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Effects of rf power and tube wall temperature on plasma stability and analyte emission in furnace atomization… Rahman, Md. Mahburbur 1994

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EFFECTS OF RF POWER AND TUBE WALL TEMPERATURE ON PLASMASTABILITY AND ANALYTE EMISSION IN FURNACE ATOMIZATION PLASMAEXCITATION SPECTROMETRYbyMD. MAHBUBUR RAHMANB. Sc. , University of Dhaka, Bangladesh, 1986A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENT FOR THE DEGREE OFMASTER OF SCIENCEINTHE FACULTY OF GRADUATE STUDIESDEPARTMENT OF CHEMISTRYWe accept this dissertation as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIAAUGUST 1994© Md. Mahbubur Rahman, 1994In presenting this thesis in partial fulfillment of therequirements for anadvanced degree at the University of BritishColumbia, I agree that the Library shall make it freely availablefor reference and study. I further agree that permission forextensive copying of this thesis for scholarly purposes may begranted by the head of my department or by his or herrepresentatives. It is understood that copying or publication ofthis thesis for financial gain shall not be allowed without mywritten permission.(Signature)___Department of__________________The University of British ColumbiaVancouver, CanadaDate OUABSTRACTFurnace Atomization Plasma Excitation Spectrometry ( FAPES ) is a relativelynew emission spectrochemical method. For analyte atomization and excitation,this method employs a graphite furnace and an atmospheric pressure plasmasustained inside the furnace. The main objective of this work was tocharacterize the plasma at high rf powers, up to 150 W, during the analyteatomization cycle.The temporal response of CO and He (I) line at different rf powers showscomplex emission characteristics during the atomization step. The intensity ofCO and He (I) emission decreases suddenly at higher furnace temperatureand higher rf powers. This sudden decrease of intensity indicates theextinguishing of plasma at higher temperature as a result of the changing inpower coupling efficiency between the load impedance and output impedanceof the rf oscillator. The reflected power level also increases with increasingforward power and does not depend absolutely on the furnace walltemperature but on the temperature of rf center electrode.The spatial distribution of analyte in the plasma shows an increase inemission intensity from the center of the furnace toward the wall, reaches amaximum at 1.25 mm from the center, followed by a decrease. Both atomicabsorption and emission experiments show a non - uniform temperaturedistribution along the length of the rf electrode. In comparison to the furnacewall , the temperature lag of the rf electrode causes analyte condensation onmthe rf electrode and subsequent re-vaporization, resulting in two peaks in thetemporal response of the analyte. Analyte condensation on the rf electrode issevere at lower rf powers but at higher rf powers, for example 125 W, the rfelectrode becomes too hot to act as a second surface and, as a result, a singlepeak is observed.The effect of rf power on analyte signal is a decrease in integrated intensityfor both emission and absorption at rf powers higher than 30 W due to severalreasons including pre-atomization loss of analyte, a change in excitationcharacteristics, and an increase in ionization of analyte at higher rf powers.Furthermore, the shape of the peaks shows that the residence time for excitedAg atoms is shorter than that for ground state atoms at rt power 50 W and more.This observation suggests that some of the ground state atoms do not becomeexcited due to quenching of the plasma which is likely because of the change inpower coupling efficiency between the load impedance and output impedancedue to rapid change of temperature and / or rapid change in thermionic electrondensity in the furnace.ivTABLE OF CONTENTSAbstract iiTable of Contents ivList of Figures viiList of Abbreviations xAcknowledgments xiiChapter 1Introduction 11.1 Historical Development of FAPES 11.2 Furnace Atomization Plasma Excitation Spectrometry 81.3 Analyte Atomization 101.3.1 Electrothermal Atomizer: The Graphite Furnace 101.3.2 Atomization in Graphite Furnace 161.4 Analyte Excitation 181.4.1 Atmospheric Pressure rf Discharge 191.4.2 The rf Discharge Characteristics in FAPES 231.4.3 PlasmaTemperature 251.5 Overview of Thesis 27Chapter 2Experimental System 282.1 Instrumentation 28V2.1 .1 The Plasma Source Work - Head 302.1.2 The Atmospheric Pressure ri Discharge 322.1.3 Spectral Isolation and Detection 322.1.4 Measurements of Spatially Resolved Intensity 332.1.5 Temperature Measurements of Graphite Furnace 342.1.6 Measurements of Atomic Absorption 342.2 Data Acquisition and Processing 352.3 Experimental Method for FAPES 36Chapter 3Investigation of the Plasma Stability Using He (I) and CO EmissionLine and Reflected Power Measurements 373.1 Introduction 373.2 Calculation of Thermionic Emission from Graphite 383.3 Experimental 403.4 Results and Discussion 423.4.1 Emission Spectra for CO Line 423.4.2 Emission Spectra for He (I) Line 453.4.3 Reflected Power 503.5 Summary 54Chapter 4Temporal and Spatial Emission and Temporal AbsorptionCharacteristics of Silver in FAPES 564.1 Introduction 564.2 Experimental 584.3 Results and Discussion 594.3.1 Spatial Effect of Plasma on Analyte Emission 59vi4.3.2 Effect of Plasma Power on Analyte Emission andAbsorption 664.4 Summary 80Chapter 5Conclusions 82Bibliography 87viiLIST OF FIGURESFIGURES PAGE1.1. Schematic representation of FANES source. (Adapted fromH. Falk, E. Hoffmann and Ch. Ludke, Spectrochim. Acta, 283,39B (1984), with permission of Pergamon Journal Inc.) 51.2. Schematic representation of HA-FANES source. (Adaptedfrom N. E. Ballou, D. L. Styris and J. M. Harnly, J. Anal. At.Spectrom., 1141, 3, (1988), with permission of the RoyalSociety of Chemistry. ) 71.3. Schematic representation of FAPES source. (Adapted from D.C. Liang and M. W. Blades, Spectrochim. Acta, 1049,44B, (1989), with permission of Pergamon Journal Inc. ) 91.4. Schematic Representation of L’vov Furnace. (Adapted from B.V. L’vov, Spectrochim. Acta, 53, 24B, (1969), withpermission of Pergamon Journal Inc.) 121.5. Schematic Representation of the L’vov Platform inside thegraphite furnace 141.6. Schematic Representation of the Massmann GraphiteFurnace. (Adapted from H. Massmann, Spectrochim. Acta,215, 23B (1968), with permission of Pergamon Journal Inc.) 151.7. Simplified Diagram of an rf Discharge System 232.1. A schematic diagram of the experimental system 29vm2.2. Schematic Diagram of the Plasma Source Work-Head 313.1. Electron flax for graphite and tungsten as a function offurnace wall temperature 393.2. Temporal emission behavior for CO at rf power of 30, 50, 75and 100 W and time-temperature profile for furnace wall 443.3. Temporal emission behavior for He (I) line at rf power of 30,50, 75 and 100 Wand time-temperature profile for furnacewall 473.4. Discharge voltage as a function of the cathodetemperature for helium at 9 hPa. (Adapted from H.Falket ai’., Prog. Analyt. Spectros., 417,11(1988), with permissionof Pergamon Journal Inc. ) 493.5. Intensity of the He3 318.774 nm line as a function of thecathode temperature at a discharge current intensity for 40mA and pressure of 9, 13, 27, 40 hPa. (Adapted fromH.Falket a!., Prog. Analyt. Spectros., 417, 11(1988), withpermission of Pergamon Journal Inc. ) 503.6. Temporal behavior for reflected power at ri powers of 30, 50,75 and 100W along with time-temperature profile for furnacewall 524.1. Temporal response of the Ag emission signal at an rf powerof 30 W for 3 ng of Ag deposited on the furnace wall whenthe monochromator is focused at 0.0 mm (A), 0.25 mm ( B ),0.75 mm (C), 1.25 mm (D), 1.75 mm (E), 2.25 mm (F) and2.75 mm (G ) with respect to the furnace center; and theTemperature-Time profile for the furnace wall (H) 63x4.2. Spatial response of the Ag emission at an rf power of 30 Wfor 3 ng of Ag deposited on the furnace wall. A : Emission(height) intensity Vs radial distance, B: Emission ( area)intensity Vs radial distance 654.3. Temporal response of the Ag atomic emission signal for 1 ngof Ag deposited on the furnace wall at an rf power of 30 (A),50 (B), 75 (C), 100 (D), 125 (E) and 150 (F) W 684.4. Effect of plasma power on emission signal for 1 ng of Agdeposited on the furnace wall at an rf power of 0, 30, 50, 75,100,125 andl5OW 694.5. Temporal response of the Ag emission and absorptionsignal for 1 ng of Ag deposited on the furnace wall at an rfpower of 0 W (A,only absorption), 30 W (B), 50 W (C), 75 W(D), 100W (E), 125W (F) and 150W (G) ; 4.5.H :TheTemperature-Time profile for the furnace wall 754.6. Ratio of absorption and emission as a function of time for 1ng of Ag deposited on the furnace wall at plasma power of30,50, 75, 100, 125 and 150W 784.7. Effect of plasma power on absorption signal for 1 ng of Agdeposited on the furnace wall at plasma power of 0, 30, 50,75, 100, 125 and 150 W 79xLIST OF ABBREVIATIONSAAS atomic absorption spectrometrya. c. alternating currentADC analog - to - digital converterAES atomic emission spectrometryAPF-CCP atmospheric pressure furnace capacitively coupledplasmaCFAES carbon furnace atomic emission spectrometryCMP capacitive microwave plasmaCRA carbon rod atomizerd. c. direct currentEIE easily ionizable elementETAAS