission: the most intense continuum background emission arises from the plasma toroid, whereas the most intense analyte emission comes from an axial plume which extends further above the load coil than the toroid. This spatial separation is evident in Figures 4.1.1. Conversely, the ICAP is more robust to matrix interference effects at high powers and low inner argon flow rates—at the expense of low, background noise dependent detection limits—because the plasma responds to these conditions by extending further above the load coil where the noise of its background continuum emission can make a greater contribution to the analytical signal Perhaps, then, the LTE interpretation works for ICAPs optimized to deal with matrix effects, whereas interpretations within the context of dynamic processes would work best for ICAPs optimized for the best detection limits. Clearly, some sort of method for determining the plasma’s state of robustness, or it’s macroscopic state is required in comprehensive experimental reports. It is by all three avenues of interpretation that the following results of Mg ratios are examined.  The Parametric Complexity  120  Magnesium Ratio Results Figure 4.3.6 shows the parametric response of the line of sight Mg ion to atom line intensity ratio for an ICAP loaded with chloroform. The shape of the axial profiles of this ratio is most clearly revealed in frame TIC. Each profile has a sharp onset in the ion to atom emission ratio, which begins at low axial positions and levels off or decreases in slope at higher positions to form a plateau. Between 20 mm and 25 mm above the coil, the profiles all merge together, then appear to converge with the horizontal axis just above 30 mm. Upon first inspection, the only four differences between all of the profiles over the twelve frames appear to be 1. the axial displacement of the initial onset, 2. the residual slope of the plateau region, 3. the axial position of their maxima, and 4. the height of their maxima. Moreover, these four profile variations appear to be related; when the initial onset of the intensity ratio shifts to higher axial positions, then the slope of the plateau region becomes steeper, up to the extreme situation where the plateau region cannot be discerned from the initial onset region. Similarly, when the position of the initial onset shifts to higher axial positions, the maximum of the intensity ratio decreases. Before attempting to interpret this behavior, it is worthwhile to examine the absolute values of the atom to ion emission ratio. Without exception, the ratio profiles display absolute values less than 20. Moreover, an absolute value of 20 is closely approached only under robust plasma conditions: low solvent load, high power, low inner argon flow rate and at viewing heights between 7 mm and 10 mm above the load coil. This is evidenced by the profiles labeled 1 through 3 in frames hA, lilA, and IIIB.  121  The Parametric Complexity 20-  IA  IA  IA  15-/  10  S  //  20-  -c  IIB  lB  15-  p10-  20-  15-  110-  .....  020-  ID  15-  r Ilic  lic  IC  hID  lID  I 10-  5-  0-  I 10  I  I  I  20  30  10  ‘  I 20  I 30  •  I 10  •  I 20  Height Above Load Coil (mm) Figure 4.3.6. Axially resolved profiles of the Ratio of Mg II emission (279.55 nm) to Mg I emission (285.21 nm) from an inductively coupled argon plasma loaded with chloroform; Forward Powers: I = 1.00, II = 1.25, III = 1.50 kiloWatts; Chloroform Loads: A = 3.2, B = 4.2, C 6.2, D = 7.4 milligrams per second; Nebulizer Argon + Sheathgas Flow Rates: 1 = 0.65, 2 = 0.70, 3 = 0.75, 4 = 0.80, 5 = 0.85, 6 = 0.90 litres per minute; The concentration of magnesium in the chloroform was 5 micrograms per millilitre.  30  The Parametric Complexity 122 If one accepts Mermet’s proposal that the Mg ratio approaches 20 for a robust plasma close to state of local thermal equilibrium, then the profiles in figure 4.3.6 indicate that LTE is only approached for low solvent loads, high power, low inner argon flow rate and sufficient heights above the load coil. One might further conclude that because the plasma appears to approach local thermal equilibrium under these conditions, vaporization and atomization processes must be complete and that non spectral, matrix interferences with the analytical performance should be negligible because the plasma is robust and energy has been efficiently transported from the plasma toroid into the aerosol channeL But it remains to be understood what happens in the plasma away from these conditions, where the ratio is much less than 20. What happens during the initial onset? What happens during the decay at higher heights? What state is the plasma in under conditions of high solvent, low power and high inner argon flow rate. The ratio values much less than 20 in the initial onset may indicate a plasma state far removed from LTE, conditions under which the vaporization and atomization of the aerosol material is just reaching completion, and under which a dynamic model of the plasma state, which accounts for vaporization and atomization may be more successful. This avenue of interpretation is supported when the onset of the Mg ratio in Figure 4.3.6 is compared with the decay of diatomic carbon emission in Figures 4.3.4—the onset of the ratio overlaps with the decay of C 2 emission. Or it is possible that the decay of C 2 emission and the onset of the Mg ratio do not overlap at all. It is quite possible that the onset of the line ofsight Mg ratio is biased to off axis positions in the plasma, where the Mg emission may be more intense than on axis, and that the tip of the C 2 emission plume lies nested within the hollow base of the Mg I and Mg II emission plumes. Only radially resolved information requiring further spatial resolution than the axially resolved, line of sight results presented here can confirm this supposition. Radially and axially resolved emission profiles presented in Chapter 5 appear to confirm this, although they do not cover quite as extensive a parameter space as the axial resolved results presented here. But the observations reported in Chapter 3 support the nested plume supposition.  The Parametric Complexity  123  Summary of Mg Ratio Results In summary, the magnesium ion to atom emission intensity ratio is an informative diagnostic in three capacities: 1., it locates the axial position where the aerosol vaporization and atomization processes have effectively reached completion; 2., it locates the downstream boundary of the plasma; and 3., it indicates how far the conditions within the plasma depart from local thermal equilibrium. It was shown that the onset of the ratio profile correlates with the decay of the diatomic carbon emission signal, presumably because the point of overlap between the two profiles roughly locates the axial position where diatomic carbon has completely disassociated and where the magnesium has been completely vaporized. An almost steplike decay (complete decay to zero over a short axial distance) correlates well with the position where the hard line emission profiles decay to zero—this is apparently the downstream boundary of the plasma where vortex entrainment of air extinguishes the plasma. The Mg ratios for a robust ICAP (low solvent load, high power and low inner argon flow rate) are remarkably close to the theoretical local thermal equilibrium values calculated using realistic electron densities.  The Parametric Complexity  124  4.6 CHAPTER SUMMARY The axial profiles reported in this chapter provide a valuable survey of the parameter space important to ICAP operation; valuable because it reveals how physical processes such as vaporization and air entrainment vary or remain constant over the entire parameter space, and under what parametric conditions partial local thermal equilibrium conditions prevail—hence where pLTE models should be realistic, how the analytical performance varies over the parameter space, and which emission signals are useful as diagnostic checks for indirectly monitoring the analytical performance. A valuable survey indeed for the analyst and researcher alike. To recapitulate, this in depth survey of the parametric response of several emission signals provided the following insight: the physical processes of vaporization and dissociation were clearly revealed by the diatomic carbon profiles and the profiles of the Mg ion to atom emission intensity ratio. The decay of the C 2 profiles correlated with the onset of the ratio profiles—their cross over point likely indicates where atomization of the aerosol material has reached completion. The response of the crossover point clearly revealed that vaporization and dissociation processes persist well into the analytical zone under a wide range of parametric conditions. This survey also revealed the significance of how far the diatomic carbon plume, or the dissociation front penetrates up through the atomic plasma: if it penetrates too far, say beyond 12 mm, then the conditions within the viewing zone deviated significantly from pLTE, presumably because not enough energy is transported into the aerosol channeL This is an important revelation to the analyst and researcher alike, because under plasma conditions far from LTE the dynamic processes associated with non spectral interference effects can be expected to be important, making interpretations in terms of collisional radiative balances difficult or unrealistic. Beyond the influence of the dissociation front, and past the analytical zone, a sharp cutoff in the Mg ratio profiles correlated with the decay of the hard line profiles, the decay of the C I profiles, and with the downstream maxima of the CN profiles—strongly suggesting that all of these features are tied together by the air entrainment mechanism which extinguishes or brings about an abrupt quenching of the plasma downstream.  The Parametric Complexity 125 4.7. CoNcLusioNs Spatial and temporal averaging imposes severe limitations on the interpretation of the axially resolved, line of sight emission profiles presented in this chapter. Further limitations arise from the complex excitation environment and the spatial distribution of the emitting matter. However, several conclusions may be drawn from the axially resolved profiles of emission intensity and emission intensity ratios reported in this chapter—conclusions regarding analytical utility and physical processes. The measurement of axial profiles are an efficient and effective means for determining the parametric response in ICAP AES. Profiles of C 2 emission and of the Mg ratio are excellent diagnostics of the dissociation processes in ICAP loaded with chloroform. It is evident that the further the C 2 dissociation front penetrates the atomic plasma, the less efficient the energy transfer into the aerosol channel. Moreover, C 2 and CN emission intensities are largely independent of the plasma conditions. They probably indicate conditions within the boundary region rather than in the plasma. In other words, C 2 emission indicates the conditions at the solvent plasma interface while CN emission indicates the conditions in the tail flame. As a result, the intensity of emission from both diatomic species is proportional to the carbon load, in contrast to C I emission, which depends on both plasma conditions and the carbon load. 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Kreuning, Spectrochimica Acta B, 42: p. 677 (1987).  8.  F.J.M.J. Maessen and G. Kreuning, Spectrochimica Acta B, 44(4): p. 387-384 (1989).  9.  G. Dube and M.I. Boulos, Canadian Journal of Spectroscopy, 22(3): p. 68-76 (1977).  10.  M.W. Blades and G. Horlick, SpectrochimicaActa B, 36: p. 861-880 (1981).  11.  A.W. Boom, M.S. Cresser and R.F. Browner, Spectrochimica Acta, Part B, 35: p. 823 (1980).  12.  G. Pan, G. Zhu, and R.F. Browner, Journal ofAnalytical Atomic Spectrometry, 7: p. 123 1-1237 (1992).  13.  J.W. Olesilc and J.C. Fister, Spectrochemica Acta Part B, 46(6/7): p. 85 1-868 (1991).  14.  M. Marichy, M. Mermet, and J.M. Mermet, Spectrochimica Acta, Part B, 45(11): p. 1195-1201 (1990).  15.  Huang, B., et al., Spectrochimica Acta, Part B, 46(3): p. 407-416 (1991).  16.  Canals, A. and V. Hernandis, Journal ofAnalytical Atomic Spectrometry 5: p. 6 1-66 (1990).  17.  Nukiyama, S. and Y. Tanasawa, Trans. Soc. Mech. Eng. Jpn.5: p. 68 (1939).  18.  Pan, C., G. Zhu, and R.F. Browner, Journal ofAnalytical Atomic Spectrometry 5: p. 537 (1990).  ,  The Parametric Complexity  127  19.  L. Ebdon, E.H. Evans, and N.W. Barnett, Journal ofAnalytical Atomic Spectrometry 4: p. 505-508 (1989).  20.  M.T. Cicerone and P.B. Farnsworth, Spectrochimica Acta, Part B, 44: p. 897 (1989).  21.  J.W. Olesik, L.J. Smith, and E.J. Williamsen, Analytical Chemistry 61: p. 2002 (1989).  22.  J.W. Olesik and E.J. Williamsen, Applied Spectroscopy 43: p. 933 (1989).  23.  J.W. Olesik and J.C.F. ifi, Spectrochimica Acta, Part B, 46: p. 869-883 (1991).  24.  G. Horlick and F. Qin. Proceedings of the Federation ofAnalytical Chemistry and Spectroscopy Societies Meeting XVII. 1990. Cleveland, OH:  25.  G. Horlick and N. Furuta, Spectrochimica Acta, Part B, 37(11): p. 999-1008 (1982).  26.  D.G.J. Weir and M.W. Blades, SpectrochimicaActa, Part B, 45(6): p. 615-618 (1990).  27.  A. Savitzky and M.J.E. Golay, Analytical Chemistry 36(8): p. 1627-1639 (1964).  28.  H.H. Madden, Analytical Chemistry, 50(9): p. 1383-1386 (1978).  29.  W.H. and S.A. Teukolsky, Computers in Physics : p. 669-672 (1990).  30.  J.W. Olesik, Analytical Chemistry, 63(1): p. 12A-21A (1991).  31.  J.M. Mermet, Analytica Chimica Acta 250: p. 85-94 (1991).  Chapter 5 The  Spatial  Complexity  of the Solvent Loaded Inductively Coupled Argon Plasma  1. INTRODUCTION  5.1.1. Objectives and Scope The observations in Chapter 3 revealed that the structure and behavior of the solvent loaded ICAP are extremely complex. Chapter 4 then revealed that the parametric behavior of the analyte and background emission is correspondingly complex. Although both chapters provided valuable insight into the workings of the discharge, the full complexity of the discharge remains hidden by spatial and temporal averaging. Moreover, both chapters failed to reveal the physical properties of the discharge. As a result, chapters 3 and 4 leave us with several tasks: We must continue to assess the spatial and temporal complexity of the discharge and then unravel its physical properties. Some might suggest that we proceed directly from here and probe the physical properties. If we were fortunate, we could identify all of the physical mechanisms and processes at work, and assess their significance in determining the analytical behavior of the discharge. Then we could formulate a model that would take all of the significant mechanisms and processes into account. Next, we could use this model to predict the analytical behavior of the discharge. If the model stood up to the experimental tests, we could finally say that we understood how the discharge worked, until further experiments proved us wrong.  The Spatial Complexity...  1 29  But such a strategy ignores the complex behavior of the discharge. Even if all of the mechanisms and processes at work in the discharge were known, it is likely that the discharge would still be too complex (spatially, temporally and parametrically) for us to devise a predictive model that was suitably realistic. Indeed, Feynman points out that it does not take very many simple processes to create a system of incomprehensible complexity [1]. This is especially true if the system happens to be inhomogenous, flowing and out of equilibrium—a system like the solvent loaded ICAP. In reality, then, our ultimate understanding of the discharge may be limited to simply knowing the fundamental mechanisms and processes at work in the discharge. We cannot discover how those processes balance, compete and interact until we have a grasp of the complexity of the discharge. Until then, we can only devise approximate models. Clearly, characterizing, developing and understanding the solvent loaded ICAP is not a simple matter of formulating predictive models and then testing them experimentally. The experimentalist must resort to the experimental strategy outlined in Chapter 1. According to that strategy, it is irrational to probe the physical properties of the discharge without some grasp of its complexity. And one way we can come closer to grasping its full complexity is by surveying the spatial and temporal complexity of the analyte and background emission. That is exactly what this chapter and the next one aim to do. This chapter considers time averaged, spatially resolved maps of analyte and background emission intensity, from both the scientific literature and our own work. The next chapter explores the temporal behavior of analyte and background emission intensity. No attempt was made to measure intensity maps resolved in both space and time.  5.1.2. Literature Review of Spatially Resolved Intensity Profiles In order to determine the most efficient and informative experimental approach towards surveying the spatial complexity, we will now critically review how investigators have already measured spatially resolved intensity profiles of emission from the ICAP We will examine how they sampled their spatial profiles, what procedures they used to reconstruct spatially resolved intensity maps, and how comprehensively they surveyed the operational parameter space. In the  The Spatial Complexity...  130  results section, we will also examine what their results revealed regarding the spatial structure and behavior of analyte and background emission features, particularly the emission from solvent pyrolysis products. The earliest ICAP literature reported vertical and horizontal emission profiles obtained by scanning the image of the discharge over the entrance aperture of a monochromator. Obviously, such a point by point approach would be impractical for an extensive survey of the spatial complexity. Example of this sort of experiment are provided by Boulos et aL [2] and Maessen et a!. [3]. In their work, the surveys were severely limited. For example, Maessen et al. (further details may be found in the literature review of Chapter 4) only surveyed one inner argon flow rate [3]. Moreover, they only sampled the radial coordinate at three viewing heights. Finer sampling of the axial coordinate was restricted to axial profiles (line of sight intensity resolved over a range of viewing heights above the induction coil). We can only speculate that these shortfalls resulted from the difficulty in measuring intensities at one spatial location at a time. In order to facilitate and extend their surveys, ICAP investigators have taken advantage of the roughly stigmatic imaging characteristics of Czerny Turner monochromators. They equipped their monochromators with one dimensional array detectors mounted vertically in the exit focal plane, as shown in Figure 5.1(c). (One dimensional array detectors are simply a row of small detector elements, say 128 to 4096 in a row. They are available as charge coupled devices or photo diodes.) This configuration allowed them to rapidly record either vertical or lateral profiles of ICAP emission for a single wavelength channel. For example, Franklin et al. [4] recorded both laterally and axially resolved profiles of calcium emission from the ICAP and studied the effects concomitant potassium. Blades and Horlick made extensive use of linear photo diode arrays to classify emission lines according to the behavior of their axial profiles [5] and to study the effects of concomitant easily ionizable elements on analyte emission [6]. Furuta and Horlick extended this work by obtaining lateral profiles with a linear array detector, and then radially inverting them [7]. Their work sorted out many of the spatial complexities of analyte emission,  The Spatial Complexity...  13 1  but their spatial sampling intervals were not optimal—more could have been revealed with less data. In all of this work, it was expedient to sample the lateral coordinate with the small interval determined by the size of the pixels on the one dimensional array detector. Such a small sample interval was appropriate for the sharp intensity gradients of the lateral profiles. However, axial profiles from both the literature and Chapter 4 reveal that a sampling interval of 0.5 mm—twenty times the size of a typical array pixel—is more appropriate for sampling the axial coordinate. Evidently, methods employing array detectors could improved by batching or binning detector pixels together in order to sample the emission profiles over larger intervals. The extra data handling capacity could then be used to estimate statistics such as the relative standard deviation. Other ICAP investigators used both photographic emulsions and two dimensional array detectors in combination with monochromatic imaging spectrometers. This configuration allowed them to record complete monochromatic images of the spectrochemical source. Briefly, a monochromatic imaging spectrometer is essentially a slitless monochromators similar to the one depicted in Figure 5.1(a) or Figure 5.1(b). With a wide entrance aperture rather than a slit, the monochromator accepts the entire image. Rather than focusing a wavelength dispersed image of the entrance slit on the exit focal plane, it focuses an entire spectrally resolved image of the spectrochemical source on the exit focal plane, where it can be captured on film or by a charge coupled device (CCD)—a two dimensional array detector. The image thus obtained is known as a image spectrogram.  