electrothermal atomic absorption spectrometryFAPES furnace atomization plasma excitation spectrometryFANES furnace atomization non-thermal excitationspectrometryFWHM full - width of half maximumGD glow dischargeGE graphite furnaceGFAAS graphite furnace atomic absorption spectrometry)HA hollow anodeHC hollow cathodeHCL hollow cathode lampHGA heated graphite atomizerICC integrated contact cuvetteICP inductively coupled plasmaICP-OES inductively coupled plasma optical emissionspectrometryLTE local thermodynamic equilibriumPMT photomultiplier tuberi radio frequencySTPF stabilized temperature platform furnaceTe electron kinetic temperatureTexe excitation temperatureTE thermal equilibriumTg gas kinetic temperatureT0 ionization temperaturev/v volume - to - volume ratioiL micro litter)ACKNOWLEDGMENTSI would like to express my deepest sense of gratitude and sincere appreciationto my research supervisor, Dr. M. W. Blades, Professor, Department ofChemistry, University of British Columbia, for his invaluable and scholasticguidance, constructive criticism, reviewing the manuscript and constantencouragement throughout the course of this projectSpecial thanks to the various members of the research group, both for theirbeneficial discussions and their camaraderie - in particular Charles LeBlancand Doug Weir.Finally , I would like to thank my wife for her patience and encouragementduring the course and preparation of this thesis.1CHAPTER 1INTRODUCTIONProgress in many areas of analytical chemistry is made possible bytechnological development often based on advancements in fundamentalscientific understanding. Analytical atomic spectroscopy is one of the importantbranches of analytical chemistry wherein the analytical spectroscopistdevelops, improves , characterizes, and then applies spectroscopic sources.The main objective of the work described in this thesis was to characterizeand to examine the stability of the radio frequency (rf) helium plasma source,operated at 13.56 MHz, in Furnace Atomization Plasma Emission Spectrometry(FAPES) as a spectrochemical source for elemental analysis.This chapter is an introduction to FAPES and some special topics related tothe work described in this thesis.1.1 HISTORICAL DEVELOPMENT OF FAPESThe classic publication of Alan Walsh on Atomic Absorption Spectrometry(AAS), describing flame AAS [1], was a revolution for atomic spectrometricmethods. As a result of this publication, flame AAS became the most widely2used spectrometric method for the determination of metallic elements, duringthe 1960’s and 1970’s. Even today, flame AAS is a useful method for a varietyof analyses. Flame AAS has some common interferences, for exampleionization , physical and chemical interferences, however these can be easilycontrolled.The first systematic investigation of Graphite Furnace Atomic Absorption(GFAAS) was carried out in 1961 by L’vov [ 2]. Unlike the flame MS method,wherein Walsh employed continuous nebulization of the sample into a flame toprovide a steady state absorption signal, L’vov introduced a small samplevolume, which was converted to an atomic vapor inside an electrically-heatedgraphite furnace. This graphite furnace method has very good detection limitsfor absolute amounts and there are very few atomic techniques which cansuccessfully compete with those detection levels [3]For many years, GEMS has been recognized as one of the most sensitiveanalytical techniques for elemental analysis [4]. On the basis of absolute mass,GFAAS detection limits are very low because sample volumes are small (5-100p1), analyte transport efficiency is high (90-100%), and analyte residence timein the observation volume is relatively long (0.1-0.5 s). A limitation of GFAASarises as a result of interferences. The interferences have been classified asinterferences due to background absorption, condensed phase interferences,vapor phase interferences, and effects due to gas expansion [5-7]. Thecombination of a thermal pre-treatment step, temporal and spatial isothermalatomization through the use of stabilized temperature platform furnaces (STPF),rapid heating cycles, probe insertion, two-step furnaces, and background3correction techniques such as Zeeman, Smith-Hieftje and the use of continuumlamps have enabled sensitive determinations of a variety of complex samples.However, interferences continue to limit the effectiveness of GFAAS and,although the use of hollow cathode lamps as primary sources provides highspectral selectivity, they introduce the limitation that restricts GFAAS to beingessentially a single element technique.There was a flurry of interest in the spectrochemical application ofcapacitively coupled plasmas (CCP) in the late 1950s and early 1960s [ 8,9],but interest shifted to inductively coupled plasmas (ICP) in about 1964-1965 asa result of the landmark papers by Wendt and Fassel [10] and Greenfield et a!.[11] describing Inductively Coupled Plasma Optical Emission Spectrometry(ICP-OES). Unlike the AAS method, emission spectroscopic methods such asICP-OES are inherently multi-elemental techniques. The ICP is also relativelyfree from interferences since refractory oxides only partially dissociated in theflame are completely dissociated at high ICP temperatures. However, on aconcentration basis, ICP detection limits are 10 to 100 times higher, and on anabsolute basis, 1000 times higher than GFAAS [3].A variety of approaches has been investigated for developing the graphitefurnace into a source capable of carrying out simultaneous, multielementanalysis [12]. The primary thrust behind these approaches has been to combinethe excellent transport and residence time characteristics of the graphitefurnace with some means of exciting the atomized analyte vapor inside thefurnace. In this way atomic emission spectrometry (AES) can be used as adetection method. Littlejohn and Ottaway [13] have described carbon furnace4atomic emission spectrometry (CFAES) which is a sensitive technique for traceanalysis using thermal excitation from normal furnace heating. However, thismethod is limited by the energy available for thermal excitation. For elementshaving resonance wavelengths below 300 nm, the detection limits are muchpoorer than those for GFAAS [14]. Furthermore, at temperatures above 2500 K,the intense emission from the furnace wall is a major source of spectralbackground in the visible region [15].Falk and his co-workers [16-18] developed a furnace atomization non-thermal excitation source (FANES or HC-FANES). This source was based onusing a low pressure, direct current (d.c.) glow discharge sustained inside agraphite furnace, which operated as a hollow cathode (HG) , and a point or ringexternal to the furnace as an anode. Figure 1.1 is a schematic diagram of theFANES source. The furnace was a Massamann-type and was graphite orpyrolytic-graphite coated graphite. Microliter volumes of sample was depositedonto the inner wall of the graphite furnace, and was vaporized into thedischarge during a high temperature atomization step . Analyte atoms wereexcited in the discharge , emitting characteristic line spectra that could be usedfor sim ultaneous multi-elemental analysis.It is well known that Penning and asymmetric charge transfer reactions areprevalent in low pressure plasma sources and this characteristic, coupled withcollisions from energetic electrons, enabled the FANES source to excite atoms(and molecules) such that a wide variety of metals and non-metals could bedetermined (for example, with an absolute detection limit for Na of 0.0007 ng/ml[17]). An additional advantage to the FANES approach was that the atomization5and excitation processes were independent of each other and could thus beindependently optimized. However, because of the low pressure requirement ofthe glow discharge operation, sample introduction is somewhat laborious andtime consuming.Graphite furnacehollow cathodeI IAnal yte-— emissiont_________AnodetGraphite furnacepower supplyFigurel.1: Schematic representation of FANES source. (Adapted from H.Falk, E. Hoffmann and Ch. Ludke, Spectrochim. Acta, 283, 39B(1984), with permission of Pergamon Journal Inc.)6Ballou et al. [19-21] described a hollow-anode plasma excitation sourcewhich was conceptually very similar to the FANES source called HA-FANES.Figure 1.2 illustrates a schematic diagram of the HA-FANES source. For thissource the graphite furnace formed the anode of a glow discharge and thecathode was a graphite rod which was oriented co-axially with the furnace andextended the entire length of the furnace. The graphite furnace was anintegrated contact cuvette and the axial electrode was a pyrolytic-graphitecoated rod. The rationale for this new design was that it simplifies electricalshielding requirements and enhances the reliability of operations. Theoperation of this source is similar to that of FANES and it suffers disadvantagessimilar to the FANES source with respect to operation at reduced pressure(<200 torr).Liang and Blades first reported an atmospheric pressure radio frequency (rf)plasma source inside the graphite furnace for analyte excitation [221. Thegeometric arrangement of the electrodes in the atmospheric pressure rf plasmawas very similar to that of the electrodes in HA-FANES. The spectrometricmethod with such an rf plasma source has been designated FurnaceAtomization Plasma Excitation Spectrometry (FAPES) [23,24]. Compared withlow pressure operation, atmospheric pressure operation was expected to offerconvenient sample introduction and increased residence time of analyte atomswithin the graphite furnace [22]. The FAPES source, which can be maintained atfrequencies of 13.56-50 MHz and rf powers of 5-600 W, is utilized to excite anatomic vapor produced from a normal graphite furnace atomization heatingcycle. When coupled with a direct reading spectrometer the FAPES sourcecould be used effectively as a means of carrying out simultaneous, multielement7determinations on small sample sizes in a manner similar to the use of GFAASwhile maintaining simi’ar detection sensitivity. A detailed description of FAPESis given in the next section.cathodeFigurel.2: Schematic representation of HA-FANES source.(Adapted from N. E. Ballou, D. L. Styris and J. M.Harnly, J. Anal. At. Spectrom., 1141, 3, (1988), withpermission of the Royal Society of Chemistry. )t integrated contactcuvette: hoUow anodeto d.c. power supply81.2 FURNACE ATOMIZATION PLASMA EXCITATION SPECTROMETRYA schematic diagram of FAPES source described by Liang and Blades is shownin Figure 1.3 [22]. The plasma source consisted of a conventional Massmanntype graphite furnace work-head (modified Instrumentation laboratory, model IL455), and a co-axial graphite electrode of 1 mm in diameter and 40 mm inlength (Ringsdorff-Werke, FRG) connected to an rf connector. The rear opticalwindow of the furnace work-head (through which light from the hollow cathodelamp is normally directed) was removed and replaced with an rf connector. Theoperating frequency was 27 MHz and the rf power delivered to the plasma wasabout 20 W, and helium was used as the plasma gas. Liang and Bladessuggested that the mode of power coupling to the plasma was primarilycapacitive in nature [22]. At first, Liang and Blades designated this plasmasource an Atmospheric Pressure Furnace Capacitively Coupled Plasma (APFCCP), however, this plasma source is now more commonly recognized as aFAPES by the acronym in the literature [25].Liang and Blades tested the source using a small brass chip weighingabout 5 mg. The emission spectra of Cu and Zn were recorded at 800 0C offurnace temperature with 20 W plasma power. They suggested that at low rfpowers, the dominant sampling mechanism was rf sputtering [22]. A relativelyhigh gas temperature and, as a consequence, a reduction in gas-phasechemical interferences compared with GFAAS, were expected for FAPES96rphite Furnece•I I I Iti’__I I I rfTo rf powersupply tTo furnace powersupplyFigurel.3: Schematic representation of FAPES source. (Adapted fromD. C. Liang and M. W. Blades, Spectrochim. Acta, 1049,44B, (1989), with permission of Pergamon Journal Inc.)Sturgeon et al. used a conventional Perkin-Elmer furnace (Model HGA-500)with a co-axial graphite rod [23]. External air was prevented from reaching theinterior of the furnace by the positive pressure of the support gas through aninternal flow through the furnace, which could be halted during the atomizationcycle, and a continuous external flow around the furnace. The power deliveredto the helium plasma was about 50 to 70 W. A 10 pi aliquot of test solution,containing Cd or Mn, was deposited on the furnace wall and subjected to theatomization cycle as in GFAAS. The plasma background emission and transientemission signals for Cd and Mn were given. The detection limits for Cd and Mnwere 36 pg and 52 pg respectively.101.3 ANALYTE ATOMIZATIONSensitivity of atomic absorption or emission analysis varies with the fraction ofthe analyte atoms that are found in the light path or emit the characteristicwavelength at one time if peak height is being measured. If peak area is beingmeasured, the sensitivity is proportional to the average residence time of theanalyte atoms. An ideal atomizer would provide complete atomization of theelement of interest irrespective of the sample matrix. For the lowest possibledetection limits, at least for absorption the atomic vapor should not be highlydiluted by the atomizer gas so that a large ground-state neutral atom populationis produced.1.3.1 Electrothermal Atomizer: The Graphite FurnaceWith a graphite furnace a discrete sample is deposited and the furnace iselectrically heated to produce a transient cloud of atomic vapor. To prevent theoxidation of graphite at high temperatures, the furnace is coated with a thinlayer of pyrolytic-graphite during manufacture and purged with an inert gas.The sample is placed on the furnace wall (wall atomization) or on a separatedevice inserted into the furnace (platform atomization). In the case of platformatomization, the platform is primarily heated by the radiation from the furnacewall. Typically, the furnace is heated in three stages in which the temperature ofthe furnace is increased progressively by passing larger currents through the11atomizer tube. The first step is drying or desolvation step, in which a sufficientcurrent causes the furnace temperature to be increased and maintained atabout 110 OC. During this stage, the solvent is evaporated leaving a solidresidue in the furnace. The second step is ash step, in which the power supplycurrent is increased, so that the furnace temperature is raised, typically to 350-1200 °C. During this stage, organic matter in the sample is ashed or convertedto H20 and CC2, and volatile inorganic components are vaporized. The finalstep is the atomization step. During this stage the temperature of the furnace(atomization temperature) can reach a maximum as high as 3000 °C and thesample is vaporized and atomized to produce a atomic vapor cloud inside thefurnace.There are two kinds of non-flame atomizers which have received extensiveexperimental study. The first successful non-flame atomizer in the form of acarbon-rod electrode and a graphite furnace described by L’vov [2,26] is shownschematically in Figure 1.4. The graphite furnace was 30 to 50 mm long with aninner diameter of 2.5 to 5 mm. The furnace was heated up to about 2500 K.Sample was introduced into the furnace on a carbon rod electrode 6 mmdiameter. The head of the electrode was shaped to fit an orifice in the wall of thegraphite furnace. The graphite furnace was heated for 20 to 30 s and theelectrode was moved into the orifice of the graphite furnace. Auxiliary electricalheating of the electrode was turned on for 2 to 3 s and absorption wasmeasured. The electrode was then lowered away from the graphite furnace andthe system was ready for the introduction of another electrode carrying thesample. To prevent penetration of the atomic vapor into the wall of the furnace,the inner wall was coated with a thin layer of pyrolytic-graphite.12Graphite furnace_1 I>LihgtpathI__I IWater cooled ends—Carbon-rod electrodeFigurel.4: Schematic Representation of L’vov Furnace. (Adapted fromB. V. L’vov, Spectrochim. Acta, 53, 24B, (1969), withpermission of Pergamon Journal Inc.)Although the power to the furnace is stepped almost instantaneously to itsselected value during the atomization step, it takes a finite time for the furnacetemperature to reach its equilibrium value. When the wall of the furnace onwhich the sample is deposited reaches a critical temperature called the“appearance temperature”, analyte vaporizes off the surface. The appearance13temperature depends on the analyte, the analyte concentration, and thesample matrix. The gas inside the tube is at a lower temperature than thefurnace walls, so that atomized analyte atoms may suffer compound formationafter vaporization. To alleviate these problems, L’vov was the first to employ agraphite platform installed inside the furnace from which the sample wasvaporized rather than from the furnace wall [27]. This graphite platform issometimes called the “L’vov Platform” or the “Stabilized Temperature Platform”(STPF) [28,29]. A schematic diagram of the L’vov platform is shown in Figure1.5. The platform is heated primarily by radiation from the furnace during itsheating cycle, so the temperature of the platform (and hence the sampletemperature) lags the furnace wall temperature. Therefore, analyte vaporizationand atomization are delayed until the gas-phase temperature within the furnacereaches the atomization temperature. In addition, due to the temperaturedifference between the furnace wall and the platform, a higher heating rate forthe platform is achieved in the initial stages of the atomization step[27].14Lvov platformGraphitefurnaceFigure 1.5: Schematic Representation of the L’vov Platform inside thegraphite furnace.Another successful non-flame atomizer is the Massmann furnace [30] shownin Figure 1.6. It is a simplified version of the L’vov graphite furnace. TheMassmann furnace consists of a straight graphite tube of 55 mm in length, 6.5mm in internal diameter and a wall thickness of 1.5 mm. The atomizer tube issupported at the ends by water-cooled electrodes. Liquid samples aredeposited onto the inner furnace wall through a small hole in the center of thefurnace by means of a micro-pipette, while solid samples were inserted fromone side of the furnace. The temperature of the furnace could reach 2900 K15within a few seconds. The furnace was enclosed in a chamber and purged withAr.Sample introductionholeFigure 1.6: Schematic Representation of the Massmann GraphiteFurnace. (Adapted from H. Massmann, Spectrochim. Acta,215, 23B (1968), with permission of Pergamon Journal Inc.)Two major