132  The Spatial Complexity  Source (a) Image Spectrogram  17 Collimator (b)  rang  Camera  Collimating Lens ce  Camera Lens  2D Array Detector  n Optics (c)  Source  Linear Array Detector  Source  Lens with Interference Filter  Focal Plane  Figure 5.1.1 Four possible optical configurations for capturing monochromatic images of a spectrochemical source.  The Spatial Complexity...  1 33  Of course, not all emission signals can be imaged this way. The emission signal must be very narrow in the wavelength domain as well as being spectrally removed (not just resolved) from other features. Molecular bandheads will smear out the image while images of closely spaced lines will overlap with each other. The best emission signals for image spectrograms are narrow emission lines spectrally removed from any other emission feature. The spectral survey presented in Chapter 3 reveals that many emission signals from the ICAP meet this requirement. Good examples of image spectrograms have been presented by Horlick and Furuta [8]. Their photographic records qualitatively reveal the spatial distinction between the hard and soft parametric behavior for emission lines from an ICAP. All the imaging problems resulting from spectral dispersion may be avoided by recording monochromatic images with camera equipped with an interference filter. Of course, a separate filter would be required for each spectral feature. This configuration is depicted in Figure 5.1(d). It is interesting to ponder how an imaging interferometer could be devised. Dittrich and Niebergall made extensive use of imaging spectrometers and cameras equipped with interference filters to record monochromatic images of high voltage sparks, d.c arcs, d.c. plasma jets and laser induced plasma plumes, all in addition to inductively coupled argon plasmas [9]. Their work should be consulted, if not to gain a wealth of insight into spectrochemical sources, then to appreciate the aesthetic beauty of their images. Essentially, they captured the monochromatic images on photographic film. The film was either mounted in the exit focal plane of an imaging spectrometer, or held in a miniature camera equipped with an interference filter. They analyzed the images by converting the blackening density of the photographic images into equidensigrams. In other word, they obtained contour maps of the image intensity. This allowed them to subtract the spectral background, and in some cases, radially invert the images. They also demonstrated that imaging spectrometers can capture time resolved images when used with high speed emulsions. But they admit that the photographic techniques involved in equidensitometry can be very laborious.  The Spatial Complexity...  134  An alternative configuration for an imaging spectrometer has been described by Hieftje and is depicted in Figure 5.1(b) [10]. Unlike the configuration used by Horlick and Furuta or Dittrich and Niebergall, this one does not focus an image of the spectrochemical source onto the entrance plane of the monochromator. On the contrary, it sends collimated light through the monochromator. The collimated beam may be stopped down at the entrance aperture so that a very narrow spectral bandwidth may be selected at the exit aperture. As a result, the spectral bandpass of the image is determined not by the size of the image, but by the dimensions of the entrance and exit apertures. Moreover, spectral dispersion does not smear out the image in this configuration. After the collimated, spectrally resolved beam leaves the exit aperture, the spatial information is then retrieved by focusing the beam onto a two dimensional detector such as a CCD array. Better spectral and spatial resolution may be thus obtained at the expense of light throughput and the number of illuminated rulings on the diffraction grating. Admittedly, fewer illuminated rulings widen the spectral bandpass, but not nearly to the extent of a complete image in the exit focal plane. Moreover, decreasing the light throughput does not present a problem for capturing monochromatic images from a light source as bright as the ICAP, especially with CCD detectors available today. Clearly, the advantages of this configuration warrant its consideration over the one depicted in Figure 5.1.(a). Figure 5.1(c). depicts a third configuration in which a thin, spectrally resolved, vertical slice of the image is accepted by the entrance slit. The spectrally resolved slice is then focused onto the exit focal plane. This solves the problem of spectrally smeared or overlapping images without sacrificing the light throughput or the number of illuminated rulings on the grating. But the entire image must be acquired slice by slice by scanning over its lateral coordinate. 5.1.3. Obtaining Spatially Resolved Maps from Line of Sight Images All of the optical configurations depicted in Figure 5.1 enable the experimentalist to acquire monochromatic images of a spectrochemical source. Such images are informative, yet because they are averaged over the line of sight, they are only resolved over two spatial dimensions rather than three. This leaves the structure of the spectrochemical source obscured.  The Spatial Complexity...  1 35  Fortunately, two properties of emission from the ICAP allow three dimensionally resolved information to be retrieved from line of sight images. First, many of the interesting emission features from the ICAP are optically thin, so line of sight images may be regarded as line integrals of emission across the source. In other words, a lateral profile of emission from the ICAP is really a set of line integrals of emission over parallel chords through the discharge. Second, the ICAP is almost cylindrically symmetric. For a cylindrically symmetric body, the value of a function along the radius can be determined from the line integrals across a set of parallel chords. This may be accomplished by radial or Abel inversion [10]. Hieftje has compared Abel inversion to peeling off successive layers of an onion [10]. The outermost line of sight intensity allows the contribution from the outermost annulus to be subtracted from each lateral location towards the centre. This is akin to peeling off the outer layer of the onion. Once the outermost layer has been peeled off, then successive layers can be stripped away in the same way until the intensity at each annulus has been determined down to the centre. The intensity at each annulus down to the centre, or the radially resolved intensity, provides complete spatial resolution for a cylindrically symmetric source. But experimental evidence shows that the ICAP is not cylindrically symmetric. On the contrary, it has some degree of bilateral asymmetry. In order to account for this, experimentalists have resorted to two other numerical methods for retrieving three dimensional information from line of sight images: asymmetric Abel inversion [11] and tomographic reconstruction [10]. Asymmetric Abel inversion first calculates asymmetry factors across the lateral profile. These factors are simply the ratio of intensity between points on the left and right of the centre of the lateral profile. A set of asymmetry factors is calculated for pairs of lateral positions from the centre out to the edge of the profile. Next, the average of the of the left and right sides of the lateral profile is radially inverted. Finally, multiplying or dividing the average radial profile by the set of asymmetry factors gives the asymmetric left and right sides of the radial profile. As one might expect, the effectiveness of this technique is limited to small degrees of bilateral asymmetry. In order to account for further asymmetry, the experimentalist may resort to tomographic reconstruction. This method assumes no axes of symmetry at all. It requires,  The Spatial Complexity...  1 36  however, much more intensity information than Abel inversion. Line of sight images must be captured from several different viewing angles in order to apply tomographic reconstruction. Fortunately, Hieftje et al. have already applied this technique to the ICAP, and have noted that while the ICAP is far from being cylindrically symmetric, the results of asymmetric Abel inversion compare favorably with those of tomographic reconstruction [10]. Apparently, asymmetric Abel inversion is adequate for retrieving spatial information from line of sight images of the ICAP. 5.1.4. Temporal Resolution All of the techniques discussed so far yield spatially resolved information, but generally fail to provide temporal resolution. Temporal resolution requires short exposure times, such as those available to gated, intensified detectors, or high speed film. An example of such technology applied to the ICAP has been provided by Olesik et al. [121. They synchronized their gated, intensified array detector to events owing to droplet vaporization. It may also be informative to phase average the experiment, as suggested by Winge et al. [13], and synchronize a shutter or optical chopper to periodic fluctuations of discharge intensity. Such experiments would yield to phase averaged rather than temporally averaged intensities. 5.1.5. The Imaging Technique Used in This Work The practical objective of this work was to survey temporally averaged, yet spatially resolved maps of analyte and background intensity.  We could not do this using the  configurations depicted in Figures 5.1.(a). or 5.l.(b). because we did not have a two dimensional array detector. Alternatively, the techniques required to radially invert photographic images were impractically complex and laborious. Moreover, two of the spectral features we planned to investigate, the CN bandhead at 388 nm and the 2 C bandhead at 516 nm, displayed very broad spectral structures which would have corrupted the spectrally resolved image. Perhaps the configuration depicted in Figure 5.1.(c). could have been used. This avenue was not pursued. We rejected the point by point method out of hand for obvious reasons. We also rejected the  The Spatial Complexity...  137  option of acquiring vertical and horizontal profiles by recording the array output directly: In the examples from the literature, the array oversampled vertical profiles excessively and did not have the flexibility to sample horizontal profiles of different widths (one would have had to change the magnification of the imaging optics or stretch the array). It also turned  Out to  be quite laborious to  sample the entire discharge in this manner. We sought a method which could circumvent all of the above shortcomings and survey spatial intensity profiles of the entire discharge with optimal efficiency. Ultimately, we batched the output from an array mounted vertically, so as not to oversample in the vertical direction, and scanned the image of the discharge across the entrance slit in order to appropriately sample the lateral coordinate. In this manner, we could record profiles of at least half of the discharge quite rapidly (in about three minutes). Moreover, the entire images were of a manageable size and the individual lateral profiles at each observation height could be radially inverted quite readily using an asymmetric Abel inversion procedure. 5.1.6. Chapter Summary This chapter surveys the spatial complexity of the solvent loaded ICAP. It presents spatially resolved maps of both analyte and background emission. Several physical phenomena are evident in the maps, notably air entrainment into the argon jet by vortex shedding, entrainment of solvent material into the outer argon stream by a recirculation eddy at the base of the discharge, and shrinking of the induction region by a thermal pinch effect. Although these phenomena are evident in the results, they are obscured by time averaging and the complexity of the discharge. Nevertheless, the results provide several guidelines for further research. They also suggest at least three ways to improve the design and methodology of ICAP-AES, 1. by using background signals to monitor the solvent plasma load, 2. by eliminating air entrainment, and 3. by using the wealth of information in spatially resolved emission maps to optimize the operating parameters.  The Spatial Complexity...  138  5.2. EXPERIMENTAL The instrumentation and procedure for igniting the plasma, generating the solvent aerosol, preparing solutions, controlling the operational parameters, and measuring spectroscopic quantities were presented in Chapter 2. Only the modifications and procedures specific to surveying the spatially resolved parametric response of the solvent loaded ICAP are presented here. In order to adequately sample the emission profile and to meet the requirements of the numerical Abel inversion procedure, up to 200 intensity samples were recorded along the lateral coordinate of the discharge at each observation height. So the spatial maps required up to two hundred times more samples of emission intensity than the axial profiles presented in the previous chapter. Further complicating the task, the optical train used to collect light from the discharge had to be stopped down in order to meet Abel inversion requirements. With this decrease in light throughput, the detector required longer integration times to provide signal to noise ratios that were limited by source noise rather than detector noise. With such large volumes of data and long measurement times, it was unrealistic to explore the sampling statistics and parametric response of the spatial emission maps to the extent that they were surveyed in Chapter 4. For practical reasons, then, the parameter space of the survey was less extensive than the one in Chapter 4, and only enough replicate measurements were made to ensure reproducibility and repeatability—not enough to reliably estimate the sampling statistics. 5.2.1. Procedure for Recording a Monochromatic Image The discharge was translated laterally across the axis of the light collection optics, so that the image of the discharge was translated laterally across the entrance slit of the monochromator. Because the monochromator was equipped with a vertical array detector, several lateral profiles— each at different observation heights—could be scanned simultaneously. As a result, an entire monochromatic image of the discharge could be recorded in a single lateral scan. The bandpass of the monochromatic image was determined by the width of the array detector and the reciprocal  The Spatial Complexity...  139  linear dispersion of the monochromator. For example, the width of the detector of 0.506 mm multiplied by the reciprocal linear dispersion of the monochromator at 516 nm (0.76 nmlmm) gave a spectral bandpass of 0.38 nm. Note that the nominal reciprocal linear dispersion for the monochromator is 0.83 nm/mm, but this value is defined by convention for a wavelength of 0 nm. With increasing wavelength, the linear dispersion increases according to a cosine function. At 516 nm, one may calculate a value of 0.76 nmlmm for the reciprocal linear dispersion. This was verified experimentally. The bandpass of 0.38 nm could be reduced by placing a slit mask on top of the detector, but this was found to be unnecessary. The sampling interval along the detector corresponded to 0.6 mm intervals along the axis of the discharge. The total axial range was 25 mm. This range extended from either 5 mm to 30 mm above the top of the load coil (the tail cone), or -15 mm to 10 mm (the induction region). In the lateral direction, the sampling interval was variable, but was generally set at 0.045 mm or 0.09 mm. In all cases, 200 lateral positions were sampled, giving a lateral range of ±4.5 mm or ±9.0 mm. This oversampled the lateral profile, but not excessively. In fact, it reduced the numerical accumulation of noise towards the centre of the radial profiles. In summary, monochromatic images of either the tail cone or induction region of the discharge were recorded with a bandpass of approximately 0.4 nm, a lateral resolution of 0.09 mm and a vertical resolution of 0.6 mm. Moreover, images of the foreground, background, and dark signal could be recorded for background subtraction. 5.2.2. Speciflcations of the Optical Train In order to generate radially resolved maps from line of sight images, the light collection envelope for each point in the image had to approximate a line integral. The optical train met this requirement as follows: A plano convex lens (see Table 2.2) was fitted with one of three apertures (5 mm diameter, 10 mm diameter or 5 mm wide x 30 mm high), and focused a 0.5X image of the discharge onto the entrance slit of a 1 meter focal length Czerny-Turner monochromator. At 516. nm, the object distance was 450 mm (from the discharge axis to the front principal point of the lens), while the image distance was 225 mm (from the back principal point of the lens to the entrance slit). The Lens Maker’s Equation for thick lenses was used to calculate image and object  The Spatial Complexity...  140  distances at other wavelengths [14]. Of course, the planar surface of the lens faced towards the entrance slit in order to minimize spherical aberration. Finally, the entrance slit was lined up with the cylindrical axis of the discharge. In practice, the cylindrical axis of the discharge veered off to one side with increasing observation heights, but this veering could be removed from the image prior to radial inversion. The spatial response of the monochromator along the entrance slit was determined experimentally. It was found to be slight over the 25 mm imaging range (± 6.25 mm along the entrance slit) and was not taken into account. Maps of the absolute light collection efficiency of the optical system were calculated using an exact ray tracing algorithm developed by Farnsworth et al. [15]. Beyond providing the light collecting efficiency of the optical train, these maps confirmed that the light collection optics met the requirements for radial inversion. The spectral response function of the detector was also determined using an irradiance standard lamp [16]. Knowing both the detector response and the light collection efficiency made it possible to calculate radially resolved maps of absolute emission intensity. In this chapter, however, only normalized intensities are presented. 5.2.3. Image Processing The monochromatic images were corrected for background or dark signal contributions by subtracting the background or dark signal images. Next, the images were smoothed with a digital filter in both the axial and lateral directions [17], [18], [19]. Because the axis of the discharge reproducibly veered to one side, the veering could be removed by shifting the lateral profile at each observation height by a predetermined number of sampling intervals (<6). Once background corrected, smoothed and straightened out, the images were radially inverted. Although oversampling the lateral coordinate improved the numerical radial inversion, 200 points in the radial coordinate were found to be excessive. So after the profiles were inverted, they were condensed from 200 x 40 intensities to 50 x 40 intensities by taking the average of 4 points in the radial direction. This rendered the images much more manageable in later analysis,  The Spatial Complexity...  141  with little or no loss of information. The corrected, smoothed, radially inverted and condensed images were then converted to topographical maps. 5.2.4. Summary of the Spectral Channels In this way, radially resolved maps were recorded for more than ten spectral channels, principally 516.56 nm (C2), 388.34 nm (CN), 247.86 nm (C 1), 257.61 nm (Mn ll), 279.55 nm (Mg Ii), 285.21 nm (Mg I), several Fell lines, An (687.13 nm), and H 1(486.16 nm). Some of these were measured for three solvents, methanol, water, and chloroform, over a range of inner argon flow rates, and a range of forward powers. The most extensively surveyed feature was the Ar I. Absolute intensities of Ar I emission are considered in Chapter 7.  Here, only an  informative sample of background features (CN, C 2 and C I) and analyte emission lines (Mn II, Mg II and Mg I) will be presented.  The Spatial Complexity...  