limitations are inherent in the Massmann-type furnaces, temporalnon-isothermality and spatial non-isothermality [30]. Temporal non-isothermalityoccurs when analyte atoms appear in the observation volume during a time inwhich the temperature in the gas-phase is low and is changing rapidly. Theatomization range within which atoms persist in the furnace (and hence thethGraphite furnaceWater coo’ed endsresidence time of analyte atoms) is dependent on the nature of the analyte and16the accompanying matrix . As a result, the degree of atomization is often lowand matrix dependent [27]. Spatial non-isothermality is the non-uniformtemperature distribution along the furnace length and is caused by the heatingcharacteristics of the water cooled ends of the furnace. This non-uniformityresults in vapor condensation on the cooler end-regions of the furnace [25]. Inaddition, recombination of the sample vapor leaving the furnace through thecooler end-regions is a major contribution to spectral interferences in AAS [26].Spatial non-isothermality of the furnace also has a severe effect in CFAES dueto self-absorption [31].Temporal non-isothermality can be removed by using L’vov platform withconventional end-heated Massmann-type furnaces but not the spatial nonisothermality. Spatial non-isothermality can be eliminated by employing a side-heated Integrated Contact Cuvette (ICC) [32]. In the ICC, the full length of thefurnace starts to heat at the same time (transverse heating), and therefore,achieves spatial isothermality. All the work described in this thesis was carriedout using an ICC type furnace.1.3.2 Atomization in Graphite FurnaceMany studies on atomization mechanisms have been reported in the literatureand various proposed mechanisms have been subjected to considerabledebate over the years. The atomization mechanisms are very complex in natureand depend on the nature of the analyte, the accompanying matrix and, of17course, on the atomizer characteristics. When the analyte sample is in a nitricacid solution, the oxide of the analyte is formed from the nitrate during theashing step or prior the atomization step. Some proposed atomizationmechanisms are summarized below.Mechanism I. Carbon ReductionReduction VaporizationMO(S) Ms1 + CO(g) > M(g)In this atomization process, the analyte oxide is reduced by carbon in thefurnace wall to form analyte atoms either in solid or liquid form which are thenvaporized to the gas phase.Mechanism II. Thermal DissociationThermal VaporizationMO(S) > M(SD > M(g)Dissociation 1/2 02In this case, the analyte oxide dissociate thermally on the furnace wallfollowed by the vaporization of the analyte atoms.Mechanism Ill. Dissociation of Oxide Vapor18Vaporization ThermalMO(S) > MO(g,) > M(g) + 1/202Here, the analyte oxides vaporizes first from the furnace wall and followedby thermal dissociation producing analyte atoms in the gas phase. The gas-phase dissociation equilibrium of the analyte may be affected by the amount ofoxygen which, in turn, is determined by the amount of CO in the gas-phase.1.4 ANALYTE EXCITATIONFor spectrochemical analysis, plasma discharges have been widely usedduring the past three decades. Plasma sources currently in use asspectroscopic sources include dc (Arc) and ac (Spark) plasmas, inductivelycoupled plasmas (ICP), microwave-induced plasmas (MIP), capacitivelycoupled plasmas (CCP), Capacitive microwave plasmas (CMP), glowdischarges (GD), flowing afterglows, Theta-pinch discharges, exploding filmsand wires and laser-produced plasmas.Without a doubt, the ICP is currently the most important plasma device usedin the field of analytical atomic spectrometry. It has been successfully utilized asa source of atomic emission, absorption, fluorescence as well as the ion sourcefor mass spectrometry. Capacitively coupled plasmas (CCP) at low pressurehave been investigated extensively during the past couple of decades, mainly19because of their widespread use in plasma processing of semiconductors.However, this low pressure plasma discharge has been largely ignored by theanalytical spectroscopy community mainly because of risk of contamination anddifficulty of operation. Atmospheric pressure discharges are less prone tocontamination and easier to operate. This was one of the main thrusts behindthe development of atmospheric pressure CCP. The atmospheric pressure CCPoperates using a variety of support gases including helium. This discharge isrelatively simple to construct and operate; offer similar characteristics tomicrowave induced plasma (MIP); and in some cases inductive coupled plasma(ICP) and in many respects is much more versatile in terms of the mode ofoperation when compared with the other plasma discharges. Recently, rfdischarge at low pressure has also been employed in mass spectrometry [33]and emission spectrometry [34].1.4.1 : Atmospheric Pressure rf DischargeWhen considering the electrode disposition with respect to the plasmadischarge, there are basically two types of capacitively coupled rf discharge:those with one or more electrodes in contact with the plasma and those withelectrodes isolated from the plasma by a dielectric wall, normally quartz orsilica glass. Schwab at el. [35-38] have studied the properties of rf capacitivegas discharge at atmospheric pressure in which the electrodes were in contactwith the plasma. He reported that these plasmas could operate in one of the twobasic modes: glow discharge and arc discharge.20The type of plasma formed, arc or glow, depends on the type of materialused for the electrodes, the condition of the surface, the current and thedischarge gas. The two can be easily distinguished visually or from the current-voltage characteristics. The negative glow attaches to the negatively poweredelectrode but since the field is reversing rapidly both the electrodes exhibit aglow region.For glow discharge, let us consider (for simplicity) two electrodes of equalarea at a certain distance apart at low pressure. If a sufficient d. c. voltage isapplied, a discharge strikes between the electrodes. In this discharge a cathodedark space and a glow can be seen. A cathode fall potential develops acrossthe dark space, leaving the glow space nearly field free. For self-sustainment(current continuity) of the discharge, a steady state electron concentration mustbe maintained. In a d.c. glow discharge , this electron concentration is causedmainly by secondary emission through positive ion bombardment on thecathode.If a low frequency alternating voltage is applied, instead of a d. c. voltage,the discharge behaves as though it has two alternating cathodes. This system isa succession of short-lived d. c. discharges, because at low frequencies there isample time for the discharge to become fully extinguished. The discharge isextinguished when the cathode potential drops below the discharge sustainvalue because of the build up of a self-bias d. c. potential on the cathode. In thecase of a d. c. discharge, the potential at the cathode is equal to the appliedpotential difference between the two electrodes but in an alternating currentdischarge, the d. c. potential at the electrodes is a self-bias voltage. This self-21bias voltage forms as a result of the differential mobility of electrons and positiveions in the discharge. Electrons collect on an electrode whenever the electrodebecomes positive with respect to the glow space. Since capacitively coupledplasmas are operated using a capacitor connected in series between the drivenelectrode and rf power supply or impedance matcher called a blockingcapacitor, the bias voltage does not bled away through the power supply.If the frequency of the applied voltage is increased, it is observed that theminimum pressure at which the discharge sustains is reduced [39]. Thisreduction indicates that there is an additional source of ionization other thansecondary electron emission from the electrodes. This additional source isresulted when electrons, oscillating in the time dependent electric field, undergocollisions with the plasma gas atoms to cause ionization. Therefore, the highvoltage electrode that is necessary in a d. c. glow discharge for the secondaryelectron emission is not required to sustain the rf discharge [40]. Furthermore,the cathode glow attached to each electrode is the same as in the d. c. case.In addition to the frequency of the discharge, the pressure is also animportant parameter and affects the discharge characteristics in two ways.Firstly, in a low pressure discharge, the mean free path of the electrons and ionsis long. The electric field in the cathode dark space causes the acceleration ofpositive ions through the dark space toward the cathode. These acceleratedpositive ions impinge on the cathode and cause sputtering of the electrodematerial and the emission of secondary electrons. However, at atmosphericpressure, the mean free path of the ions is short and therefore, ions can be22considered as stationary. Secondly, the high pressure discharge is essentially alow current glow discharge. Transition from glow to arc discharge can beproduced by an increase in current under constant pressure (or by an increasein pressure at constant current) and a considerable fall in the discharge voltagewhich is associated with a change in the electron emission mechanism from thecathode [41]. In the case of an arc discharge, electron emission from thecathode is mainly thermionic and field emission.In an atmospheric pressure discharge, a positive space-charge region isbuilt up in front of the appropriate electrode in each negative half-cycle. Atfrequencies above 1 MHz, the length of this space-charge region does notexceed 5x103 cm (in air), neither does the mean free path of an ion [36]. Thevoltage of the rf discharge depends on the nature and the distance between theelectrodes. The time scale of the application of the rf voltage is such thatfrequencies in the order of 1 MHz and above result in a pseudo-continuousplasma. The reignition voltage in each half cycle is dependent on the electrodedistance. It has also been reported that the reignition voltage drops at a certainelectrode distance [38]. This observed drop in the reignition voltage wasattributed to the residual charge carriers at longer electrode distances and wasnot due to the space charge effect [38].If the current density increases, an ri arc discharge can be observed which isbrighter in appearance than a glow [35] and usually it does not maintain astable position, but moves around on the electrode surface. The rf glow can turntemporary into an arc at any time. The transition to the arc is also favored by23conditions that facilitate electron emission, for example rough electrode surfaceand salt deposits. For analytical purposes, a glow discharge is preferred sinceexcitation conditions are more uniform and stable, and hence the precision andaccuracy should be superior. Figure 1.7 shows the main components of an rfdischarge system which consists of an rf power supply, a matching network andthe discharge.Figure 1.7: Simplified Diagram of an ii Discharge System1.4.2 The rf Discharge Characteristics in FAPESThe FAPES source enables the formation of He or Ar plasmas at atmosphericpressure inside an otherwise normal graphite furnace atomizer. The plasma isformed and sustained at atmospheric pressure by placing high voltage rf24excitation on a conductive electrode located inside the graphite furnace with aco-axial geometry, while maintaining the furnace at virtual ground. The plasmasource within the graphite furnace is a bright region surrounding the ri electrodeand a less intense plasma fills the remainder of the graphite furnace volume. ftis also observed that when the rf power coupled to the plasma is increased, thebright region surrounding the electrode extends along the rf electrode beyondthe length of the graphite furnace. The appearance of the extended rf glowalong the rf electrode marks the onset of arcing between the rf electrode and thefurnace wall. Voltage-current characteristics of the rf discharge in FAPES arenot known yet and a mathematical model describing the discharge has notbeen developed.It is interesting to note the use of rf discharge at atmospheric pressure inatomic spectrometry. Firstly, the discharge inside the graphite furnace is usedonly for the analyte excitation process. The analyte is introduced into theexcitation volume during the high temperature atomization step of the furnace.Secondly, the discharge contains unequal electrode areas causing differentcurrent densities and electric field strengths on each electrode during each halfcycle. Therefore, this FAPES plasma source is not radially symmetric along thefurnace length. Finally, the discharge contains hot electrodes with varyingtemperature up to about 3000 OC depending on the experimental conditions.However, depending on the rf power and the furnace temperature, the rf plasmaappears to change into an rf arc with the evolution of the thermionic electrons.251.4.3: Plasma TemperatureThe density of excited analyte atoms in a particular energy state in a source isdetermined by the dissociation equilibrium ( Guldberg-Waage Distribution ),population factor ( Boltzmann Distribution ) and degree of ionization ( SahaDistribution ). Temperature is the most important parameter governing theabove equilibria. Temperature also changes the full-width of half-maximum(FWHM) of line profile which affects the sensitivity and linear dynamic range ofa calibration curve for AAS. Both temporal and spatial isothermal operation isimportant for GF-AAS to obtain freedom from interferences, and to have highsensitivity, and precise analysis. Temperature is an important parameter in thecontrol of the processes of diffusion, convection, and gas expansion.Furthermore, background intensity and signal-to-noise ratio are dependent onthe temperature in the atomizer and source.Relatively high temperature plasma sources such as ICPs give rise tointense line emission for the analyte and low matrix interference effects in theanalytical determinations. This is why high temperature plasma sources areconsidered to posses superior analytical merit compared with those of lowtemperature plasmas. A unique temperature for a system can be specified onlyif the system is in a state of thermal equilibrium.If thermal equilibrium is not established, a single temperature can not beassigned to the plasma. This non-thermal equilibrium leads to several differentdefinitions of temperature in the plasma depending on the species consideredwhich are : Electron Kinetic Temperature (Te, from the Maxwell velocity26distribution), Gas Kinetic Temperature (Tg, from the Maxwell velocitydistribution) Excitation Temperature (Texe, from the Boltzmann energy statedistribution) and Ionization Temperature (T0, from the Saha equation).However, when the source is in a state of local thermodynamic equilibrium(LTE), a unique temperature can be defined for each point in the source butallowing for the possibility of different temperatures at different points. A morecomprehensive description of plasma temperatures and the relevant distributionfunctions are available elsewhere [42, 43].271.5 OVERVIEW OF THESISThe work described in this thesis is focused on further characterization ofFurnace Atomization Plasma Excitation Spectrometry as a spectrochemicalsource for elemental analysis. In the next chapter a brief description of theexperimental system used for this thesis is given. This experimental system canbe used to acquire two data channels simultaneously.Chapter three deals with the investigation of the plasma stability. Theemission behavior of a He (I) and CO line are studied at different rf powers.Reflected power for different forward rf powers are measured. The effect ofthermionic electron emission and furnace wall temperature on the rf plasma arealso discussed in this chapterSpatial and temporal emission characteristics of the rf plasma for Ag arediscussed in chapter four . Both atomic absorption and emission are measuredto study the effect of rf power on the temporal response of the analyte emissionsignal . The effect of ri power on integrated emission signal and the quenchingof plasma at higher rf power is also discussed in this chapter.In chapter five, the conclusions are presented, some limitations are pointedout and some recommendations are made to improve the presentinstrumentation of FAPES.28CHAPTER 2EXPERIMENTAL SYSTEMAn experimental system capable of measuring simultaneous atomic emissionand atomic absorption was used to study the plasma stability for FAPES as wellas to study the time-resolved and spatially-resolved behavior of the analyte. Abrief discussion of instrumentation, data acquisition, data processing and aswell as experimental method for FAPES is presented in this chapter.2.1 INSTRUMENTATIONA schematic diagram of the experimental system is depicted in Fig. 2.1. Themain components of the FAPES source assembly were an rf power supply, an rfmatching network, a plasma source work-head, and a furnace power supply forresistance-heating the graphite furnace. In addition, two lenses ( Li and L2 ), amonochromator, a photomultiplier tube ( PMT ), and a current amplifier wereused for the spectral isolation and detection along with an analog-to-digitalconverter (ADC) and a computer ( PC/AT) for the data acquisition.In figure 2.1 the ADC data channels are labeled as Cl and C2. When theatomic absorption signal was measured, a lock-in-amplifier was employed todetect the hollow cathode lamp (HCL) signal from the PMT output. For other-E_(3.jMonochromatorPMTII-Amp.:TRG”C2PtIADCIPlesmeSourceWork—heed—fHCLPovrSup.]ILock-in-AmpUferClPC1ATFig.2.1:ASchematicDiagramoftheExperimentalSystem30experiments, Cl was connected to the output from the corresponding signaltransducer, for example; optical pyrometer, to acquire the data. The “TRG” inthe figure represents the trigger channel for the ADC.2. 1. 