142  5.3. RESULTS AND DISCUSSION 5.3.1. Format of Presentation The spatially resolved intensity maps presented in the following sections may be regarded as emission from the planar slice through the discharge shown in Figure 5.3.1(a) The plane of each map corresponds to the plane in the figure, with most maps bounded by z =5 mm to z =30 mm and r mm to r  =  =  -7 mm to r  =  +7 mm, and a few bounded by z  =  -15 mm to z  =  +10 mm and r  =  -7  +7 mm. Obvious shadows of the induction coil reveal which maps extend into the  load coils (that is. those bounded by z  =  -15 mm to z  =  +10 mm). There is some error in the  radial intensities for the induction region because the turns of the load coil were not horizontal— they were helical. Intensity from the planar slice is represented by topographical isocontours as shown in Figure 5.3.1(b) The isocontours always depict the normalized intensity ranging from 0.1 to 1.0 in increments of 0.1. These isocontours were either normalized to the maximum intensity for individual maps or the overall maximum for a set of maps.  143  The Spatial Complexity  S S a) C) C Ca Cd)  C Ca  S S 0)  15.0 10.0 5.0 .ooo 0 . Radial Distance, mm  Figure 5.3.1 The reference frame and contour intervals for the spatially resolved intensity maps. (a) How the image plane corresponds to a vertical slice through the discharge. (b) Typical contour map of the image intensity with a contour interval of 0. 1X the maximum intensity.  The Spatial Complexity...  144  5.3.2. An Overview of Spatially Resolved Analyte and Background Emission Figure 5.3.2 depicts representative intensity maps of all the analyte and background species considered in this chapter. The maps were measured for a ICAP operated at 1.25 kW, with an inner argon flow rate of 0.81 11mm. and a chloroform load of 4.5 mg/s. From left to right, the first map depicts emission from the CN bandhead at 388.34 nm and second map depicts the C 2 bandhead at 516.56 nm. They reveal the boundary regions of the plasma. The CN map reveals the downstream boundary region known as the plasma mantle. This is where air entrains into the plasma jet. In effect, the CN map reveals the interface between the hot atomic plasma and the cold room air. Within the induction coil, the C 2 map partially reveals the upstream boundary region of the plasma. This region surrounds the base of the plasma and lines the inside of the aerosol channel. The C 2 emission may be regarded as a dissociation front between the hot atomic plasma and the relatively cold mixture of argon and undissociated solvent vapour. It is interesting to note the relative thickness of the C 2 and CN boundaries. In general, the CN boundary was significantly wider than the C 2 boundary. Only the cap of the C 2 region was of comparable thickness to the CN boundary. This may indicate that the temperature gradients were steeper across the C 2 region except at its cap. Steeper temperature gradients would result in a narrower region over which the C 2 molecule was stable. Alternatively, the C 2 cap and the CN boundary may have been broadened by time averaging. Varicose instabilities resulting from vortex shedding may have blurred or broadened the CN boundary while droplet vaporization may have blurred the cap of the C 2 map. Further evidence for time averaging is provided by the extent of overlap between the molecular boundary region and the atomic plasma region. The overlap between the molecular boundary region and the atomic plasma region is greatest where the thickness of the boundary regions is greatest. It turns out that shedding ring vortices, or varicose instabilities in the flowfield, are the most plausible explanation for this broadening and overlap. Varicose instability will be discussed in later sections.  Radial Distance, mm  C.  2 C  C) C  -5  -15  Cu  -lOö  Cl)  E  o  S  CI  c  Radial Distance, mm  Mn II  Mg  Mg  q  00  5  10  ,.,  1  20  25  30  Figure 5.3.2 An overview of spatially resolved maps of analyte and background emission from the solvent loaded ICAP.  CN  M9il Mg I  .  0  C Cu 0  E  S a 0  LM  The Spatial Complexity...  146  The next three maps in Figure 5.3.2 depict Ar I and C I emission. The C I and Ar I emission maps define the volume of the atomic plasma (where atomic emission predominates) and reveal how it nests within the molecular boundary region (where molecular emission predominates). Two views of the atomic plasma are provided, within the induction coil and above the torch rim. Within the induction coil, the C I isocontour reveals the toroidal induction region of the plasma. Above the torch rim, both Ar I and C I maps reveal that the toroidal induction region coalesces into a tail cone. As mentioned earlier, the isocontours of atomic emission overlap with the boundary layer emission to a greater extent downstream from the torch rim than within the induction coil. It is possible that the temperature gradient downstream is more gradual, but more likely that the downstream boundary is time averaged. At any rate, the dimensions of the atomic region, the overlap between the atomic and boundary region, and how these spatial features influence analyte emission all deserve special attention. The three narrow maps on the right side of Figure 5.3.2 reveal the intensity distribution of analyte emission. The first two maps reveal the plumes of emission typically displayed by hard lines (atomic ion lines or atom lines with excitation potentials  >  6 eV). Shown are the intensity  distributions of Mn 11(257.6 nm) and Mg II (279.5 nm). The upstream base of the hard line plume begins at approximately 5 mm to 10 mm above the top of the induction coil. From this location, the plume extends 15 to 20 mm downstream, up to 25 mm above the induction coil. Typically, the hard line plume varied in length, width and intensity when one varied the operating parameters. In spite of this, the plume retained its distinctive oval shape with no obvious tapering or constrictions upstream or downstream. The oval plume always displayed an unambiguous axial maximum residing well downstream at 10 to 15 mm above the induction coil. The plume extended radially to approximately 3 mm. In general, this maximum radius was found at approximately 15 mm above the induction coil. In contrast, the third narrow map reveals the structure displayed by soft lines (atom lines with excitation potentials <<6 eV). This map depicts the spatial distribution of emission from the Mg I (285.2 nm) line, an atom line with an excitation energy of 4.35 eV. The most conspicuous  The Spatial Complexity...  147  differences between this soft line structure and the hard line plumes to its left are the relative positions of their maxima and their characteristic shapes. The maxima for the soft line structure lies significantly further upstream, or at a lower height above the induction coil than the hard line maxima which reside above 10 mm above the induction coil. The bases of the hard line plumes also lie well downstream so that the plumes display a characteristic oval shape. In contrast, the base of the soft line plume appears to stretch out into a narrow haft which extends into the torch, giving it the characteristic club shape of spatially resolved soft line emission. The last, rightmost frame of Figure 5.3.2 shows a map of the ratio of Mg 11(279.55 nm) and Mg 1(285.21 nm) line emission. It reveals the characteristic trough in the thermal conditions along the axis of the discharge. The trough is most pronounced upstream and disappears further downstream. This reflects the decay of thermal gradients in the plasma downstream from where the solvent material has completely dissociated. Chapter 4 discusses how this ratio reveals how thermal conditions in the plasma approach local thermal equilibrium. Discussion of how the ratio indicates the robustness of the plasma may also be found in Chapter 4. The spatial maps presented in this chapter extend our understanding of how robust the plasma is, and how it approaches local thermal equilibrium. Incidentally, Figure 5.3.2 clearly shows the value of recording complete spatial maps of intensity from the analyte plume, the atomic plasma and the plasma boundary region. Readers can immediately see both the boundaries and overlap between the different regions of the discharge. If all researchers reported maps like this, then comparing results from different laboratories would be greatly facilitated.  The Spatial Complexity...  148  5.3.3 Comparison with Emission Maps Reported in the Literature Figure 5.3.3. presents Maessen and Kreuning’s diagram of the general emission distribution from pyrolysis products in the ICAP loaded with organic solvent [3]. Clearly, the results presented in Figure 5.3.2 are consistent with those of Maessen and Kreuning, who arrived at their diagram after recording more than 100 radial profiles of CN, C I and C 2 emission over a range of observation heights. The structure of their CN, C I and C 2 regions are all consistent with those in Figure 5.3.2., except that much of their spatial detail appears to have been obscured.  Figure 5.3.4. reproduces Dittrich, Brauer and Niebergall’s diagram for a water loaded ICAP derived from equidensigrams of Ar I (360.6 nm), N2+ (391.4 nm) and Cu I (324.8 nm) emission [9]. Significantly, these were equidensigrams of line of sight intensity, not radially inverted intensity. In fact, Dittrich et al. radially inverted only a limited sample of their intensity data. Even so, Figure 5.3.4. extends our view to ICAPs loaded with aqueous solvent, revealing many similarities between the effects of organic and aqueous solvent loading. It also provides a reliable basis for interlaboratory comparison, because it reveals the location of boundary regions, atomic plasma regions as well as analyte plumes.  Spatial Complexity...  (a)  149  Chloroform, Solvent Load = 15 pmol/s 10-  10-  10 ‘  C 5 mm , 2 above coil  CN, ‘20mm ; above I coil -  S  ..  I  ‘ C,5mm ‘ above S coil  ‘  5 I  44 -  •0  00  .  10  0• 10 0  0• 10 0  I  •_‘I  I  -  10  Toluene, Carbon Load =50 iimol/s 1010 :  above coil  I I I  0- \,5mm I 0• 0 10 0  CN, ‘.2Omm ‘-a bove coiI  ,  C,5mm above ‘ coil  .  I ‘ -I  ..I  I  ‘4.  0. 10 0  s—I  —  10  Radial Distance (mm) —s  (b)  I  30  c  0  CCN  20  .4’  I  I  ill  I  a ill •‘IiI •  o 10  ‘I’ I I 1  -10  f•  i‘ci £  -0  I  I  0  10  Radial Distance (mm) Figure 5.3.3 Maessen and Kreuning’s spatially resolved profiles of emission from solvent pyrolysis products. (a) A sample of their radial profiles of C2, C I and CN emission. (b) Their general spatial distribution based on more than one hundred profiles similar to those shown in (a). These figures are roughly adapted from the figures in Ref. [3].  Spatial Complexity...  150  (a) 30  20  10 -10  0  10  -10  0  10  10  Distance from Axis (mm)  (b)  Figure 5.3.4 A summary of Dittrich, Niebergall and Brauer’s equidensitometry results. + emission. (b) An overlay of the lowest intensity 2 (a) equidensigrams of Ar I, Cu I and N equidensites from (a).  151  The Spatial Complexity  0 0 0  0 0 0 (a)  (b)  (c)  Figure 5.3.5 The characteristic structure of emission plumes for (a) soft, (b) intermediate, and (c) hard line emission from the inductively coupled argon plasma. The bold line indicates the outer boundary of the plasma. -  The Spatial Complexity...  152  The spatial structure of analyte emission plumes revealed Dittrich et aL [9], Horlick et at. [8], [6]and Franklin et al. [4] is summarized in Figure 5.3.5. Soft lines, or atomic lines with excitation potentials much less than 6 eV, display a conical plume low in the discharge (Figure  5.3.5.(a).). This cone resides close to the inner boundary between the aerosol channel and the plasma. The inner plume may be surrounded by an outer halo, similar to those revealed by Ditthch et at. Evidence for this outer halo has also been provided by Franklin et at. [4] and by profiles of Fe I emission in Chapter 9. The outer halo appears as an outer wing on lateral emission profiles. In contrast, the image spectrograms and lateral profiles reported by Horlick et at. reveal that outer halos may not always be present [8]. This indicates that the outer halo depends on the individual experimental setup. Dittrich et at. have suggested that air entrainment, which depends on the torch geometry and ventilation of the room air, may be responsible for such large discrepancies between experimental results [9]. In contrast to the soft line plumes, the hard line plumes depicted in Figure 5.3.5.(c). occupy the hot apex of the tail cone. Intermediate line display intermediate plumes (Figure 5.3.5.(b).). The literature is replete with much more spatial information regarding the emission from the ICAP, although no reports are as comprehensive or detailed as this report, or those by Dittrich et at. [9], Horlick et at. [8] and Maessen et al. [3].  The Spatial Complexity...  153  5.3.4. The Parametric Response of Emission Maps  The Parametric Response of Diatomic Background Emission: The Response of Emission from the Plasma Boundary Regions  The eighteen spatially resolved maps of emission from the CN bandhead at 388.34 nm in Figure 5.3.6 reveal how CN emission responds to chloroform load and forward power. Note the two distinctive structural features of CN emission, the central plume and the conical outer mantle: The central plume is absent under conditions of high power and low solvent load (in the lower left hand plots) but increases in height and extends upward to meet the outer mantle with increasing load and decreasing power (in the upper right hand plots). On the other hand, the outer mantle (which caps the atomic plasma or resides at the boundary between the plasma and the surrounding air) appears to collapse inward and downward with decreasing power and increasing solvent load. Also note the intensity response of the CN emission: From left to right, or with increasing chloroform load, the CN intensity of the mantle increases almost linearly with chloroform load at all locations. The response of CN intensity to power is more ambiguous (power increases from the top row plots to the bottom row plots). Although the CN intensity increases with forward power at specific locations (for example, z  10 mm and r  =  6 mm), a  linear increase at all locations is not evident. Indeed, at some locations the CN intensity appears to decrease with forward power.  4.2 mg/s  6.2 mg/s 7.4 mg/s 8.6 mg/s  5.0  10.0  15.0  20.0  25.0  30.0  Radial Distance, mm  otcO  10.0 mg/s  •  Figure 5.3.6 Isocontour maps of CN (388.34 nm) emission intensity for a chloroform loaded ICAP. The inner argon flow rate was 0.81 11mm. The contour interval is 0.1X the maximum intensity for the entire set of eighteen maps. Evident in the maps are an outer, conical mantle and an inner plume.  1.50kW  1.25 kW  1.00  3.4 mg/s  The Spatial Complexity...  155  In order to reveal the response of CN intensity to chloroform loading and forward power, one may first remove the spatial confounds by integrating the intensity over all space. This is essentially what Browner et a!. have done [20]. Alternatively, one may choose a spatial location where the structure of the plasma remains fairly constant even when the forward power and chloroform load are varied. On the axis and beyond the tip of the plasma(z  =  30.0 mm, r = 0.0  mm), the intensity response appears to be largely determined by how far the atomic plasma extends downstream. Here the effects of power and solvent load are severely confounded by the spatial response. Similarly, at a position on the axis and at an intermediate observation height (z 15.0 mm, r  =  =  0.0 mm), the intensity is also determined by the spatial response. Here it is  determined by how far the inner plume extends downstream, and whether or not the plume extends past z  =  15.0 mm. In contrast, the structure of CN emission near z  =  15.0 mm and r  -5.2 mm appears to be relatively independent of power and solvent load. Because the structure here appears to be constant, this is a good location to examine the response of CN intensity to forward power and solvent plasma load. At 1.5 kW, the intensity increases almost linearly with solvent load, indicating that excitation conditions in the CN mantle are constant and that the amount of solvent material determines the CN intensity. At lower powers, the CN response departs from direct proportionality and displays a maximum at intermediate loading. Evidently, high levels of solvent plasma load sap enough energy from the discharge to lower the CN emission intensity. Moreover, it is likely that vortex shedding entrains air into the argon stream, effectively mixing N 2 with the argon and solvent material, so that the solvent carbon combines effectively with the atmospheric nitrogen, and the thermal conditions in the boundary region are intermediate between the plasma and the air—according to the respective gas temperatures and heat capacities. Further discussion of the mechanism behind this response is reserved for later sections. Note, however, that one can optimize the CN signal as a working diagnostic by selecting the right viewing location. z= 15.0 mm and r = -5.2 mm appears to be the best place to monitoring solvent plasma during routine analysis. A similar analysis can be applied to C I and C 2 emission maps.  The Spatial Complexity...  156  In contrast to CN emission maps, C 2 emission maps reveal the upstream boundary of the discharge. This boundary is characterized by solvent pyrolysis and a steady recirculation eddy rather than air entrainment and vortex shedding. Both processes are evident in the C 2 emission maps presented in Figure 5.3.7. This figure depicts C 2 emission within the torch for an ICAP loaded with meta-xylene. For these maps, the meta-xylene load was 0.2 mg/s. the forward power was set at 1.25 kW, and the inner argon flow rate was varied from 0.6 to 1.1 Jlmin in 0.1 1/mm increments. The inner argon flow rate increases from left to right. At low inner argon flow rates, the C 2 emission wraps around the base of the plasma while the central plume of C 2 emission only extends a short distance along the axis. It is unlikely that diffusion could account for the C 2 emission around the base of the discharge. On the contrary, a recirculation eddy near the base of the discharge, predicted by computer simulations, could account for convective mixing of solvent material with the outer argon stream. (This recirculation eddy is illustrated in Figure 1.3.) Evidently, such a recirculation eddy entrains solvent material quite effectively into the outer argon stream at low inner argon flow rates, thus reducing the load on the axial channel. However, when the inner argon flow rate is increased, the eddy is less effective at entraining solvent material. Interestingly, computer simulations predict that extremely high inner argon flow rates, the inner argon stream may actually sweep the recirculation eddy away. Consequently, the load on the axial channel increases, and the central plume extends farther downstream, while the outer C 2 emission intensity decreases. Moreover, the entire profile settles down into the torch. This indicates that the plasma translates axially when the outer argon flow is loaded with solvent material. C I emission maps provide further insight into this apparent translation.  0.71/mm 0.81/mm  I 1 Ip  0.91/mm  -  IDII  1.01/mm  -  -18.0  -13.0  -8.0  -o.v  2.0  7.0  Radial Distance, mm  0.0.000  loIt  1.1 I/rimn  C) C) C  Figure 5.3.7 Isocontour maps of C 2 (516.56 nm) emission intensity for a meta-xylene loaded ICAP. The inner argon flow rate was varied from 0.61 11mm to 1.11 11mm in 0.10 1/mm increments. The contour interval is 0.1X the maximum intensity for the entire set of six maps. Evident in this set of maps is the distribution of solvent material over the argon stream.  L: :  0.61/mm  L1  The Spatial Complexity...  158  Maps of C 2 emission from an ICAP loaded with other solvents at different rates of solvent plasma load are all consistent with the behavior depicted in Figure 5.3.7. In response to methanol loading, the peripheral component of C 2 emission was more intense than the central plume, and the central plume extended a shorter distance downstream. Evidently, the inner argon stream laden with methanol vapour could not penetrate the recirculation eddy as effectively as an inner stream laden with heavier solvent molecules, because the methanol laden stream would have had less momentum. Indeed, chloroform loading displayed the opposite response. In summary, two features of the flowfield are evident in the parametric response of C 2 and CN emission maps, vortex shedding associated with air entrainment, and a recirculation eddy at the base of the discharge which entrains solvent material into the outer argon stream. The extent to which solvent material is entrained depends on the flow properties of the inner stream. In addition, the C 2 and CN emission maps provide guidelines for using C 2 and CN signals as control diagnostics during routine anaiysis.  5.3.4 The Parametric Response ofAtomic Background Emission Contrasting quite sharply with the CN results is the response of spatially resolved emission from atomic carbon (from the C I line at 247.9 nm). The fifteen maps shown in Figure 5.3.8 reveal how C I emission responds. Once again, the top row corresponds to the lowest power of 1.00 kW, the middle row to the intermediate power of 1.25 kW, and the bottom row to the highest power investigated of 1.50 kW. From left to right the chloroform load increases from X mg/s to X mgls, so that the rightmost 15 maps of CN emission correspond precisely with those shown here.  d’o  Radial Distance, mm  OD  qp000  5.0  10.0  15.0  20.0  25.0  30.0  Figure 5.3.8 Isocontour maps of C I (247.61 nm) emission intensity for a chloroform loaded ICAP. The inner argon flow rate was 0.81 1/mm. The contour interval is 0.1X the maximum intensity for the entire set of +jfteen maps. Evident in each map is the toroidal remnant of the induction region that coalesces into a cone.  1.50kW  1.25 kW  1.00kW  I  The Spatial Complexity...  