1 The Plasma Source Work-HeadA schematic diagram of the plasma source work-head is depicted in Fig. 2.2.The work-head was a five-way hollow cube made of aluminum containing apyrolytic-graphite coated integrated contact cuvette (ICC), a tungsten co-axialrod, and a high current furnace support structure made of copper and MacorTM.The hollow five-way cube was 6 x 6 x 6 inch with 5 inch diameter “O”-ringsealed aluminum flange. The ICC was 19 mm long, 5.7 mm in internal diameterand 7.1 mm in outer diameter. The co-axial rod was 0.9 mm in diameter andextended up to the full length of the graphite furnace.The furnace support structure and the water cooling system of the graphitefurnace were similar to the method described by Ballou et a!. [1 9].The graphitefurnace was resistance-heated by using a furnace power supply ( Model lL-655,formerly Instrumentation Laboratory; now Thermo-Jarrell Ash, Waltham, MA,USA). The plasma gas was directed into the work-head through a small inlet onthe flange with the furnace support system and out through a small outletlocated on the opposite flange.The co-axial rod was attached to the rf connector using a “vertical mount”,where it was powered using two vertical metal blades ( as in Figure 2.2 ). Thiskind of arrangement is suitable for simultaneous measurements of absorption31and emission signal. The alternative arrangement, when the co-axial rod isdirectly connected to the rf connector in line is called the “horizontal mount”.Figure 2.2: Schematic Diagram of the Plasma Source Work-HeadThe plasma was viewed through a one inch diameter quartz window on thefront-side flange. On the opposing backside, a similar quartz window allowed alight source ( hollow cathode lamp) to be directed through the graphite furnacefor atomic absorption experiments, or a horizontal-mount can be installedinstead of the quartz window if necessary. The analyte sample was deposited32onto the inner furnace wall through a small screw-top port mounted on the topflange.2. 1. 2 The Atmospheric Pressure ri DischargeInside the graphite furnace, a helium gas discharge was sustained byemploying a 13.56 MHz rf generator ( Model RFX-600, Advanced Energy, FortCollins, CC, USA ), an automatic power tuner ( Model ATX-600, AdvancedEnergy, Fort Collins, CC, USA) and an impedance matching network ( Model501 7-000-G, Advanced Energy, Fort Collins, CC, USA). The rf electrode wascoupled with the output of the matching network through a variable 1-10 tHinductor. With this arrangement a helium discharge over the power rangebetween 10 and 150 W could be ignited and sustained. At an rf power above 50W, occasional arcing was observed between the rf electrode and the furnacewall.2. 1. 3 Spectral Isolation and DetectionA 0.35 m Czerny-Turner monochromator ( Model 270, Schoeffel-MacPherson,MA, USA) with a holographic grating of 2400 lines/mm was used for spectralisolation. The entrance slit of the monochromator was 50 tm wide. Signals weredetected with a photomultiplier tube ( PMT, Model R955, Hamamatsu,Middlesex, NJ, USA ).The PMT was operated at 700 V. The output of the PMTwas amplified using a current amplifier ( Model 427, Keithley, Middlesex, NJ,33USA). The gain and the rise time of the current amplifier was i07 and 0.3 msrespectively, unless otherwise noted.For atomic absorption experiments, a 1: 1 image of the hollow cathode lightwas formed at the center of the furnace by using a 20 mm diameter, 75 mm focallength fused silica lens ( Li; Melles Griot, Irvine, CA, USA). Another 1: 1 imageof the furnace plasma was formed at the entrance slit of the monochromator byusing a 50 mm diameter, 150 mm focal length fused silica lens ( L2; MellesGriot, Irvine, CA, USA). The plasma source work-head, furnace and the hollowcathode lamp ( when necessary ) was aligned with the entrance slit of themonochromator by using a HeNe laser ( Melles Griot, Irvine, CA, USA).2. 1. 4 Measurements of Spatially Resolved IntensityThe plasma source work-head was mounted on a post which in turn wasmounted on a crank-driven linear translation platform. This platform allowed thework-head to be moved literally relative to the detection system. For spatiallyresolved emission intensity measurements, the work-head was translated inincrements of 0.25 mm. A displacement indicator gauge ( Model 2047-il,Mitutoyo, Japan ) was used to monitor the platform movement precisely.342. 1. 5 Temperature Measurements of Graphite FurnaceThe temperature of the graphite furnace was measured my monitoring theradiation emitted from the furnace during the atomization step. The emission ofthe radiation was monitored by using an optical pyrometer ( Ircon Series 1100,Model 11 x 30, IL, USA) which viewed the sample introduction hole through a 1inch diameter quartz window on the top flange of the work-head. The opticalpyrometer was mounted on a support-arm which in turn was fixed to the plasmasource work-head by a vertical post. The pyrometer could be rotate around thevertical post which allowed it to be focused onto a selected position on thegraphite furnace.The pyrometer output was amplified by an amplifier ( Electrical ServicesShop, Department of Chemistry, UBC ) and then digitized. The digitized datawere converted into absolute temperature by using the calibration dataprovided by the pyrometer manufacturer and fitted to a 8 th order polynomialleast square fit. The graphite furnace was assumed as a gray body radiator withan emissivity of 0.7.2. 1. 6 Measurement of Atomic AbsorptionDuring the measurement of atomic absorption, the output of the current amplifierwas fed into a lock-in-amplifier ( Model 121, PAR, Princeton, NJ, USA) as inputsignal. The reference signal of the lock-in-amplifier was from the HCL sourcewhich was modulated at 200 Hz by using a pulsed power supply (ElectricalServices Shop, Department of Chemistry, UBC). This pulsed power supply can35be modulated up to 500 Hz and can deliver up to 100 mA of current. The outputof the lock-in-amplifier was passed to a data acquisition system throughchannel number 1 (Cl). The output was saved and absorbance was calculated.2.2. DATA ACQUISITION AND PROCESSINGThe graphite furnace supply triggered the analog-to-digital-converter (ADC) bya trigger signal as soon as it started the atomization step, to start the dataacquisition. The signal from the single or two channels was digitized with 12-bitresolution by using a sixteen channel ADC ( Model ADM12-10, Quatech, Akron,OH, USA). The ADC was capable of operating at a maximum sampling rate of30 KHz. The data were stored by using a 12 MHz IBM PC/AT compatiblecomputer.The data acquisition software allowed two signals, analyte and background,to be acquired and stored in the computer. The data acquisition rate ( maximumof 250 Hz ) and the number of data points per channel ( maximum of 2000points ) were software selectable and were limited by the free conventionalmemory allocated by the version 4.0 of MSDOSTM.The data processing software facilitated the calculation of diagnosticinformation such as peak height, peak area and the peak width of the temporalresponse of the emission signal. It also calculated the absorption from the HCLsignal. Other options included were background correction, signal averaging,smoothing, displaying and generating plots for the HPTM plotter.362.3. EXPERIMENTAL METHOD FOR FAPESThe analyte sample or the blank solution was deposited onto the furnace wallusing an eppendorf digital pipette ( Model No. 4710. Eppendorf, German ) andwas subjected to the thermal treatment. The furnace temperature program wasin auto mode and the duration of heating steps were selected. After the ashingstep there was 10 s lag. Within that lag the rf power was applied to the rfelectrode, and the plasma was ignited. This lag allowed sufficient time to ignite,and to stabilize the plasma before the start of the atomization step. At thebeginning of the atomization step the data acquisition was automaticallytriggered. The minimum atomization step was limited to 5 s by the furnacepower supply and the signals were collected for 8 s. The cooling of thegraphite was the next step after which the system was ready for the nextsample. The sufficient cooling of the furnace support structure, made of copper,during the atomization step and removal of the hot gases from the plasmasource work-head after the atomization step were two important considerationsfor the usable atomization temperature with this experimental system. Thesample throughput was 4 to 6 samples per hour.37CHAPTER 3INVESTIGATION OF THE PLASMA STABILITY USING He(I) AND COEMISSION LINE AND REFLECTED POWER MEASUREMENTS3.1 INTRODUCTIONDischarge stability is one of the most important considerations for the successfulutilization of a plasma source in emission spectrometry. In addition , for FAPESthe furnace temperature is an important fundamental property which is closelyassociated with analyte atomization and excitation . In high temperatureplasmas, analytes produce intense atomic or ionic line spectra that can be usedfor analytical determinations. The emission intensity of an analyte depends onthe total number of atoms or ions in the volume from which the signal iscollected and the fraction of atoms or ions that are in the excited state.In this study, the stability of the He plasma source at 13.56 MHz in FAPESwas investigated. Temporal profiles of the He ( I ) line, CO line, and thereflected power as a function of temperature are presented in this chapter.383.2 CALCULATION OF THERMIONIC EMISSION FROM GRAPHITEDuring the atomization cycle , the temperature of the furnace wall becomeshigh enough ( 2200 K ) to emit thermionic electrons. The effect of thermionicelectrons on the properties of the plasma discharge and on the analyte signal isnot known at this time.The phenomenon of thermionic emission is related to the ejection ofelectrons or positive ions from a solid when it is heated to a sufficiently hightemperature. The current density, j, of electrons emitted from a uniform surfaceof a pure metal can be expressed in terms of the metal temperature, T, byRichardson equation [44]= A (1 - r)T2eø1TIn this equation A is a constant which is the result of a combination offundamental physical constantsA = 4tmk2eh3 = 120 amp. cm2. deg2where