160  In contrast to the outer mantle and inner plume of CN emission, the spatial distribution of C I emission fills in the conical void underneath the CN mantle and around the inner plume: It assumes the shape of a toroid just above the rim of the torch and coalesces downstream into a cone. This shape characteristic of the plasma region where atomic line emission predominates over molecular emission, indicating that the C I emission emanates from hot, atomic plasma and not from the molecular, flame like conditions of the boundary regions. Nevertheless, it is important to note that the spatial distribution of C I emission overlaps with the spatial distribution of CN emission. In other words, it does not fit perfectly into the conical void. Again, one may turn to the complex dynamic processes at work in the tail flame to explain this overlap. It is clear that a steady state explanation would be inadequate. Of course, there could be gradual, steady state transitions between molecular and dissociated species, with the transition described by equilibrium concentrations, but the dynamic processes discussed above most not be ignored. The gradients in these C I maps qualitatively reveal how energy is transported from the outer regions of the plasma into the axial channel. It is not appropriate to describe these time averaged gradients in detail, except to note that the gradients decay downstream from the point where the solvent material has completely dissociated. Comparing the inner plume of CN emission with the C I emission nicely illustrates this point.  The Spatial Complexity...  161  Figure 5.3.9 depicts C I emission from the induction region of the ICAP. The four maps on the left reveal the response of C I emission to inner argon flow rate and methanol load, while the four on the right reveal the response to inner argon flow rate and chloroform load. In each set of four maps, the inner argon flow rate increases from top to bottom, while the solvent plasma load increases from left to right, and the isocontours are normalized to the overall maximum intensity. The maps reveal that the volume containing the C I emission shrinks in the axial direction in response to an increase in solvent load or decrease in inner argon flow rate. In fact, the volume shrinks in the axial direction by 10 mm when the methanol load is increased and the inner argon flow rate is decreased. Observations reported in Chapter 3 indicate that the C I emission volume corresponds closely to the plasma volume, so the redistribution of carbon cannot account for the response. Moreover, Figure 5.3.8 reveals that vertical translation cannot account for this response. Electron density measurements presented in Chapter 8 reveal that an enormous increase in electron number density accompanies the axial shrinking. For methanol loading, the electron density between the top and middle turns of the induction coil increases from 8.0 x 1015 cm 3  to 1.3 x 1016 cm 3  .  Evidently, the maps in Figure 5.3.9 reveal a thermal  pinch effect. The effect is less conspicuous for chloroform loading because chloroform is less effectively entrained into outer argon stream, for reasons discussed earlier. Alternatively, the bond dissociation enthalpy of the C—O bond in methanol may contribute much more significantly to the thermal pinch effect than C—Cl or C—H bonds. In summary, plasma decay, time averaging of the plasma boundary and the thermal pinch effect are all evident in the parametric response of C I emission maps.  1.0 mg/s methanol 3.2 mg/s CHCI 3  Q  Radial Distance, mm  cOO  Ii” :  -15.0  -10.0  5.0  10.0 E E  Figure 53.9 Isocontour maps of C I (247.61 nm) emission intensity within the induction region. The four maps on the left depict the response to methanol load and inner argon flow rate. The four maps on the right depict the response to chloroform load and inner argon flow rate. The isocontours in each set of four have been normalized to their respective maxima—the contour interval is 0. lx the maximum for each set. Evident in these maps are axial and radial shrinking of the induction region.  (OQ 0  0.3 mg/s methanol  LJ  The Spatial Complexity...  163  5.3.5 The Parametric Response ofAnalyte Emission Maps In general, two distinctive responses were observed for maps of analyte emission. One response was observed for hard lines, or atom lines with excitation potentials  >  6 eV and all  atomic ion lines. This response reduces to the hard line behavior discussed in chapter four when spatially averaged by line of sight optics. The other response was observed for soft lines, or atom lines with excitation potentials <<6 eV. The response of hard line emission Figure 5.3.10 reveals how hard line emission responds spatially to solvent load (Mg II 279.55 nm). At the lowest attainable solvent load depicted in the left hand frame, the hard line plume displays the lowest intensity, presumably because the low condenser temperature required to trap the solvent vapour has also lead to significant sample loss. The entire plume and its intensity maximum also sit the furthest upstream at the lowest solvent load. With an increase in solvent load, shown in the second frame, the overall intensity increases significantly, presumably because the condenser no longer traps a significant amount of analyte. The intensity maximum also moves marginally upstream, whereas the plume lengthens significantly so that its tip resides at 25 mm above the top of the induction coil. A further increase in solvent load results in the plume shown in the third frame. With this increase in solvent load, the top of the plume extends no further than previously, and the intensity maximum only moves marginally upstream by perhaps 1.5 mm. On the other hand, the base of the plume moves downstream by 3 mm and clear of the torch, while The overall intensity of the plume decreases significantly. In addition to these major changes, the 0.1 isocontour has become narrower, possibly indicating that the analyte is confined closer to the axis. (plot all four with individual normalization) Further increasing the solvent load to the maximum tolerable load (in this case, for chloroform) results in the plume shown in the right hand frame. With this increase in solvent load, the plume continues to decrease in intensity and become narrower, while the downstream tip of the plume once again extends no further than 25 mm above the top of the induction coil. It is interesting to note that this downstream limit for the tip of the plume overlaps the downstream limit for the atomic  Radial Distance, mm  3 7.4 mg/s CHCI  0•  a  3 8.6 mg/s CHCI  Figure 5.3.10 The response of a hard line plume (Mg II 279.55 nm) to chloroform plasma load.  3 4.2 mg/s CHCI  q  5.0  10.0  15.0  20.0  25.0  30.0  4-  Cu  a  0  0  The Spatial Complexity...  1 65  plasma, as revealed by the C I maps and by the 0.1 isocontour for the CN maps. This suggests that analyte plumes are time averaged as well. In order to interpret the response of the hard line emission plume to solvent load, several physical processes must be taken into consideration. These physical processes determine the local concentration of atomized or ionized analyte, and the energy available to excite the analyte so that it emits: Relatively far upstream from the plume, the analyte is essentially confined to the aerosol stream because undesolvated particles and droplets must follow the gas stream owing to their minute inertial moments compared to their immense viscous drag. However, once the analyte begins to desolvate and vaporize, it becomes free to diffuse across the stream lines of the plasma flow. It can disperse radially as it flows across the boundary region of the aerosol channel and into the plasma. Once in the plasma, the analyte continues to diffuse across the streamlines. As the analyte is transported upward and out from the axis by convection and diffusion, energy is transported upward and in toward the axis from the toroidal energy loading region by convection, radiation and heat conduction across enormous thermal gradients. As a result, the time averaged density of analyte and the time averaged density of energy available to excite the analyte vary enormously throughout the plasma. To a first approximation, only two properties determine the local emission intensity of hard line species. These are the time averaged density of analyte and the time averaged density of energy available to excite the analyte. Consequently, the maximum in hard line emission resides where the maximum amount of energy is available to excite the hard line species, and where the hard line species has not dispersed appreciably by any mass transport process. By similar reasoning, the limits of the hard line plume reside where the energy available to excite the hard line is cut off, or where the local concentration of hard line species is very low. Accordingly, the upper boundary of the hard line plume coincides with the plasma boundary, the radial limits of the plume are determined by the radial transport of analyte, and the base of the plume is determined by vaporization, atomization, and ionization processes that convert the analyte into hard line species.  The Spatial Complexity...  1 66  The response of soft line emission The contrasting response of a moderately soft line, Mg I (285.21 nm) is depicted in Figure 5.3.11. In contrast to hard line plumes, the soft line plumes depicted here do not extend past the plasma boundary. On the contrary, they appear to be nested within the hard line plumes. In fact, it appears as though the soft line emission occupies a cooler temperature band in the plasma than the hard line plumes, a band that surrounds the hollow, inner boundary. This suggests that the geometry of soft line plumes can be explained in terms of norm temperatures, as Blades et al. have pointed out previously [5]. The norm temperature for an optical transition is simply the temperature at which the emission intensity for that transition displays a maximum. In a thermal plasma, one generally encounters a single maximum for line intensity with increasing plasma temperature because of two competing processes. First, with increasing temperature, electron collisions increasingly populate an excited bound state, for a particular line, according to the Boltzmann function. As a result, the emission intensity for that line increases with temperature. Second, at sufficiently high temperatures, the atomic species begins to ionize into the next ionization stage, thus depopulating the excited state. Alternatively, emitting molecules dissociate with increasing temperature, thus depopulating molecular excited states. Overall, atomic, ionic and diatomic emission from the ICAP can be roughly characterized by a norm temperature (if one ignores dynamic processes).  6.2 mg/s CHCI 3  Radial Distance, mm  0  o  Figure 5.3.11 The response of a soft line plume (Mg I 285.21 nm) to chloroform plasma load.  4.2 m!s CHCI 3  0  0 C  5.0  10.0  15.0  2  20.0 E E a,  25.0  30.0  The Spatial Complexity...  168  As it happens, the electron kinetic temperature in the plasma, the temperature which largely governs electronic excitation of plasma bound states, ranges from approximately 6000 K to 9000 K. In general, the norm temperatures for molecular species fall below this range by approximately 1000 K whilst the norm temperatures for soft lines fall within this range, and the norm temperatures for hard lines exceed this range by approximately 1000 K (see, for example, Dittrich or Olesik). Consequently, molecular emission generally occupies the plasma boundary, soft line emission occupies diffuse temperature bands within the plasma (time averaged bands, of course), and the most intense hard line emission may generally be found where the most plasma energy is available for electronic excitation.  Magnesium Line Intensity Ratios Maps of the ratio of ion line to atom line intensity (Mg II 279.55 nm and Mg I 285.21 nm) are presented in Figure 5.3.12. The reader may verify that they agree with the line of sight, axial profiles presented in Chapter 4. It is apparent in these maps that the cool regions lie close to the inner boundary, and that the excitation environment grows hotter towards the downstream limit, where the maps become discontinuous.  7.4 mg/s CHCI 3  Radial Distance, mm  3 6.2 mg/s CHCI  0  0 0  8.6 mg/s CHCI 3  0  5.0  10.0  15.0  20.0  25.0  30.0  Figure 5.3. 1Z The response of the magnesium line intensity ratio (Mg II 279.55 nm I Mg I 285.21 nm) to chloroform plasma load.  3 4.2 mg/s CHCI  •  ci  U)  0’ C  E E  The Spatial Complexity...  170  5.4. DISCUSSION  5.4.1 Physical Properties Revealed by Emission Intensity Maps  Physical processes including diffusion, heat conduction, radiation, and convection are all evident in the maps presented in the preceding sections, but they are all obscured by their complex interactions with each other and by temporal averaging. For example, convectional processes and huge thermal gradients obscure the diffusion of neutral species and the ambipolar diffusion of electron ion pairs, if diffusion constants or rates of diffusion are to defined at constant temperature. Vortex shedding obscures diffusion beyond the exit of the torch while recirculation of the argon obscures diffusion at the upstream end of the discharge. Moreover, the huge thermal gradients between the induction region and the axial channel obscures diffusion near the axis of the discharge. True, the recirculation eddy and vortex entrainment is not immediately evident in the time averaged maps, but they can be inferred from the results of computer simulations and experimental work reported elsewhere. Indeed, all of the maps were consistent with the presence of a recirculation eddy and vortex shedding, so these convectional features cannot be dismissed. If one accepts the recirculation eddy and the vortex shedding mechanism for air entrainment, one will note that they have profound effects on the physical characteristics of the solvent loaded ICAP. For example, vortex shedding determines the downstream boundary of the plasma. It cuts the plasma off by folding cold room air into the argon stream and thus extinguishing the plasma. As a result, the tail cone of the plasma cannot be regarded as a region where the plasma decays gradually and steadily owing to microscopic processes such as three body recombination or radiative loss. On the contrary, one must consider the possibility of an abrupt, fluctuating, discontinuous limit at the downstream boundary of the plasma, more akin to the bounds of the potential core in a round jet.  The Spatial Complexity...  17 1  The recirculation eddy has similarly profound effects on the discharge. It determines how the solvent load is distributed over the argon stream. Hence, it determines whether the axial channel or the induction region will be heavily loaded with solvent material. The balance between these two extremes determines the temperature and density profile of the plasma gas downstream from the torch. Hence it determines how energy is transported to the analyte. Clearly, both the recirculation eddy and vortex shedding must be taken into consideration because they are central to the solvent load problem. Beyond mass transport by convection and diffusion, the effects of heat conduction and heat capacity are evident in the maps of C I emission from the induction region. Inspection of these maps reveals that the plasma volume shrinks in the axial direction in response to solvent loading. This indicates that the discharge responds to solvent loading with a thermal pinch in the axial direction rather than a simple translation downstream. Although such an effect may be obscured in the C I maps by the mass transport of carbon in the argon stream, the thermal pinch effect is corroborated by observations, comparison with similar effects reported in the literature, and by electron density measurements presented in Chapter 8. Phenomena related to heat conduction are also evident beyond the torch rim. For example, maps of the Mg II to Mg I intensity ratio show that solvent dissociation consumes the energy transported from the outer regions into the axial channel. Much of the energy is thus consumed at the very boundary, so the very center is kept relatively cool. Only once all the solvent material has dissociated does an appreciable amount of energy flow from the outer region into the very center of the axial channel. However, the mechanisms for energy transfer from the outer regions into the channel are not clear, nor obvious. Even so, the maps provide no reason to invoke anything beyond thermal transport or heat conduction. Any further argument about radiation trapping, the transport of metastable species or other nonthermal channels of energy transport must be regarded as speculative—after all, the results presented here are obscured by temporal averaging.  The Spatial Complexity...  172  Beyond heat conduction within the plasma, the intensity of CN, C I and C 2 maps indicate that those emission features may provide effective avenues for radiative loss of energy from the discharge. Although all the emission maps and absolute intensity calibrations provide all the information required to assess the degree of radiative loss by solvent dissociation products, the problem was not pursued in this work. In summary, several physical processes and phenomena are evident in the emission maps presented in this chapter. Moreover, the emission maps begin to unfold the complexity with which these processes interact. But the quantitative physical characteristics of the solvent loaded ICAP remain obscure. Several experiments for removing the ambiguity and obscurity suggest themselves, further demonstrating the value of surveying the spatial complexity of the spectrochemical source. Those experiments will be discussed in the following chapters.  The Spatial Complexity...  173  5.4.2. The Air Entrainment Mechanism Vortex Shedding Vortex shedding has been observed for flame-jets as well as for round jets of cold gas, and is most readily observed when the jets are excited acoustically. Becker and Massaro have reviewed the literature on acoustically excited jets up to 1967 [21]. Their review brought them back to 1858, when Leconte[22j reported that a coal gas flame-jet jumped in response to certain notes from a violoncello and suggested that, “We must look upon all jets as musically inclined.” The jumping flame was just a manifestation of vortex shedding. Soon afterwards, it was found that combustion was inessential for acoustic sensitivity in a jet—round jets of cold gas made visible by smoke particles behaved in a similar manner. Later on, Lord Rayleigh analyzed the instability problem [23], and employed stroboscopic illumination to study it [24]. He found that both varicose and sinuous instabilities could be acoustically excited in round jets. The varicose instabilities took the form of symmetric swelling and constriction of the jets diameter synchronous with the exciting tone while sinuous instabilities took the form of rhythmic undulation or twisting of the jet (vortex evolution is associated with the varicose instability). Varicose instabilities predominate in jets with a flat, or top hat velocity profile with a thin boundary region. Becker and Massaro’s literature review also reveals that vortex shedding and the acoustic sensitivity of jets has been extensively studied in more recent times and that the associated theory has also been developed extensively. But that is beyond the scope of this thesis. Becker and Massaro themselves have presented photographic records and detailed observations of vortex evolution in a round jet [211. Their study focused on an axisymmetric jet of cold gas from a nozzle with a flat flow velocity profile at the nozzle mouth except for a thin boundary layer of laminar flow near the nozzle wall. Their study is very informative, because it spans a wide range of Reynolds numbers for the axisymmetric jet, including the range typically encountered for ICAPs. They divided the complete range of Re  =  600 to 20,000 into eight flow  The Spatial Complexity...  174  regimes. (Incidentally, the ICAP (Re <1450  =  100 to 600 ) resides in the first flow regime of 600  <  Re  [251, [26]. Indeed, displays behavior remarkably similar to Becker and Massaro’s cold  jet in the first regime.) They found that successive vortices shedded off the jet according to a general frequency law (or wave velocity law). In general, -Econstant, 0 U where  (5.1)  f is the vortex shedding frequency, 2 is the wavelength of the varicose disturbance,  and  U m 0 ay be regarded as the centre line velocity of the jet. The constant is approximately 0.5,  50  vortices shed off the jet at approximately half the velocity of the jet stream. Vortex shedding followed this frequency law both in the presence and absence of acoustic excitation. Interestingly, they found that when the vortex shedding was excited by pure acoustic tones, discrete frequency jumps were observed which turned out to be related to the resonant properties of the nozzle tube. (Would similar resonances in the ICAP be dependent upon torch dimensions?) Among other things, they found that varicose instabilities were prevalent for thin boundary layers, whereas sinuous instabilities were prevalent for fully developed laminar flows. One further point worth mentioning is that they observed they transition from vortex shedding to turbulent flow (note that vortices need not be turbulent). In certain flow regimes, successive vortices would collide, then break up into turbulent eddies. In other flow regimes, the onset of turbulent flow resided upstream from the tip of the potential core. Which flow regime the ICAP is found, if it indeed displays similar phenomena, is not known. The ICAP displays many of the characteristics of an axisymmetric round jet in which one would expect vortex shedding. Before the hot argon jet flows out of the confinement tube into the relatively stagnant air of the torch box, it’s flow is essentially laminar rather than turbulent. Its flow profile also appears to have a thin boundary layer and a top hat velocity profile, except for the axial channel [27]. Moreover, the argon exists as unionized gas—as a coolant stream—close to the torch wall. So at least the outer flow bears similarities to a round jet prone to varicose instabilities. On the other hand, it may be dangerous to assume that the flow dynamics of hot plasma bear any similarity to those of cold argon. It should also be noted that the ICAP has both  The Spatial Complexity...  175  axial and tangential velocity components of flow. It is not understood how tangential components affect varicose instabilities. In spite of these departures from a cold, axisymmetric jet with a flat flow profile, we can develop the following conjecture: if the ICAP indeed behaves as a round jet, the argon flow remains laminar just beyond the exit of the torch, but varicose instabilities arise in the cylindrical surface of discontinuity in the flowfield. As a result, the surface of discontinuity eventually folds into a toroidal vortex (as shown in Figures 6.1.1 and 6.1.2—see next chapter, pages 191 and 192) much like a breaking wave. As this vortex moves downstream, it entrains air and radically convolutes the cylindrical interface between the air and the argon. As it continues on its course downstream, it grows in thickness, folds in on itself and continues to entrain more air into the argon jet at its interior; As the toroidal vortex moves upward, its inner edge makes contact with the hot plasma and likely folds cool material in with the plasma gas, likely quenching the plasma. In other words, the plasma is probably confined to the potential core of the jet, where the potential core is simply the region of the flowfield unperturbed by air entrainment. In short, the plasma boundary is likely defmed by a modulated limit or cutoff determined by vortex entrainment of cold air rather than by thermal or radiative dissipation of energy. The time averaged picture of this modulated plasma boundary is revealed by the 0.1 isocontours of CN intensity in Figure 5.3.6. In the outer boundary region, the CN emission maps and the tail flame observed above the plasma are probably time averaged pictures of the vortex shedding. Within this region, the interface between air and plasma—over which mass transport by diffusion takes place—becomes radically convoluted, even though the flow may still be laminar (experiments with cold gas jets and diffusion flames reveal that the flow eventually becomes turbulent as the vortices collide with each other and disintegrate, at downstream distances of more than two jet diameters from the nozzle). The net result of the vortex shedding is complex, pulsating mixing mechanism, with a frequency corresponding to the shedding frequency of the vortex rings, which modulates the outer diameter of the plasma. It seems reasonable to conclude, then, that the overlap of the contours for CN and C I maps above the  The Spatial Complexity...  176  torch rim is a consequence of temporal averaging, while the minimal overlap between C I and C2 isocontours within the torch is characteristic of the steady plasma boundary there. To our knowledge, detailed pictures of this vortex shedding process are only available for cold jets and diffusion flames and not for plasma jets. But reliable experimental evidence shows that the ICAP also displays vortex shedding. This evidence includes high speed movie frames in which vortex structures are plainly visible and noise power spectra in which bands corresponding to vortex shedding frequencies are unmistakable [13]. This and other evidence is discussed in further detail in Chapter 6. Lastly, one of the most unforgettable observations of the ICAP is that at high power and outer argon flow rates(2.25 kW, 15.0 1/mm.), the discharge begins to sing, emitting a very clear acoustic tone.  The Spatial Complexity...  177  5.4.3. Implications for the Analytical Peiformance Physical explanations and conjecture aside, the structure and behavior of the analyte plumes are critical to the analytical performance of the ICAP. Of particularly relevance are the spatial relationships between the analyte plumes and background emission, because the ratio between analyte signal and background emission is something the analyst would like to maximize. Note that one may predict the signal to background ratio from emission maps. One need only multiply the light collection efficiency for each spatial location by the intensity of analyte or background emission, where the light collection efficiency can be calculated by exact ray tracing. After integrating over all space, one obtains analyte and background signals. In this way, one could calculate the signal to background ratio for a variety of light collection configurations, and optimize the signal to background ratio numerically. Also relevant to the analytical performance is the influence of vortex shedding and air entrainment Air entrainment influences the analytical signal in several ways. It introduces flame like conditions to the periphery of the analyte plumes, rendering the analyte signal susceptible to all of the matrix interference effects normally encountered with flames. It also introduces noise by modulating the analyte plume. It may even corrupt the analytical blank by entraining dust or other pollutants. Finally, vortex shedding and air entrainment are sensitive to acoustic excitation and changes in the flow dynamics of the room air. Consequently, environmental sound and changes in the flowfield of the room air may corrupt the analytical signal. Evidently, it would be beneficial to eliminate air entrainment altogether.  The Spatial Complexity...  178  5.4.4 Suggested Improvements for the Design and Methodology ofICAP-AES The fmal step in the rational strategy, proposed in Chapter 1, called for improving the design and methodology of the analytical technique after gaining insight into the complexity and physical properties of the discharge. Here, we have only surveyed the parametric and spatial complexity, yet two improvements immediately suggest themselves: 1. Eliminate vortex shedding by tailoring the flow dynamics of the discharge, and perhaps eliminate molecular background. For example, an outer sheath of argon and oxygen could suppress vortex shedding, 2 formation and CN formation. It may also be possible to manipulate the plasma shape using C flow dynamics and improve the analytical performance. 2. Optimize the light collection envelope with ray tracing calculations and maps of analyte and background emission. For example, the signal to background ratio could be optimized. 5.4.5. Diagnostic Utility of Spatially Resolved Intensity Maps The diagnostic utility of radially resolved, monochromatic images of emission intensity maps, for both physical diagnostics and control diagnostics, is evident in this chapter. The maps reveal that emission from C , CN and C I may all be used as control diagnostics for solvent 2 plasma load, provided they are viewed at the appropriate location. For example, the C 2 intensity is proportional to solvent load when viewed down the axis, while CN and C I intensity are proportional to solvent load when viewed off axis. On the other hand, for physical diagnostic work, the C I maps within the induction region reveal that the plasma shrinks both axially and radially in response to solvent load. C 2 emission within this region indicates how solvent material is distributed over the argon flowstream. Chapter 7 underscores the utility of radially resolved, monochromatic images by presenting maps of electron density calculated from maps of the absolute intensity of a single argon line.  The Spatial Complexity...  179  5.4. CoNcLusioNs  From a physical perspective, a downstream, conical limit is evident in all maps of emission intensity at locations above the torch rim. This limit, and the overlap between isocontours of different emitting species, may be attributed to time averaged, varicose instabilities in the plasma jet. In contrast, the flowstream within the induction region is apparently steady and unperturbed by fluctuations. In maps of this region, it is evident that the distribution of solvent over the argon stream depends on the inner argon flow rate and the properties of the solvent. Briefly, less solvent material is distributed over the outer argon stream when the momentum (flow rate x density) of the inner stream increases. This response may be attributed to a recirculation eddy at the base of the discharge. It also evident in these maps that the plasma responds to solvent plasma load by shrinking both axially and radially. More about this shrinking will be revealed in Chapter 8. From an analytical perspective, the emission maps reveal the analytical utility of emission from argon and solvent pyrolysis products. In particular, the three dimensional information reveals the observation zones where the emission intensity of these species is proportional to solvent load and plasma excitation conditions. In terms of the rational strategy proposed for this thesis work, the spatial maps presented here indeed offer many useful guidelines for pursuing physical diagnostic investigations. Those guidelines are reviewed in Chapter 7.  The Spatial Complexity...  1.  1 80  R.P. Feynman, R.B. Leighton, and M. Sands, The Feynman Lectures on Physics. Vol. Volume I. 1965, Reading, Mass.: Addison-Wesley Publishing Company, Inc (1965).  2.  G. Dube and M.I. Boulos, Canadian Journal of Spectroscopy 22(3): p. 68-76 (1977).  3.  F.J.M. J. Maessen and G. Kreuning, Spectrochimica Acta, Part B, 44(4): p. 387-384 (1989).  4.  S.R. Koirtyohann, C. Baber, and M. Franklin, Spectrochimica Acta, Part B, 31: p. 589587 (1976).  5.  M.W. Blades and G. Horlick, Spectrochimica Acta, Part B, 36: p. 86 1-880 (1981).  6.  M.W. Blades, and G. Horlick, Spectochimica Acta, Part B, 36(9): p. 881-900 (1981).  7.  N. Furuta and G. Horlick, Spectrochimica Acta, Part B, 37(1): p. 53-64 (1982).  8.  0. Horlick and N. Furuta, Spectrochimica Acta, Part B, 37(11): p. 999-1008 (1982).  9.  K. Dittrich and K. Niebergall, Progress inAnalyti cal Atomic Spectroscopy 7: p. 315372 (1984).  10.  G.M. Hieftje, Spectrochimica Acta, Part B, 47(1): p. 3-25 (1991).  11.  Y. Yasutomo, et al., IEEE Trans. Plasma Sci. PS-9: p. 18 (1981).  12.  J.W. Olesik, J.W. and J.C.Fister III, Spectrochimica Acta, Part B, 46: p. 869-883 (1991).  13.  R.K. Winge et al., Journal ofAnalytical Atomic Spectrometry 3: p. 849-855 (1988).  14.  Hecht, E. and A. Zajac, Optics. 1 ed., Reading: Addison-Wesley. 565 (1979).  15.  P.B. Farnsworth, B.W. Smith, and N. Omenetto, Spectrochimica Acta, Part B, 45(10): p. 1151-1166 (1990).  16.  R.S. Stair, W.E. Schneider, and J.K. Jackson, Applied Optics 2(11): p. 1151-1154.  17.  A. Savitzky and M.J.E. Golay, Analytical Chemistry 36(8): p. 1627-1639 (1964).  18.  Madden, H.H., Analytical Chemistry 50(9): p. 1383-1386 (1978).  19.  Press, W.H. and S.A. Teukolsky, Computers in Physics: p. 669-672 (1990).  20.  Pan, C., 0. Zhu, and R.F. Browner, Journal ofAnalytical Atomic Spectrometry 5: p. 537 (1990).  21.  Becker, H.A. and T.A. Massaro, Journal of Fluid Mechanics, 31: p. 435-448 (1968).  The Spatial Complexity... 22.  J. Leconte, Phil. Mag. 15: p. 235 (1858).  23.  Lord Rayleigh, Phil. Mag. 15: p. 235 (1879).  24.  Lord Rayleigh, Phil. Mag. 17: p. 188 (1884).  25.  Benoy, D.A., Modelling of ThermalArgon Plasmas. Ph.D Dissertation, Technische Universiteit Eindhoven (1993).  26.  J. Davies and R.D. Snook, Journal of Analytical Atomic Spectrometry 1: p. 195-201 (1986).  27.  P. Yang and R.M. Barnes, SpectrochimicaActa, Part B, 13(4): p. 275-309 (1990).  181  Chapter 6 The Temporal Complexity of the Solvent Loaded Inductively Coupled Argon Plasma  6.1 INTRODUCTION 6.1.1 Objectives The previous chapters surveyed both the parametric and structural complexity of the discharge. In doing so, they revealed where solvent load effects occur in parameter space and physical space. They also set the stage for exploring the physical properties of the discharge— except for one thing—they did not survey temporal complexity. In fact, the results of the previous chapters mask the temporal complexity because their results are averaged over time scales ranging from 1 second up to 10 minutes. True, time averaged measurements would suffice if the discharge had both a steady flow field and a steady temperature field. Then one could assume that time averaged results were equivalent to time resolved results. One might even assume that downstream events were temporally related to upstream events. This would allow one to regard gradients across the stream lines as evolving in time as one translated the frame of reference downstream. Moreover, one could assume that local thermodynamic properties were constant in time. Then one could measure them simply by increasing the spatial resolution until thermal gradients became insignificant. Indeed, many such assumptions—which could only be validated by a steady flowfield—have been relied upon in the past. But fluctuations are known to disrupt the flowfield of the discharge. Perhaps the two  The Temporal Complexity...  1 83  most important fluctuations result from vortex shedding beyond the exit of the torch, and droplet vaporization within the axial channel. First consider vaporizing droplets. Relatively large droplets (approximately 25 im in diameter) of an aqueous aerosol may actually survive in the plasma all the way up to the analytical viewing zone (z  =  15 mm above the induction coil). Although these large droplets are statistically  few in number, they are significant in volume, so they contain a significant amount of analyte and solvent material compared with the rest of the aerosol. When such large droplets are swept along by the argon stream, their solvent evaporates and cools small regions of the surrounding plasma. When such regions of local cooling pass through the viewing zone, they cause the emission signal to fluctuate aperiodically on the sub millisecond time scale. Similar fluctuations result when relatively large particles left behind by the large droplets fly through the viewing zone because they are surrounded by local concentrations of analyte vapor. In general, the fluctuations resulting from aerosol vaporization are aperiodic and drastically alter plasma conditions on the sub millisecond time scale. In contrast to vaporization events, vortices set up periodic fluctuations when they shed off the plasma jet.  In fact, they modulate the emission signal at approximately 200 Hz.  Experimental evidence suggests that vortices modulate both the outer boundary of the discharge and the flowfield within the plasma. In fact, it appears as if vortices peristaltically pinch the flow of the axial channel as they shed off the plasma jet. At any rate, it is clear that both vortex shedding and aerosol vaporization disrupt the flow field and temperature field of the discharge. Consequently, time averaged measurements of plasma properties must be regarded as biased. This bias may be avoided by completely resolving measurements over both space and time. But such an approach has not met much success to date. Alternatively, one could survey the complexity of the temporal behavior to determine whether other approaches, say phase averaging, would suffice. The objective of this chapter is to survey the temporal complexity of the discharge. This survey, in combination with the spatial and parametric surveys of previous chapters, provides a rational basis for probing the physical properties of the discharge.  The Temporal Complexity...  1 84  Surveying the temporal complexity of the discharge essentially equates to identifying, then explaining the significant fluctuations in the flow field and temperature field. A number of experiments have been pursued to do just that. The following sections review these experiments, experiments including the measurement of noise spectra, the study of vaporizing droplets and desolvated particles in the discharge, and flow diagnostics of the discharge. 6.1.2 Noise Spectra Among the first to explore the temporal behavior of the ICAP, Walden [1], and Horlick and Belchamber [2] studied the noise in the analyte signal by examining its noise power spectrum. A noise power spectrum may be obtained by sampling the emission signal at an appropriate rate, and then taking the Fourier transform of the time varying part. Next, different frequency components of the noise, such as 1/f noise, white noise, whistle noise, and power line noise, can all be identified and then traced back to their respective sources, such as a defective power supply, the gas flow dynamics of the discharge or nebulizer drift. Incidentally, Talmi et al. [3] had already applied this approach to other spectrochemical sources. They found that it was a powerful method for isolating sources of noise in spectrochemical methods. Following Talmi’s lead, Horlick and Belchamber obtained noise power spectra of the analyte emission signal from an ICAP and identified several distinct components of the noise. For example, they identified a low frequency component which they attributed to sample flicker, or 1/f fluctuations of the analyte transport efficiency. They also identified an acoustic component which they concluded was due to plasma rotation. They were led to this conclusion by the results of an interesting experiment. Essentially, they used two monochromators and detectors to simultaneously monitor signal fluctuations from an ICAP via two optical channels. One optical channel viewed the discharge at  900  with respect to the other. Remarkably, the acoustic  fluctuations for the two channels were 90° out of phase. Evidently, the plasma was rotating, or so Beichamber and Horlick concluded. Winge et al. later demonstrated that the acoustic frequency noise at  200 Hz resulted  The Temporal Complexity...  1 85  from vortex shedding [4]. This conclusion has been corroborated by the noise power spectra reported by Houk et aL [5], Hieftje et al. [6], [7], Furuta et al. [8], Snook et al. [9] and Loos Vollebregt et al. [10]. Of all this work, Winge et al. [4] provide the most convincing evidence for the vortex shedding phenomenon. They found that the acoustic noise band at  200 Hz  disappeared when the outer tube of the ICAP torch was extended by 6 mm. The extension prevented air from entraining into the argon stream and hence suppressed vortex shedding. Consequently, the acoustic noise band at  200 Hz disappeared from their noise power spectra for  the extended torch. They went further by capturing images of shedding vortices with a high speed movie camera. The shedding frequency revealed by the movie frames matched a peak in their noise power spectra. Although this finding proved that vortex shedding was responsible for the acoustic bands discovered earlier by Walden [1], and Horlick et al. [2], one must note that vortex shedding and plasma rotation are not mutually exclusive. For example, if the vortices were asymmetric, then the rotation imparted to them by the tangential argon flow would be evident as a phase shift. Hence, the shedding of asymmetric vortices is consistent with the results of Horlick and Belchamber?s dual optical channel experiment [2]. Aside from the vortex shedding phenomenon, noise power spectra may reveal the effects of droplet vaporization. In fact, noise spectra reported by Antanavicius et a!. [11] demonstrate that broad band noise (0 to 2.5 kHz) in ion line signals may be attributed to aerosol vaporization. In typical noise power spectra, however, this broad band noise is buried beneath the baseline of white noise. But Antanavicius et aL lowered the white noise level in their spectra by suppressing the shot noise of their photomultiplier [11]. In order to do that, they used high concentrations of test analyte. These concentrations yielded large radiation fluxes for the analyte signal and allowed them to suppress the photomultiplier shot noise by turning down the photomultiplier gain. After suppressing the white noise, they compared noise spectra for test analyte introduced as vapor with those for test analyte introduced in an aqueous aerosol. They found that for the frequency range between 0 to 2.5 kHz, the noise for aerosol introduction exceeded the noise for vapor introduction by an order of magnitude. In short, they demonstrated that aerosol vaporization may be investigated with noise power spectra, provided large concentrations of test analyte are used.  