e is the absolute value of the electron charge, k is Boltzmann’s constant,and h is the Planck’s constant. r is the reflection coefficient for electrons crossingthe potential barrier at the metal surface when the electric field just outside themetal is zero. For pure metals, r is around 0.05. is usually called the electronicwork function and is defined so that e is a characteristic amount of workrequired to remove an electron from the interior of the metal to a position just39Figure 3.1.IIIElectron flux for graphite and tungsten as a function offurnace wall temperature.Graphite- -- Tungstenoutside the metal surface. In general, is dependent to some extent ontemperature and the normal component of the electric field at the metal surface.10x109987CM606Cl)c25•Z5ci)LI.i40.30z210—1500 1000 1500Temperature (K)200040A calculation of electron emission from graphite and tungsten was done atdifferent temperature and is plotted in Figure 3.1. The calculation was carriedout using Richardson equation and using the work function of graphite is 5 eVand that of tungsten is 4.55 eV . The figure shows the variation of electron fluxwith temperature where the number of electrons increases exponentially withtemperature for both graphite and tungsten. In the case of graphite (havinghigher work function ), the exponential increase of electron flux happens at ahigher temperature than for tungsten (having lower work function ).In the case of FAPES , the emitted electrons from the graphite furnacewall and from the graphite ( or tungsten ) electrode may change the powercoupling efficiency between the load impedance and output impedance of the rfoscillator. The alteration of impedance matching may change the plasmacharacteristics and may even cause plasma shut-off ( Sections 3.4.1 and3.4.2). Thermionic emission from the furnace wall and the rf electrode may alsocauses an increase in reflected power ( Section 3.4.3).3.3 EXPERIMENTALThe experimental system described in Chapter 2 was employed in thepresent study. The plasma source work-head was a Massmann type fitted with apyrolytic-graphite coated rf electrode [22]. This furnace work-head was used tocollect spectra without ingress of atmospheric gases when it was purged and41sealed effectively. This source was operated at rf powers of 25, 50, 75 and100 W.A 0.35 m Czerny-Turner monochromator ( Model 270, Schoeffel-McPherson,MA, USA) equipped with a holographic grating with 1200 lines/mm was usedfor the measurement of the He and CO line spectra from the plasma source.The 1:1 image of the plasma was formed at the entrance slit of themonochromator by using a fused silica lens ( Oriel, Stratford, CT, USA ) with150 mm focal length and 50 mm diameter. Measurements of the plasmareflected power levels were obtained from the appropriate I/O port on the rear ofthe RFX-600 rf power supply (Advanced Energy, Fort Collins, CC, USA).A PMT ( Model R955, Hamamatsu, Middlesex, NJ, USA ) was used as adetector. The output from the PMT was amplified by using a current amplifier(Model 426, Keithley, Middlesex, NJ, USA). The amplified signal was convertedto digital form by using an ADC ( RC Electronics, Santa Barbara, CA, USA) andstored in a 12 MHz IBM PC/AT compatible computer for further processing. Thedata acquisition was accomplished by the software provided by RC ElectronicsCo.423.4 RESULTS AND DISCUSSION3.4.1 Emission Spectra for CO’ lineWith a helium plasma source, the dominant background emission spectra arefrom CO, OH, NH, N2 and N2-- and He. In the 220 to 270 nm region , thespectra are dominated by emission bands of CO [451. It is well known that COis readily excited in a helium discharge as a result of selective excitationaccording to the following reactions [46]He+2He > He2 + HeHe2 + CO > CO( B 2-) + 2He (Charge Transfer)He (2 3S) + CO > CO( A 211) + e + He (Penning Ionization)The potential energy range of He2 (18.3 - 20.3 eV) is mainly responsible forexcitation of CO (B 2+) state by a resonant charge-transfer mechanism [47].Typical emission intensities for CO line at 219 nm for different plasmapowers at 13.56 MHz are provided in Figure 3.2. Four replicate measurementswere carried out for each determination. These were then averaged andpresented in the Figure. The variation of emission intensity for CO wasmeasured by a non-sample ( no aqueous sample was deposited ) atomizationstep in order to examine the effect of furnace wall temperature on the COemission intensity. The furnace wall temperature was measured43simultaneously using an optical pyrometer ( Ircon Series 1100, Model 11 x 30,IL, USA). In the figure, the temperature axis does not represent the actual walltemperature for the first 2.4 second and from 5th to 6.4th second of theatomization cycle due to the pyrometer sensor used during the experements.At this time, it is not clear whether the oxidation of carbon from the rfelectrode and furnace wall, or desorption of carbon from the rf electrode andfrom the furnace wall is the major source of carbon in the gas-phase. However,in an atmospheric pressure helium plasma, it is most likely that CO is mainlyformed from CO which results from the oxidation of carbon , by residual °2 inthe gas phase. The gradual degradation of the rf electrode is the evidence ofthis process. The formation of CC, in the presence of a plasma, is an additionalsource of CO which is not found in GFAAS.As seen, in the figure, there is a small increase in the emission intensity atthe very beginning of the atomization step (around 1260 K), followed by adepression and then a dramatic increase in the emission intensity which isagain starts to depress dramatically at a furnace wall temperature around 1450K . This complex response of the signal may be due to random variation of theamount of CO in the furnace during the different stages of the atomizationcycle. The initial increase in the emission intensity is most likely due to increasein the amount of CO in the plasma as a result of increased desorption of COfrom the rf electrode and the furnace wall. The density of thermionic electronsfrom the heated rf electrode and furnace wall increases with the furnace walltemperature . These thermionic electrons might recombine with CO and>Cl)a)CC0C)C,)2w+00be responsible for the depression of CO emission intensity. This observeddepression may also be due to the recombination of He2 with the thermionicelectrons, and hence, change in the excitation characteristics for COP. At hightemperature region, the rapid changes of furnace wall temperature might causethe change of power coupling efficiency between the load impedance andoutput impedance of the rf oscillator. This alteration of impedance matching44P=lOOw• . - .-. P=75w• -- P=50wP=30W— Temp.20001 8001 60014001200100080060040020001 700-116001500 .CCD1400 -13000 1 2 3 4 5 6 7 8Time (Sec.)Figure 3.2. Temporal emission behavior for CO(219 nm) at ii power of30, 50, 75 and 100 W and time-temperature profile for furnacewall.45could shut-off the plasma, and, hence decrease the CO emission intensitydramatically.3.4.2 Emission Spectra for He(I) lineThe emission of He (I) lines at 388.8 nm for a 50 W plasma (frequency was notgiven ) [23] and at 388.8, 447.1, 471.3, 492.2 and 501.6 nm for a 50W plasmaat 13.56 MHz [48] were reported by Sturgeon et al. . In a subsequentpublication, some He (I) lines (447.1, 471.3, and 492.2 nm) observed for a13.56 MHz helium plasma [48], were absent in the helium plasma at 40 MHz[49]. Hettipathirana etal. also reported the emission of He (I) line at 501.6 nmfor a 16W, 18W, 22W, 26W and 30W plasma at 13.56 MHz [45].A typical temporal emission intensity for He (I) line at 501.6 nm fordifferent plasma powers at 13.56 MHz is provided in Figure 3.3. Four replicatemeasurements were carried out for each determination which was thenaveraged and presented on the Figure. The variation of emission intensity forHe (I) was measured by a non-sample ( no aqueous sample was deposited )atomization step in order to examine the effect of furnace wall temperature onthe He (I) emission intensity as well as to determine the stability of He plasmawith furnace wall temperature. The furnace wall temperature was alsomeasured simultaneously.46From the Figure 3.3 , it is clear that there is an increase in the emissionintensity at the beginning of the atomization step with increasing rf power. Thisincrease in the helium emission is due to the increase in excitation when the rfpower is increased. For each rf power there is also an initial increase in theemission intensity with increasing furnace temperature. This increase might bebecause of change in the He-excitation temperature. However, Sturgeon at el.reported that, for a 100 W plasma, the He-excitation temperature wasunaffected by the furnace wall temperatures between 500 and 2500 K [50].Figure 3.3 also shows that the initial increase in the emission intensityfollowed by a dramatic decrease in emission intensity which is dependent on rfpower as well as on furnace temperature. For higher rf power, the decrease inintensity appears earlier (in terms of time and temperature) than that for thelower rf power. This dramatic decrease happens at around 1375 K for rfpower 30 W and 1200 K for rf power 100W. After the dramatic decrease thebehavior of the helium intensity is random and inconsistent. When theatomization cycle is over, the furnace wall temperature starts decreasing andthe