The Temporal Complexity...  1 86  They suggested that the time scale of the vaporization event (500 ps) corresponded to the inverse of the noise bandwidth (2.5 kHz), and offered two explanations. A time scale of 500 ms may correspond to the pulse relaxation time of the analyte signal. In that case, the wave form of the pulse would be determined by the vaporization of analyte particles and the pulse duration would follow Poisson statistics. On the other hand, 500 ms could be the observation time of the flowing aerosol (after Eckert [12]). In that case, one must assume that emitting particles survive the entire distance over the observation window. Interestingly, one obtains a particle velocity of 20 m/s when one divides their observation distance (slit height) of 10 mm by the time scale of 500 ps. This velocity agrees quite well with typical centerline velocities in the ICAP. In short, some doubt surrounds the correct assignment of the broad band noise. Does it correspond to particle lifetimes or observation times? Recall that similar doubt surrounded the assignment of the acoustic band at 200 Hz—did the acoustic band correspond to plasma rotation or vortex shedding? In summary, noise power spectra can reveal the presence or absence of temporal fluctuations owing to aerosol vaporization and vortex shedding, but beyond that, noise power spectra do not reveal the processes or mechanisms behind the fluctuations. 6.1.3 Studies of Vaporizing Droplets and Particles Farnsworth et al. [131 and Olesik et al. [14], [15], [16], [17]looked beyond noise power spectra and examined the form of the transient signal itself. They did this for Mg I, Ca I and Ba I emission lines, and discovered conspicuous emission spikes on the sub millisecond time scale. Their discovery led them to investigate the relationship between aerosol droplets and the time resolved emission of low energy atom lines and atomic ion lines. By correlating both dips in the ion signal and spikes in the atom signal with laser light scattering off incompletely desolvated droplets, they proved that incompletely desolvated droplets can exist in a water loaded ICAP, at least when the aerosol stream has not been modified by heating or desolvation. Moreover, both droplets and desolvated particles induce temporal fluctuations in the plasma.  The Temporal Complexity...  1 87  Olesik et al. pursued these experiments much further. They surveyed the parametric response of droplet induced fluctuations [16]. They also established relationships between droplet phenomena, emission profiles resolved in time and space, and time averaged axial profiles similar to those presented in Chapter 4 [17]. These experiments allowed them to demonstrate that the parametric behavior of analyte emission could be explained in terms of dynamic phenomena rather than the popularly unquestioned thermal characteristics of an ICAP in steady state. Horlick et al. followed a different tack [18]. They examined the auto-correlation of droplet modulated signals.  They did this for different entrance slit geometries on their  monochromator. This allowed them to determine the size of droplet induced disturbances in the plasma. The disturbances turned out to be about 1.5 mm across. In short, several experimental methodologies for exploring the dynamic behavior of the ICAP have evolved from the original experiment of examining the wave form of the analyte emission signal. 6.1.4 Flow Diagnostics Further experimental techniques for exploring the dynamic behavior of the ICAP have been borrowed from experimental fluid mechanics. Techniques such as high speed photography, particle tracking and anenometry have been applied to the ICAP, while flow visualization techniques using Schlieren photography or Mie scattering have not been applied as far as we know. Nevertheless, good examples of high speed photography have been reported by Winge et a!. [4], who have examined vortices shedding off the ICAP with a high speed movie camera. Their high speed movie frames clearly reveal vortex structures in the tail section of the ICAP. The wavelength of the vortex structure, the position where the varicose waves roll over into vortices, and periodic constriction of the analyte plume are all evident in the movie frames. Moreover, the vortex shedding is consistent with noise power spectra of the same discharge. The frequency of a prominent peak in the noise power spectrum corresponds to the frequency of vortex shedding.  (All of these results compelled Winge et al. to suggest that diagnostic  measurements of the ICAP should be phase averaged rather than time averaged.) The findings of Winge et al. were corroborated by high speed videos of the ICAP recorded by Furuta [19] and  The Temporal Complexity...  188  high speed photography of the ICAP by Houk eta!. [5] and Grey [20]. In addition to vortex phenomena, the photographs reported by Houk et al. [5] capture images of a single aerosol particle vaporizing. In addition to high speed photography, anenometry and particle tracking have provided further insight into the temporal fluctuations and flow dynamics in the ICAP. Particle tracking experiments have revealed that the flow velocity along the centerline of the discharge is approximately 25 mIs. Cicerone and Farnsworth measured velocities between 20 m/s and 40 m/s within 1 mm of the axis. Interestingly, they found that the velocities increased with power and viewing height. Olesik et al. found that this centerline velocity was independent of the inner argon flow rate, a finding that suggests that the axial channel dilates in response to an increase in flow rate, in order to keep the axial flow velocity constant. More about the dilation of the axial channel will be said in Chapter 8. Beyond particle tracking, Barnes et al. [21], [22] have inserted anenometry probes into the jet to measure its velocity profile. For an ICAP confined by an extended torch, they found that the velocity profile was essentially flat except for a thin boundary layer and an axial maxima corresponding to the axial flow. These results indicate that an unconfined ICAP would be prone to vortex shedding because vortex shedding typically arises in jets with flat velocity profiles. On the other hand, jets with fully developed flows (the boundary thickness approaches the jet radius), are prone to sinuous instabilities [23]. 6.1.5 Axisymmetric Jets The work reviewed in the previous sections provides insight into the velocity profile of the argon jet, the effects of desolvating droplets and vaporizing particles, the acoustic noise resulting from vortex shedding, and the gross structures of shedding vortices in the tail flame of the discharge. Moreover, these experimental findings have been extended by numerical simulations of the flowfiekl and temperature fields [24]. But neither simulations nor experimental results available to date provide insight into how the flowfield of the discharge may be perturbed by vortex shedding. Consequently, the temporal complexity of the discharge remains unclear. Fortunately, a wealth of insight into vortex shedding has been provided by experimental and  The Temporal Complexity...  1 89  theoretical work with axisymmetric jets and diffusion flames. Several reports of this work are worth noting. While Prandtl [25] provides a good introduction to fluid mechanics, Becker and Massaro’s [23] paper on vortex evolution in a round (axisymmetric) jet provides a good starting point. They illuminated an axisymmetric jet through a slit, in order to obtain images of the flow structure for a narrow slice through the jet. Because the jet was laden with oil condensation smoke, and because the light was stroboscopically synchronized with the vortex shedding, they were able to obtain sharp, phase averaged photographs of the sagittal cross section of the jet. These photos clearly reveal how the vortex structures evolve as they travel downstream. Moreover, they reveal how vortex shedding varies as the Reynolds number is increased from 600 to 20000, and how the structured, laminar flow of the vortices break down into turbulence (the ICAP occupies a flow regime of Re  =  200  —  600). In fact, the phase averaged photos of the  smoke laden jet are far clearer than high speed photos of the ICAP, because the exposure is more even (neither under nor overexposed) and the boundaries of the flow structures are sharp. Complementary photos are provided by Subbarao and Cantwell [26], Longmire and Eaton  [27], Roquemore et at. [28] and Dahm et al. [29]. Dahm et at. [29] supply laser ,  induced fluorescence images of coaxial jets. These images reveal how vortex formation may be attenuated by surrounding the jet with an outer sheath flow. Roquemore et at. [28] supply 2D photos of a diffusion flame illuminated by a sheet of laser light. The laser light was scattered off 2 particles formed by TiCl Ti0 4 in the fuel and air reacting with combustion products. Their images reveal flow structures both at the outer boundary of the jet (visualized by Ti0 2 particles) and close to the axis (visualized by combustion). Interestingly, the inner combustion region and outer vortex structures of a diffusion flame resemble the pinched analyte plume and shedding vortices of the ICAP. Further insight into the inner flow field is provided by Longmire and Eaton’s study of a particle laden jet [27]. Although the flow regimes they studied corresponded to Reynolds numbers much higher than in the ICAP, the velocity and density information they obtained from their laser Doppler anenometry experiment reveal much about how the axial flow of a jet interacts with ring vortices shedding off the periphery. Finally, Subbarao and Cantwell provide stroboscopic Schlieren photographs of jets with Reynolds numbers in the immediate  The Temporal Complexity...  190  vicinity of the ICAP [26]. Although their photos only reveal the flow at the interface between their helium jet (the jet was buoyant) and coflowing air, the sharp Schlieren images reveal the vortex structure clearly. The experimental work cited above is complemented by the numerical simulations conducted by Martin and Meiburg [30]. Not only is their report lucidly written, it supplies beautiful and informative 3D graphics of their simulation results.  Also complementing  experimentally recorded images are power spectra of the acoustic fluctuations in axisymmetric jets. These have been reported by Bridges and Hussain [31], and Kyle and Sreenivasan [32], and bear many similarities with the noise spectra of emission from the ICAP. Although the theory developed on the structure and evolution of vortices is beyond the scope of this thesis, the reader is urged to consult the references cited above because they provide valuable insight into the vortex shedding phenomenon discussed in this chapter. Moreover, it is possible to construct a conceptual model of vortex shedding in the solvent loaded ICAP once the work on axisymmetric jets as been consulted. Such a model is illustrated in Figures 6.1.1 and 6.1.2. Figure 6.1.1 depicts a three dimensional wave that forms at the surface of discontinuity of the flow field. As the wave crests, analyte material is drawn into the crest and pinched away from the trough, in much the same way that particles behaved in Longmire and Eaton’s experiment. Figure 6.1.2 reveals how this process takes place in the ICAP. The six frames in the figure correspond to different phases of the vortex train. The drawing were based on the photographs reported by Winge et al. [4], and compare the typical time averaged view of the discharge with the phase averaged view. Evident in the figure is the peristaltic pinching of the analyte plume, a process which probably accounts for much of the overlap between the time averaged images reported in Chapter 5 of analyte emission and molecular emission from the boundary regions.  191  The Temporal Complexity  Y  A  Figure 6.1.1 The 3D varicose wave that rolls off the argon jet into a ring vortex. The peristaltic bunching of the axial stream has been exaggerated to illustrate how the varicose disturbance interacts with the axial stream.  0 0 0  Figure 6.1.2 Vortex shedding from the inductively coupled argon plasma. The phase averaged view is shown on the left side of each frame. The right side of each frame shows the time averaged view. The analyte plume is shaded grey, the plasma is white, and the downstream boundary region is hatched.  I  The Temporal Complexity...  193  6.1.6 Chapter Summaiy Out of all the powerful experimental techniques designed to explore the temporal complexity of the discharge, the measurement of noise power spectra is the most appropriate for this work. Rather than being specifically tailored to droplet vaporization or vortex shedding, noise spectra can test for the presence of several phenomena. Quantitative information may also be extracted from noise power spectra (for example, frequencies and noise levels), and when the spectral density is scaled to the right units, results can be compared with a growing body of noise power spectra reported in the literature. Moreover, the spectra contain information relevant to both the analytical performance and physical characteristics of the discharge. Finally, noise power spectra can be easily measured over a wide range of parameters, so one may rapidly survey the temporal behavior of the discharge over a wide range of experimental conditions. Such a survey, in combination with a parametric survey and spatial survey, can provide one with a solid basis from which to investigate the physical characteristics of the discharge. This chapter presents the salient features of noise spectra measured over three frequency ranges: 0- 50 Hz, 0- 500 Hz, and 0- 100 kHz. For the 0 500 Hz and 0- 100 kHz ranges, three -  parameters were explored: solvent, solvent load, and observation height. Only the peristaltic pump rate was studied for the 0  -  50 Hz range. Noise spectra were measured for low energy  atom lines and ion lines of test analytes, and for molecular bandhead emission of diatomic species in the boundary region. Droplet disturbances are evident in both the noise spectra and transient waveforms for the low energy atom lines. Evidence for vortex shedding is evident in all spectra that span acoustic frequencies above 100 Hz.  The Temporal Complexity...  194  6.2. EXPERIMENTAL  6.2.1 Noise Spectra All transients were detected with a Hammamatsu R955 photomultiplier fitted to the 1 meter Czerny Turner monochromator described in Chapter 2. The voltage applied to the photomultiplier was set to 600 V (yielding a photomultiplier gain  —  10), and the output was  amplified by a Kiethley 428 current amplifier. The amplifier gain was generally set to 106, but gains of i0 5 and i0 7 were also used. The built in low pass filter of the Kiethley 428 current amplifier prevented high frequency noise from being aliased into the noise spectra. The cut off frequency was taken as the reciprocal of the filter rise time, and the signal was sampled at the Nyquist frequency, or twice the cut off frequency. For example, when the 0 to 500 Hz frequency range was investigated, the filter rise time was set to 1 msec, giving a cutoff frequency of 1 kHz. Consequently, the signal was sampled at 2 kHz. For these settings, the -3 dB point of the filter response was at 350 Hz, and the filter response rolled off at -60 dB per decade (-18 dB per octave). Note that much sharper cut off filters are available, but the built in filter was adequate. Data were sampled using the RC Electronics Compuscope software and ISC-16 data-acquisition board. The DADiSP worksheet ( DSP development corporation) was used to calculate the noise power spectra in batch mode with a command file. In general, transients 16K long were sampled, then divided into 16 records of 1024 samples each. For each 1K record, the d.c. component was first removed by subtracting the mean signal. Then the time varying component was apodized with a Hanning window. A fast Fourier transform was then taken, and the sum of the squares of the real and imaginary parts gave the noise power spectrum. An average of the 16 resulting noise power spectra gave a good estimate of the noise power spectrum for the original 16K transient. Finally, the noise power, or  The Temporal Complexity...  195  noise spectral density S, was expressed in a variety of units so the average noise spectra could be compared with literature results. Beyond expressing the noise power S in arbitrary units, the noise amplitude A could be obtained by taking the square root of S, and then expressed in photocurrent units (A s). It was also useful to nonnalize the spectral density to the intensity of the d.c. component in order to correct for differences in signal strength. A variety of unit scales were available to do this. The results could be expressed in decibels, where the decibel scale represents the ratio of the noise amplitude A to the d.c. photocurrent A , dB 0  2Olog(A/). This facilitated  comparison with ref. [4]. An alternative decibel scale was proposed by Houk et at. [5] and represents the ratio of the noise amplitude A to the amplitude of noise approached asymptotically at high frequencies (essentially the photomultiplier shot noise level). For comparison with the results in ref. [11], the spectral density was expressed as noise power S, divided by the d.c. photocurrent A , while for comparison with the results in ref. [7], the spectral density was 0 expressed in reduced units of noise amplitude A divided by the d.c. photocurrent A . In total, 0 noise spectra in the literature are reported according to more than six different unit scales. 6.2.2 imaging The discharge was imaged onto the entrance slit with an image to object ratio of 2:1, and the slit height was 1.5 mm, so the projected aperture of the slit height was 3 mm. Winge et al. [4] discuss how this dimension limits the spatial sampling frequency of periodic plasma fluctuations traveling past the observation zone. They point out that if the wavelength of a periodic plasma fluctuation were equal to the projected slit height, then the fluctuation would be totally attenuated to the d.c. level. As it happens, a projected slit height of 3 mm yields an effective spatial sampling frequency of approximately 3000 Hz, when the wave velocity of typical periodic plasma fluctuations (10 m/s) are taken into account. Ref. [4], should be consulted for further discussion of how both the spatial filtering of the slit-height aperture and the time domain filtering determine the overall frequency response of the measuring system.  The Temporal Complexity...  1 96  The observation height was varied by translating the 1.5 mm slit-height aperture over the entrance slit. Neither the spatial response of the photomultiplier, nor the spherical aberration of the lens were taken into account. 6.2.3 Emission Channels Emission signals of the Mn II (257.61 nm) line and the CN (388.34 nm ) bandhead were sampled at 2000 Hz to investigate the 0 to 500 Hz frequency range where vortex fluctuations can be expected. Emission signals of the Ca I (422.70 nm) line and the Mg I (285.22 nm) line were sampled at 200 kHz to investigate both the form of sub millisecond transients and the 0 to 100 kHz frequency range where droplet vaporization effects can be expected. Finally, the emission signal from the C2 (516.56 nm) bandhead was sampled at 100 Hz to investigate low frequency pulsations owing to the peristaltic pump. 6.2.4 Test Solutions and Solvent Load Loading by three solvents was studied: water, methanol and chloroform. The chloroform load was varied from 3.2 to 10 mg/s. the methanol load from 0.3 to 0.7 mg/s while desolvated aqueous aerosol (0.15 mgls) from a MAK nebulizer was compared with undesolvated aerosol from a Meinhard concentric nebulizer (0.32 mg/s). The metal concentrations were 20 pg/ml (20 ppm) in chloroform and methanol (as 2,4 pentanedionates), and 50 ig/m1 (50 ppm) in aqueous solvent (calcium carbonate in dilute HC1, magnesium chloride and manganese sulfate).  The Temporal Complexity...  197  6.3 RESULTS  Noise spectra covering three frequency ranges are presented here. First the 0 500 Hz -  range reveals the acoustic features owing to vortex shedding. Then the 0 100 kHz range reveals -  the broad band noise resulting from vaporizing droplets and particles. Finally, the 0  -  50 Hz  range reveals the low frequency noise associated with sample introduction. 6.3.1 The 0 to 500 Hz Frequency Range: Vortex Shedding The following four figures depict noise spectra for Mn 11(257.6 nm) line emission and CN (388.3 nm) bandhead emission from a solvent loaded ICAP. Noise spectra are shown at three viewing heights above the induction coil: z  =  6 mm, z  =  12 mm and z  =  18 mm. Figure  6.3.1 depicts the response of Mn II (257.6 nm) noise spectra to chloroform load; Figure 6.3.2, the response of CN (388.3 nm) noise spectra to chloroform load; Figure 6.3.3, the response of Mn II (257.6 nm) noise spectra to methanol load; and Figure 6.3.4, the response of CN (388.3 nm) to methanol load. Clearly, all of the noise spectra evolve in the same way with viewing height. In general, an acoustic noise band at  200 Hz dominates the spectra at all viewing heights. A subharmonic  and two harmonics of the 200 Hz band emerge at 12 mm, then become comparable in magnitude to the 200 band Hz at z  =  18 mm. This evolution of acoustic noise with height is typical of the  vortex shedding phenomenon for ICAPs sustained in short torches.  The Temporal Complexity (a)  I  I  z.  •V  .iO  C  .20  I  I  •30 .40  I  I  •50  100  0  Z  200  300  400  (b)  I’  100  0  Z  200  300  400  C”  I /C’,c::. III  Z  100  0  300  200  t—CZ•  400  Frequency (Hz)  Figure 6.3.1 The effect of chloroform plasma load on the 0 500 Hz noise spectra for the Mn II line at 257.6 10 nm: (a) z 18 mm, (b) z = 12 mm, and (c) z = 6 mm above the induction coil. -  199  The Temporal Complexity (a)  100  200  300  400  0  100  200  300  400  0  100  200  300  400  0  (b)  Z  1: Z  Frequency (liz)  Figure 6.3.2 The effect of chloroform plasma load on the 0 500 Hz noise spectra for CN emission at 388.340 nm: (a) z = 18 mm, (b) z = 12 mm, and (c) z = 6 mm above the induction coil. -  200  The Temporal Complexity (a)  100  0  Z  400  300  200  (b)  I  100  0  200  I Z  400  300  I  I  0  100  200  300  400  Frequency (Hz)  Figure 6.3.3 The effect of methanol plasma load on the 0 500 Hz noise spectra for the Mn II line at 257.6 10 nm: (a) z = 18 mm, (b) z = 12 mm, and (c) z = 6 mm above the induction coil. -  201  The Temporal Complexity (a)  0  .o .io .20 .30•  .4o. I  I  400  300  200  100  0  0•  I E L__ 400  300  200  100  0  (c)  I z  000  C  200  300  Frequency (Hz)  400  ‘V  &  Figure 6.3.4 The effect of methanol plasma load on the 0 500 Hz noise spectra for CN emission at 388.340 nm: (a) z = 18 mm, (b) z 12 mm, and (c) z = 6 mm above the induction coil. -  The Temporal Complexity...  202  The f/2 subharmonic of the 200 Hz band first emerges in the z becomes more intense at z  =  =  6 mm spectra. It  12 mm, and is joined by 3/2 f and 2 f harmonics at 18 mm. An  explanation for the f/2 subharmonic was found by Winge et al. [4]. Their high speed, black and white movies revealed that vortex shedding alternates between strong and weak fluctuations. On the other had, the 3/2f and 2 f harmonics emerge when the varicose wave begins to crest. This happens immediately before the wave rolls over into a vortex. Note that additional harmonics are required to describe the wave as it crests, because its waveform clearly departs from a simple sinusoidal function. Above z  18 mm, the cresting wave rolls over into a vortex. When this  happens, Winge et al. [4] have found that 5/2 f and 3 f harmonics emerge. Undoubtedly, these additional frequency components are required to describe the evolving wave form as it departs even further from a simple sinusoidal function. All of the 0- 500 Hz noise spectra evolved this way. The only difference between the CN and Mn II spectra was a flicker component below 100 Hz displayed by the Mn II spectra at z =6 mm. This 1/f noise probably resulted from sample flicker (drift and low frequency fluctuations in the aerosol transport efficiency), but not from droplet vaporization. The next section reveals that droplet vaporization results in broad band noise that spans a much greater frequency range than the 1/f components displayed here. In any case, the 1/f components in these spectra are far weaker than those reported in the literature. Interestingly, the noise spectra reported in the literature are for sample introduction without desolvation. Evidently, the low levels of 1/f noise in Figures 6.3.1  -  6.3.4 corroborate what Maessen et al. [33], [34] observed—that the desolvating  condenser filters out the sample flicker noise. Of course, one must also remember that sample flicker depends quite sensitively on the design of the pneumatic nebulizer. Perhaps the MAK nebulizer is inherently quiet [35]. Apart from the 1/f component, the similarity between the noise spectra for Mn II emission and CN emission was unanticipated. While the CN noise reveals fluctuations in the boundary region where one would expect vortex shedding to prevail, one would expect the Mn II  The Temporal Complexily...  203  emission from the analyte plume to be removed from processes ii the boundary region. On the contrary, vortex shedding modulates both the analyte plume and the boundary region to the same extent. Evidently, the varicose instability is not confined to the boundary regions, but penetrates  right down to the axial channel. For insight into how this happens, one must go beyond noise spectra. The high speed photographs reported by Winge et al. [4] reveal that shedding vortices pinch the analyte plume of the ICAP. A clearer picture of how shedding vortices affect the flow field is provided by the large body of work on axisymmetric jets. This work reveals that ring vortices constrict the flow along the axis of the jet. Between successive vortices, the axial flow is forced radially outwards. In fact, experimental results suggest that shedding vortices bunch the material flowing along axis into packets.  One experiment revealed that particles in an  axisymmetric, particle laden jet are bunched together between successive vortices [27]. Although the vortex shedding frequency is independent of viewing height in Figures 6.3.1 through 6.3.4, Figure 6.3.5 reveals that it decreases with solvent plasma load. Moreover, the decrease is far steeper for methanol load than for chloroform load. In order to explain this response, and reconcile it with reports that the frequency increases with r.f. power, outer argon flow rate, and a short extension (1 cm) of the outer tube, we propose the following model. The wave velocity of the varicose disturbance is roughly half the velocity of the coolant stream. Consequently, any variation in the operating parameters that increases the velocity of the coolant stream also increases the shedding frequency. An increase in the outer stream velocity can be explained in terms of two properties: 1., how much argon is carried by the outer stream; and 2., how effectively the hot plasma transfers heat to the argon in the outer stream and thereby accelerates the outer stream. Increasing the r.f. power increases the shedding frequency by accelerating the coolant stream—more heat is transferred to the coolant stream. Increasing the outer flow increases the shedding frequency at first, because a greater volume of coolant argon must flow through an outer annulus of constant area.  204  CL) 0  0.0  .—  :  1.0  2.0 3.0  methanol load  4.0  6.0  7.0  Solvent Plasma Load, mg/s  5.0  8.0  chlciro form load  9.0  10.0  11.0  Figure 6.3.5 The effect of solvent plasma load on the vortex shedding frequency. The error in the shedding frequency of ± 3 Hz was taken from the peak width at half maximum for the acoustic band. The error in the solvent plasma load of ± 5 % was determined by the precision of the continuous weighing method described in Chapter 2.  172  1 76  180  1 84  188  1 92  196  ci) 200  CD  (3  208  212  216  220  12.0  I  The Temporal Complexity...  205  But with further increase, the outer flow ultimately levels off. It may even reverse as more energy is convectively lost from the discharge, and the induction region can no longer accelerate the outer flow as effectively by transferring heat to it. Similarly, an increase in solvent plasma load consumes plasma energy and decreases the plasma’s ability to accelerate the outer stream. This effect is more pronounced for methanol load because the plasma lifts and contracts axially (see Chapter 5). As a result, the axial distance over which the coolant stream comes into contact with the induction region decreases with methanol load. Far less heat can be transferred to the outer stream over shorter distances. The converse argument applies to extending the outer tube by 1 cm (Winge et al. observed that this increases the shedding frequency [4]). A 1 cm extension of the outer tube confines the outer flow next to the hot plasma for a longer distance, so the outer flow accelerates more than in a shorter torch. Alternatively, the frequency dependence could be explained in terms of the area of the unobstructed annulus through which the outer stream must escape past the plasma. This area would vary with the dimensions of the plasma. In reality, the true mechanism by which the outer stream velocity and shedding frequency vary is probably a combination of both the heat transfer efficiency and the geometry of the discharge. 6.3.2 The 0 to 100 kHz Frequency Range: Droplet Vaporization In contrast to vortex shedding, it is not quite as simple to observe fluctuations owing to vaporizing aerosol, for two reasons. First, Olesik et al. [14], [15], [16], [17] have pointed  Out  that droplet disturbances may be so numerous that they actually overlap and swamp one another out and yield an apparent d.c. signal. Second, Antanavicius et al. [11] pointed out that the waveform resulting from particle vaporization is typically buried beneath the photomultiplier shot noise. However, appropriate measures may be taken to expose the effects of aerosol vaporization in the noise power spectra. Here, we consider how Antanavicius brought the broad band noise owing to aerosol vaporization above the baseline of white noise in their noise spectra. Their strategy was essentially to use large concentrations of test analyte in order to increase the radiation flux of the analytical line. This allowed them to turn down the voltage applied to their  The Temporal Complexity...  206  photomultiplier, thus lower the photomultiplier gain and lower photomultiplier shot noise—a significant component of the total white noise. Consequently, using large concentrations of test analyte allowed them to lower the white noise in their final signal and see the broad band noise attributable to aerosol vaporization. Essentially, the photomultiplier shot noise S, contributes significantly to the total white noise, but scales down with the square of the gain, g, so the total white noise may be reduced quite considerably by decreasing the photomultiplier gain. More precisely, =  n’, 2 2e f g  (6.1)  where e is the electron charge, n is the number photoelectrons ejected from the photocathode, and F’ is a factor that accounts for the fluctuations in the gain owing to statistical nature of amplification (1.1 < F’ <1.4)  [11]. Antanavicius et al. were interested in bringing the signal  fluctuations owing to aerosol vaporization above the level of photomultiplier shot noise. Consequently, they wished to know how the ratio of the shot noise to signal (anodic current) scaled with the photomultiplier gain. Noting that the signal, or the anodic current, is given by the cathodic current multiplied by the gain, or I=1 g 0 =neg,  (6.2)  where I is the anodic current and I is the cathodic current, one obtains the ratio of shot noise SSh(,  to the anodic current by dividing equation 6.1 by equation 6.2. =  2egF’.  (6.3)  Evidently, the ratio of shot noise to the anodic current scales proportionally with the photomultiplier gain. Typically, values for SShO,’ exceed iW’ 3 A s when the photomultiplier gain exceeds 106. It turns  Out  that such high levels of  mask the broad band noise owing  to aerosol vaporization. But by using high concentrations of analyte which resulted in high radiation fluxes, Antanavicius et al. were able to decrease the photomultiplier gain to 1 doing so, they brought the white noise down below 5  x  By  1014 A s and unmasked the broad band  The Temporal Complexity...  207  noise in the signal for ion line emission (Call ). While the noise in the ion line signal that results from aerosol vaporization may lie buried in the photomultiplier shot noise, the noise in low energy atom line signal that results from droplet vaporization is far more conspicuous.  Consequently, the arguments applied by  Antanavicius to particle vaporization should also apply to droplet vaporization. The noise power spectra of low energy atom line emission should reveal the broad band noise attributable to aerosol vaporization. This can be ensured by using large concentrations and low photomultiplier gains. On the other hand, it is possible for droplet disturbances to overlap and yield an apparent d.c. signal. The way around this problem is to extend the survey over a range where droplet events are separated or absent. Then the conditions where droplet events coalesce should be evident, if it occurs. In this way, it was established that incompletely desolvated droplets are not present in the ICAP when the aerosol has been desolvated. Moreover, the broadband noise attributable to aerosol vaporization was only encountered at low observation heights for an ICAP loaded with water, without desolvation, as the following paragraphs describe.  The Temporal Complexity...  208  Figure 6.3.6 depicts noise resulting from droplet vaporization in a temporally resolved signal for emission from a low energy atom line (Ca I 422.70 nm). The signal is characterized by intense spikes superimposed on a weak d.c. component. The d.c. component results from atom line emission from the unperturbed plasma. In the unperturbed plasma, such atom line emission is usually weak for elements with low ionization potentials because the thermal conditions are such that the analyte is  >  99 % ionized. However, when a droplet vaporizes to  create a region of local cooling, the analyte’s degree of ionization within the region of local cooling decreases from  >  99% down to 50% or even 0 %. Consequently, the density of neutral atoms is  multiplied, say by 10 to 50 times, and an intense emission spike in the atom line signal is observed as the region of local cooling is swept through the observation zone. Of course, this switch-like, ionization effect is most conspicuous when the atom line has a low excitation potential and the neutral analyte has a low ionization potential. Because they meet these criteria, atom line signals from the alkaline-earth elements are the most sensitive indicators of droplet perturbations in the plasma. It is recognized that the number of droplet disturbances traversing the observation zone per unit time may be so high that the emission spikes could coalesce into an apparent d.c. signal. In our studies, however, no spikes were observed under the typically robust operating conditions of our 1CM?. Spikes only began to emerge when the power and viewing height were decreased, or when the water load and inner argon flow rate were increased. In other words, emission spikes began to appear as the plasma grew less robust. In fact, we could follow the increase in the frequency of emission spikes, and their density never reached the point of coalescing into an apparent d.c. component.  D  -  >%  LIA time, ms  11  12  13  and the discharge was loaded with water at 0.32 mg/s (without desolvation).  Figure 6.3.6 The transient signal for the Ca I line at 422.673 nm. The viewing height was z  1000-  2000-  =  1161  8192O  6 mm above the induction coil,  14  C  The Temporal Complexity...  21 0  In general, most observed emission spikes have a peak width on the order of 100 JIs. In view of the work done by Cicerone and Farnsworth [13] and Olesik et al. [14], [15], [16], [17], and considering that the projected slit height was only 3 mm, this time scale is undoubtedly the observation time of the droplet disturbance rather than the life time of a vaporizing droplet. In other words, the droplet disturbances survive for long distances compared to 3 mm, and fly through the observation zone with a velocity on the order of 30m/s, a typical centerline velocity for the ICAP, so the observation time  3 mm / 30 mIs  100 ps. Moreover, if a noise power  spectra were taken of a transient with emission spikes, then one would expect broad band noise from 0 to approximately 10 kHz  —  the reciprocal of 100 ts.  Figure 6.3.7 depicts exactly that sort of broad band noise in an ICAP loaded with 0.32 mg/s water, without a desolvating condenser interposed between the spray chamber and the torch. Significantly, the broad band noise rapidly subsides with an increase in viewing height, and was only observed for water loading without desolvation. Evidently, of the three solvents considered in this work, the effects of desolvating droplets are confined to high levels of water loading. Further discussion of aerosol vaporization effects may be found in Chapter 4. Beyond the effects of desolvating droplets, further conclusions cannot be drawn from the noise spectra measured in this study. But the approach taken by Antanavicius et al. [11] offers promise for investigating the effects of vaporizing particles.  0  Cl)  ci)  Li,  C.)  h,  CL)  Cl)  •1..  :3 C.) 0 0  ci)  o  10  0 5  Frequency, Hz  10  6 mm  15  Figure 6.3.7 Noise spectra for the Ca I line at 422.673 nm. The discharge was loaded with water at 0.32 mg/s (without desolvation).  1 0-14  1 0-13  1o 12  1  1  3 20x10  The Temporal Complexity...  21 2  6.3.3 The 0 to 50 Hz Range: Sample Introduction Figure 6.3.8 reveals that fluctuations in sample delivery are evident in emission signals. Specifically, the pulsations of the peristaltic pump are evident in C 2 emission. Moreover, the peak corresponding to the pulse frequency is the dominant component of low frequency noise, and its amplitude decreases with an increase in the pumping rate. Similar effects were observed for analyte emission by Loos-Vollebregt and Goudzwaard. They point out several ways of dealing with noise, including standard addition [Myers and Tracy] and modulated sample introduction [Steele and Hieftje]. Here, we only point out that the C 2 emission signal can be used to monitor low frequency noise.  0  C/I  c  1 I  -.  I  L1  C/I  C- C  i-..  .  0 (ii  b  0  01  0  -‘  r\) I\) —. .  0 0 0 0  01  0  P  (71  0  01  9)  0  9)  01  0  0  C)1  0  (71  9)  0  9)  01  I\.)  CJ1  0  P  0  01  0  o  P  0  P  0  0  0)  01  91  0  1 01  (Ti  9) 0  1 (11  0)  0  0  1  01  0  0  0  01  91  0  3  xlO r c o 0  9’  01  3  (71  0  0  xlO  C),  0  01  C.)  C.)  0  0  0  0  o  C  Co  0  0  Noise Power, arbitrary units  01 (71 0 0000  xlO  3  o  1\) 0  I)  o  01  I  II  o  C.) 0  The Temporal Complexity...  214  6.4. CONCLUSIONS The physical properties, particularly the thermal state of the ICAP, are modulated by two macroscopic processes. Aerosol vaporization modulates the temperature field aperiodically while vortex shedding modulates both the temperature and flow fields periodically. Incompletely vaporized droplets in the plasma can be eliminated by desolvation. It is not clear what the effects of desolvation have on the vaporization of desolvated particles. However, it is clear that vortex shedding is always present torches without extension tubes. Moreover, the vortex shedding frequency depends on the solvent and the solvent plasma load, and modulates both the plasma boundary and analyte plume of the discharge to a similar extent. In light of vortex shedding, future diagnostics should be phase averaged as Winge et al. have already suggested [4]. Clearly, the survey of the temporal complexity of the discharge presented in this chapter is a valuable contribution to the rational strategy proposed in Chapter 1—it provides an important guideline for further diagnostic work—that measurements should be phase averaged, or that their  interpretation should at least account for varicose modulations and aerosol vaporization. Once again, improvements in the analytical technique suggest themselves at an early stage in the strategy. For example, CN emission, a potential source of spectral interference, could be eliminated by modifying the flow dynamics of the discharge to prevent air entrainment and vortex shedding. Perhaps a coflowing, outer sheath of argon and oxygen could be tailored to eliminate the formation of diatomics that emit in the ultraviolet and visible regions.  The Temporal Complexity...  21 5  6.5. REFERENCES  1.  G.L. Walden, Spectrochimica Acta, Part B, 35: p. 535-546 (1980).  2.  R.M. Beichamber and G. Horlick, SpectrochemicaActa Part B, 37(1): p. 17-27 (1982).  3.  Y. Talmi, R. Crosmun, and N.M. Larson, Analytical Chemistry 48: p. 326 (1976).  4.  R.K. Winge, D.E. Eckels, E.L. DeKaib and V.A. Fassel, Journal ofAnalytical Atomic Spectrometiy 3: p. 849-855 (1988).  5.  R.K. Winge, R.K., J.S. Cram, and R.S. Houk, Journal ofAnalytical Atomic Spectrometry 6: p. 601 604 (1991). -  6.  N. Furuta, C.A. Monnig, P. Yang and G.M. Hieftje, Spectrochemica Acta, Part B, 44(7): p. 649-656 (1989).  7.  N.N. Sesi, P.J. Galley, and G.M. Hieftje, Journal ofAnalytical Atomic Spectrometry 8: p.  65-70 (1993). 8.  N. Furuta, Journal ofAnalytical Atomic Spectrometry 6: p. 199-203 (1991).  9.  J. Davies and R.D. Snook, Journal ofAnalytical Atomic Spectrometzy 1: p. 195-201 (1986).  10.  M.P. Goudzwaard and M.T.C. de Loos-Vollebregt, Spectrochemica Acta Part B, 45(8): p. 887-901 (1990).  11.  R. Antanavicius, P. Serapinas, and P. Sirnkus, Journal ofPhysics D: Applied Physics 22: p. 254-257 (1989).  12.  H.U. Eckert, Spectrochimica Acta, Part B, 40: p. 145 (1985).  13.  M.T.. Cicerone and P.B. Farnsworth, SpectrochimicaActa, Part B, 44: p. 897 (1989).  The Temporal Complexity...  21 6  14.  J.W. Olesik, L.J. Smith, and E.J. Williamsen, Analytical Chemistry 61: p. 2002 (1989).  15.  J.W. Olesik, and E.J. Williamsen, Applied Spectroscopy 43: p. 933 (1989).  16.  J.W. Olesik, and J.C. Fister ifi, Spectrochemica Acta Part B, 46(6/7): p. 85 1-868 (1991).  17.  J.W. Olesik, and J.C.Fister III, Spectrochimica Acta,, Part B, 46B: p. 869-883 (1991).  18.  G. Horlick, G. and F. Qin, Federation ofAnalytical Chemistry and Spectroscopy Societies Meeting XVII. Cleveland, OH (1990).  19.  Furuta, N.J., Journal ofAnalyticalAtomic Spectrometry 6: p. 199 (1991).  20.  Gray, A.L. Journal ofAtomic Analytical Spectrometry 7: p. 1151-1153 (1992).  21.  R.M. Barnes and R.G. Schleicher, SpectrochimicaActa, Part B, 36: p. 8 1-101 (1981).  22.  R.M. Barnes, R.M. and J.L. Genna, Spectrochimica Acta, Part B, 36: p. 299-323 (1981).  23.  H.A. Becker and T.A. Massaro, Journal of Fluid Mechanics 31: p. 435-448 (1968).  24.  P. Yang and R.M. Barnes, Spectrochimica Acta Review 13(4): p. 275-309 (1990).  25.  Ludwig Prandtl, Essentials of Fluid Dynamics. London: Blackie and Son Limited (1952).  26.  E.R. Subbarao and B.J. Cantwell, Journal of Fluid Mechanics 245: p. 69-90 (1993).  27.  E.K. Longmire and J.K. Eaton, Journal of Fluid Mechanics 236: p. 2 17-257 (1992).  28.  L-D. Chen, L.P. Goss, W.F. Lynn and W.M. Roquemore The Structure ofJet Diffision Flames, in Turbulent Reactive Flows, S.N.B.Murthy, R. Borghi, Editors., SpringerVerlag: Berlin (1987).  29.  W.J.A. Dahm, C.E. Frieller, and G. Tryggvason, Journal of Fluid Mechanics 241: p. 371-402 (1992).  The Temporal Complexity...  21 7  30.  J.E. Martin and E. Meiburg, Journal of Fluid Mechanics 230: p. 271-318 (1991).  31.  J. Bridges, J. and F. Hussain, Journal of Fluid Mechanics 240: p. 469-501 (1992).  32.  D.M. Kyle and K.R. Sreenivasan, Journal of Fluid Mechanics 249: p. 6 19-664 (1993).  33.  F.J.M.J. Maessen, G. Kreuning, and J. Balke, Spectrochimica Acta, Part B, 41: p. 3 (1986).  34.  F.J.M.J. Maessen and G. Kreuning, SpectrochimicaActa, Part B, 44(4): p. 387-384 (1989).  35.  H. Anderson, H. Kaiser, and B. Meddings. Proceedings of the Winter Conference on Plasma Chemistry. 1980. San Juan: Heydon, London (1981).  The Temporal Complexity...  21 8  Notes  Chapter 7 A Parametric Survey of Electron Density in the Solvent Loaded Inductively Coupled Argon Plasma  7.1 Introduction  Drawing on the insight provided by the previous chapters, this one begins to explore the physical characteristics of the plasma region of the discharge; It surveys the electron number density ne (the density of unbound plasma electrons) in the tail cone of the discharge. The survey covers a range of inner argon flow rates, solvent load, and forward power. It extends over the entire volume of the plasma beyond the exit of the torch, from z  =  5 mm to 25 mm, and r  =  ±8  mm. Moreover, it explores three different solvents—water, methanol and chloroform—and then compares them with a pure argon ICAP. In order to facilitate such an extensive survey,  e  was  determined from the absolute intensity of a single argon line. This method required only two emission channels, one for the line intensity and one to subtract the continuum background. Consequently, the amount of data required to cover a given set of experimental parameters was far less than for multichannel diagnostics, such as those based on line profile measurements. This convenience made it practical to explore a very extensive parameter space. The results of the survey reveal how  e  ,  the thermal conditions, and the transport of energy respond over an  extensive, previously unexplored range of experimental parameters.  A Parametric Survey ofElectron Density  220  Before proceeding to examine how electron density varies over experimental parameters, we will first review the guidelines offered by the previous chapters for exploring the physical characteristics of the discharge. Then we will consider why three particular plasma parameters, the electron density ne, the electron temperature Te and the non equilibrium parameter b, are particularly informative in describing the physical characteristics of the ICAP. After briefly reviewing how these plasma parameters have already been investigated for ICAPs loaded with water, we will consider the theoretical basis for determining the electron density from the absolute intensity of a single argon line, and discuss why this technique holds promise for the ICAP. Finally, we will examine the results of an electron density survey based on the absolute intensity of a single argon line.  7.2 Guidelines Offered by Previous Chapters for Investigating the Physical Characteristics of the Discharge The previous four chapters fulfilled the first step in our rational strategy—they revealed the parametric, structural, and temporal complexity of the inductively coupled argon plasma. In doing so, they provided guidelines for investigating its physical characteristics. In fact, the guidelines provided by the previous chapters suggest how one may most effectively explore the temporal and spatial characteristics of the discharge. Moreover, they reveal which spectral features and which spectral windows would be most useful for spectroscopic diagnostics, and what physical processes would be worthwhile investigating. For example, the noise power spectra presented in Chapter 6 reveal periodic fluctuations in the plasma flowfield suggest. If spectroscopic diagnostic measurements were time averaged over those fluctuations, then the results could well be biased. But if the measurements were phase averaged or phase locked to the fluctuations (i.e.., sampled at discrete intervals along the period of the fluctuation), then the bias owing to time averaging could be avoided.  A Parametric Survey of Electron Density  221  Beyond guidelines for exploring the temporal characteristics, guidelines for exploring the spatial characteristics were provided by the observations of Chapter 3, and by the radially inverted images of Chapter 5. Although the results presented in these chapters are all time averaged, they reveal the specific locations in the discharge where the effects of solvent loading are most extreme, hence where further investigations should be directed. For example, they reveal that the induction region is extremely sensitive to changes in solvent plasma load. Obviously, diagnostic measurements should directed there. On the other hand, the spatial survey reveals that the plasma decay region further downstream must also be investigated—it is evident that plasma decay downstream from the torch rim follows the response of the induction region in a complex manner. In any case, it is clear that measurements cannot be restricted to a single viewing height or radial position—the discharge does not respond as a static entity whose thermal conditions can be understood simply in terms of energy consumption, dissipation and mass transport. On the contrary, further investigations should follow the guidelines offered by Chapters 3 and 5, and extend the diagnostic investigation over a wide range spatial dimensions, ideally over the entire discharge, in order to account for spatial translation and distortions. Further investigations should also follow the guidelines offered by the spectral survey in Chapter 3 a survey which revealed spectral windows and emission features which hold promise ,  for physical investigations. This survey extends from ultraviolet to near infrared emission, and reveals where one may find spectral regions free from complex background emission. In those spectral windows, diagnostic measurements of analyte intensities would be free of spectral interference from background signals. The spectral survey also reveals several intense atomic argon lines, as well as other spectral features that may yield diagnostic information. Significantly, the spectral survey reveals that many argon lines are free from spectral interference. On the other hand, the survey reveals that the intensity of the H line, a useful spectroscopic diagnostic signal for electron number density measurements, is much weaker in ICAPs loaded by solvents containing small proportions of hydrogen (e.g., CHC1 ) than for those 3 loaded with water or methanol. Moreover, the H line suffers interference from C2 emission.  A Parametric Survey of Electron Density  222  Clearly, diagnostic measurements based on the intensity and spectral lineshapes of atomic argon lines offer advantages over other the H method examined in the next chapter. Lastly, the spatial results of Chapter 5 reveal that molecular emission from C , Nj and 2 CN are not suitable for probing the physical characteristics of the solvent loaded argon plasma. In fact, chapters 4 and 5 revealed that the most intense molecular emission originates from the boundary region or outside of the atomic plasma ( see also ref. [7] of Chapter 6 and refs. [3] and [9] of Chapter 5). Consequently, molecular intensities contain information from the boundary region, but generally do not reveal the physical characteristics within the atomic plasma. This is particularly true for line of sight measurements. Diagnostic measurements based on molecular emission should therefore be reserved for plasmas supported in molecular gases, or confined to situations in which the molecular emission is intense enough to supply information from the plasma region. Apart from offering guidelines for investigating the physical characteristics of the discharge, the emission intensities reported in previous chapters offers qualitative insight into the physical properties  .  Even though the information they provide is only qualitative, it gives a  good indication of the fluid dynamics, at least when compared with the results of computer simulations reported in the literature. In fact, simulation results and temporally resolved measurements led to the proposal of flowfield model of both steady and unsteady regions. Similarly, some indication of the transport processes in the boundary regions of the discharge was provided by spatially resolved emission maps, as the C2 emission maps of Chapter 5 will attest. They reveal how solvent material is entrained into the outer argon stream. But such insight is only qualitative because emission intensities depend on both the local density of the emitting species and the local excitation conditions, properties which are species specific and unknown in general. Consequently, emission intensities alone leave us far from understanding the physical characteristics of the discharge quantitatively. In fact, they raise more questions than they answer.  But at least they bring us to the next step in our rational strategy—  investigating the fundamental physical characteristics of the discharge.  A Parametric Survey ofElectron Density  223  In summary, the previous chapters offer valuable guidelines for further investigation. They even provide qualitative insight into the physical properties of the discharge. Now the problem is to decide which physical properties or plasma parameters should be explored to obtain an adequate, quantitative, physical description of the discharge.  7.3 An Accurate Physical Description of the ICAP: Plasma Parameters A complete physical description of the ICAP must account for enormous thermal gradients, inhomogeneity, complex energy balances and the four states of matter (solid, liquid, gas, plasma) assumed by the sample material. Moreover, it must describe the fluid dynamics, the mass and energy transport processes, the collisional-radiative processes, and the thermodynamic state of the discharge. Such a description would be impractical—if not impossible—to obtain for a system as incomprehensibly complex as the ICAP. Consequently, the most reasonable question to ask right now is—what do we need to know? Within the context of this thesis, this question breaks down into the following four questions: 1. What physical properties of the plasma determine the analytical signal? 2. What plasma parameters define those physical properties? 3. How are they modulated by dynamic processes? 4. And finally, how do those plasma parameters respond to operating parameters, including solvent plasma load? We already know of two dynamic processes that disrupt the steady flow and temperature fields—aerosol vaporization and vortex shedding.  In Chapter 6, the effects of droplet  vaporization were found to be limited to ICAPs loaded with undesolvated, aqueous aerosol. The effects of particle vaporization  ( as distinct from droplet vaporization) have been shown to be  significant by other workers [11, [2]. That leaves vortex shedding to be dealt with, possibly by phase locking or phase averaging.  A Parametric Survey of Electron Density  224  Beyond macroscopic disturbances, we already now a great deal about thermal state of the discharge. (By thermal state, we simply mean the state of how energy quanta are distributed over matter according to thermal, or statistical distribution functions.  ) This knowledge is very  useful because the distribution of energy quanta over matter determines the analyte and background signal. It turns out that thermal state of the plasma is dominated by kinetics of the plasma electrons. This is true because inelastic collisions between atomic species and plasma electrons redistribute energy quanta over bound states more effectively than any other collisional or radiative process. In order to see exactly how the plasma electrons collisionally redistribute the bound states, we first need a description of the kinetic energies of the plasma electrons. Experiment has confirmed that in the ICAP, the kinetic energies of the plasma electrons follow a Maxwellian distribution [3]. Theoretical work supports this finding [4]. If the kinetic energies of the plasma electrons indeed follow a Maxwellian distribution, then their kinetic energies may be described by only one parameter—the electron kinetic temperature Te populations of bound states governed by the kinetics of the plasma electrons?  But how are the  A Parametric Survey ofElectron Density  225  When 1. the internal energy states of atoms or molecules in the gas state are rapidly redistributed by inelastic collisions, 2. the colliding particles follow a Maxwellian velocity distribution, and 3. the particles do not interact with each other beyond their collisions, then it is a consequence of classical, Maxwell Boltzmann statistics that the internal energies of the atoms and molecules are distributed according to the Maxwell Boltzmann distribution [5], [6]. According to this distribution, in the state of maximum thermodynamic probability, the number of particles n in the ith energy level is given by  g  u./kT  (7.1)  where u, is the energy of the ith energy level, Z is the partition function for bound states in the species, k is Boltzmann’s constant, T is the temperature, and g 1 is the degeneracy, or statistical weight of the energy level (an integer that specifies the number of different internal states that have the same energy). The three prerequisites listed above for the Maxwell Boltzmann distribution are closely met by electrons and atoms in the ICAP. (For example, the electron density is far too low,  <  cm, and the temperature far too high, 17 1O  >  5000K, for quantum  interactions between electrons—consequently Maxwell Boltzmann statistics apply rather than Fermi Dirac statistics). Moreover, the internal states are limited to the electronically excited, atomic bound states.  A Parametric Survey of Electron Density  226  In the ICAP, then, the Boltzmann distribution for bound states within a single ionization stage may be written  /exp,_EJ,  (7.2)  where n, and flq are the respective populations of the p and q excitation states, E and Eq are their respective excitation potentials, g and gq their statistical weights, k the Boltzmann constant, and Texc the electronic excitation temperature. In the ICAP, the Boltzmann balance is maintained by excitation and deexcitation of the bound states by electron impact:  Xq + e <  Boltzmann  Eq K.E.(1) where  X,is an atom in state p  ,  >  X, + e  (7.3)  E K.E.(2)  and Xq is the same atom in state q, and e is an electron with an  unspecified kinetic energy of K.E.(l) or K.E.(2), where K.E.(1) differs from K.E.(2) by AE = (E  —  Eq). As a result of these collisions, the excitation temperature Texc which describes the  Boltzmann distribution of bound states approaches the electron temperature of the plasma electrons.  A Parametric Survey ofElectron Density  227  If these collisional processes prevail over any other excitation or deexcitation process, and Txc indeed equals Te, then we can say that the plasma is in local Boltzmann equilibrium (LBE). In that case we may write  =  y  ex[;  Eq)]  (7.4)  Here, Te was written in place of Tec to emphasize that the Boltzmann balance (equation 7.3) is dominated by the kinetics of the plasma electrons. Beyond the Boltzmann balance, the atomic state distribution functions are more generally described by the Saha balance, which determines how energy quanta are distributed over the bound states of successive ionization stages. Similar to the Boltzmann balance, the Saha balance is maintained by inelastic collisions between plasma electrons and atomic species. Consequently, the ionization temperature I for the Saha distribution approaches the electron temperature. Unlike the Boltzmann balance, however, the Saha balance is maintained by collisional ionization and three body recombination rather than collisional excitation and deexcitation. The Saha distribution for bound states may be written  fle =  2 gitJ(22r1nekTe  )  exp  g,  (7.5)  where n is the ion ground state density, n 1 the density of the ith neutral excited state, g 1 and g the respective statistical weights, E the ionization potential, me the electron mass, n the density of excited state i for the neutral, E its excitation potential, and h Planck’s constant. ,  A Parametric Survey of Electron Density  228  Here, Te was written in place of T to emphasize that the Saha balance (equation 7.6) tends to be dominated by the kinetics of the plasma electrons. In fact, the Saha balance is maintained by collisional ionization and the corresponding reverse process of three body recombination:  1 X  +e (  Saha  E K.E.(l)  >  X  +  e  +  e  (7.6)  E K.E.(2)  where X is an atom in state i, X is the corresponding ground state ion, and e is one of two electrons with an unspecified kinetic energy of K.E.(1), K.E.(2) or K.E.(3), where K.E.(l) differs from K.E.(2)  +  K.E.(3) by AE  =  (E  —  E). If this balance prevails, and  ‘rn  indeed equals Te,  then we can say that the plasma is in local Saha equilibrium (LSE). Beyond LBE and LSE, another convenient reference state for describing the thermal state of the plasma is LIE, or local isothermal equilibrium, in which the kinetic temperature of heavy particles (atomic species) T 5 equals the kinetic temperature of the plasma electrons Te. If the ICAP were in the theoretical state of local thermal equilibrium, we would have LBE, LSE, and LIE. In other words, Texc,  ion’ 1  Te , and Tg would all be equal.  Departures from LTE It turns  Out  that the theoretical state of local thermal equilibrium (LTE) is quite useful in  describing the thermal state of the ICAP, because the ICAP approaches LTE very closely. But in order obtain an accurate description of the thermal state of the plasma, and consequently an accurate description of how excited states of analyte and background emitters are populated, we need to know how the thermal state departs from LTE.  A Parametric Survey of Electron Density  229  Departure from LIE In reality, the kinetic temperature is much cooler for the heavy particles than for the electrons. This is true because the different particles experience different forces and collisional interactions according to their mass and charge. In contrast to the heavy argon atoms and ions, unbound electrons absorb most of their energy directly from the electromagnetic field between their collisions