helium emission intensity begins to increase47200019001 800170031600 -oCD1500140013001 2001100100020001 9001 8001 700 a;131 6000CD15001 4001 3002001100100028002400Temp.P=50WP=30WCD>,14-,Cl)Cci)4-,Cci)1‘4’I4 lV80040000 1 2 3 4 5 6 7 8Temp.P=100wP=75W2800240020001600Cl)CCD 12004-,C‘ 800.3)I 4000Figure 3.3.0 1 23rime4(sec)5 6 7 8Temporal emission behavior for He (I) line at rf power of 30,50, 75 and 100 W and time-temperature profile for furnacewall.48The dramatic decrease of He (I) line emission could be the result of plasmashut-off, which might arise because of changes of power coupling efficiencybetween the load impedance and output impedance of the rf oscillator. Thisimpedance matching might have been altered by the thermionic electronsemitted from graphite surface [51] at high temperature. At atmospheric pressure,the mean free path for electrons in helium is in the jim region [52]. Thermionicelectrons originating from the surface of the center electrode are likely to have agreater effect on plasma processes than those generated some distance awayat the furnace wall where the field is less intense. It is generally accepted thatthere are two populations of electrons in plasma with different temperature [53,54]. A high temperature group promotes excitation and ionization whereas alower temperature, higher density fraction is involved in collisional de-excitationprocesses. Thermionic emission from the furnace wall may serve to flood theplasma with a large number of low-energy electrons, thereby altering theimpedance matching.This observation of He (I) line is comparable with the observation of Falk etal. [53] for FANES source. Falk and his co-workers reported the characteristicsof the discharge voltage as a function of the cathode temperature , where thefurnace acts as the cathode, for different current intensity and 9 hPa helium gaspressure which is depicted in Figure 3.4 . As seen in the figure the dischargevoltage drops dramatically at around 1700 K.49300> 200‘I100Figure 3.4. Discharge voltage as a function of the cathodetemperature for helium at 9 hPa. ( Adapted fromH. Falk et a!., Prog. Analyt. Spectros., 417,11 (1988),with permission of Pergamon Journal Inc. )In the same report they also showed the He (318.774 nm) line intensity as afunction of cathode temperatures with various applied gas pressure. Accordingto their observation with rising cathode temperature the intensity at firstincreases and then decreases drastically at around 1700 K ( for 9 hPadischarge pressure) with the emission of electrons from the cathode surfacewhich is depicted in Figure 3.5 as well.504030 ii20 ii770 1273 1770 2270tempercture/ K50Figure 3.50>‘CCIntensity of the He 318.774 nm line as a function ofthe cathode temperature at a discharge currentintensity for 40 mA and pressure of 9, 13, 27, 40 hPa.(Adapted from H. Falk et aL, Prog. Analyt. Spectros.,417, 11 (1988), with permission of Pergamon JournalInc. )3.4.3 Reflected PowerReflected power is an important variable because of the fact that changes inreflected power suggest changes in the amount of power coupled to theplasma and hence changes in the characteristics of plasma. In FAPES, thegraphite furnace heats up during the atomization step, and the plasma150130110go7°503010470 720 970 1220 1470 1720 1970 2220 2470cath. temp./ KHe 31&77nr4CmA9 hPa13 h Pa27 h Pa40 hPa51impedance changes as a result of the evolution of therm ionic electrons from thehot graphite surfaces. This change in the plasma impedance necessitates theuse of an impedance matching network to maintain the reflected power at aminimum and to protect the rf power supply.The change of reflected power during the atomization step due to furnacewall temperature was studied at rf power of 30, 50, 75 and 100 W at 13.56MHz. Four replicate measurements were carried out for each determinationwhich were then averaged and presented on the Figure. Measurements of theplasma reflected power levels were obtained from the appropriate I/O port onthe rear of the RFX-600 rf power supply (Advanced Energy, Fort Collins, CC,USA ). This 0-5 V signal was directly compatible with the data acquisitionsystem. The furnace wall temperature was also measured simultaneously.The results of this experiments are given in Figure 3.6. Initially there is nosignificant change in reflected power, but at a higher furnace wall temperature,the reflected power increases sharply for 50, 75 and 100 W plasma power. For30 W plasma power, the increase in reflected power is gradual starting ataround a furnace wall temperature of 1450 K , however, a sharp increase inreflected power happens at around 1550 K, 1650 K and 1700 K of the furnacewall temperature for the plasma power of 100, 75 and 50 W respectively. Thereflected power level rises to about 88, 65, 40 and 20 W for the forward powerof 100, 75, 50 and 30 W respectively.52The change in reflected power level is apparently determined by thetemperature. It is also evident from this figure, however, it is not the absolutetemperature of the furnace wall that accounts for the time dependence of thereflected power level. This is clear from the nature of reflected power where theP=100wP=75W--. P=50W- .-.- P=30W— Temp.1 00908070605040L.0ci)C.)ci)ci)20101 9001 8001 7001 60015001 40013001 200110010000-CD3CD-ICDTemporal behavior for reflected power at rf powers of 30 , 50,75 and 100 W along with time-temperature profile for furnacewall -0 1 2 3 4 5 6 7 8Time (Sec)Figure 3.6:53change of reflected power does not follow the changing pattern of the change offurnace wall temperature . Thus, it appears that it is the temperature of thecenter rf electrode that influences the magnitude of the reflected power level,the latter being heated by radiation from the furnace wall. The reason for a risein reflected power and, hence, a decrease in the efficiency of the coupling of therf energy into the system is not yet clear. At high temperature, however,thermionic emission of electrons from the graphite surface occurs [51] whichmight be responsible for the alteration of the impedance matching and, hence,causes a rise in reflected power.543.5. SUMMARYStability of the helium plasma at 13.56 MHz was studied for the FAPES sourceby examining the He (I) and CO emission lines as well as the reflected powerat different rf power. A calculation is also carried out using the Richardsonequation to determine the thermionic electron flux for graphite and tungsten.The thermionic electron flux increases exponentially with furnace walltemperature.Initially, the CO emission intensity increases with ii power and with furnacewall temperature followed by a dramatic decrease at around 1450 K . Theevolution of thermionic electrons might be responsible for the depression ofCO emission intensity by recombining with CO or with He2, and, hencechanging in the excitation characteristics for COP.A time - resolved study of the He (I) emission line showed complex emissioncharacteristics during the atomization step. Like COP, the emission intensity forHe (I) also shows an initial increase with rf power and with furnace walltemperature which is followed by a sudden decrease with increasing furnacewall temperature. This sudden decrease happens earlier for higher rf powersthan for lower ones. This dramatic decrease is indicative of the plasma shut-offat higher temperature as a result of the changing of power coupling efficiencybetween the load impedance and output impedance of the rf oscillator.55Reflected power also shows a significant change at higher temperatureThe reflected power level rises at about 88, 65, 40 and 20 W for the forwardpower of 100, 75, 50 and 30 W respectively. The magnitude of the reflectedpower level does not depends absolutely on the furnace wall temperature buton the center rf electrode which being heated from the furnace wall by radiation.It is suggested that thermionic emission of electrons from the graphite surfacemight be responsible for the alteration of the impedance matching and, hence,cause an increase in reflected power.56CHAPTER 4TEMPORAL AND SPATIAL EMISSION AND TEMPORAL ABSORPTIONCHARACTERISTICS OF SILVER IN FAPES4.1 INTRODUCTIONFor GFAAS, the temporal response of the analyte absorption is an importantdiagnostic tool and has often been combined with kinetic and thermodynamiccalculations in order to determine the analyte atomization mechanisms duringthe high temperature atomization step [55-58]. The temporal response of theanalyte signal is also an important factor that affects the analyticalcharacteristics; for example, sensitivity, detection limit, precision, and linearrange as well as the matrix effects for analytical determinations by GFAAS [28,59]. For FAPES, the temporal response of the analyte signal is likely to bedifferent from that in GFAAS mainly because of two factors. Firstly, for FAPES,the presence of the electrode and plasma inside the graphite furnace maycause a difference in the atomization characteristics from those in GFAAS. Thepresence of the plasma may affect atom formation processes by participating inthe equilibria for condensed and gas phase species. Gas phase concentrationsof different species, which influence the atomization characteristics of analytesmay be different due to the presence of plasma, compared with those whereplasma is absent. Secondly, due to the presence of the rf electrode inside thefurnace, the vaporization characteristics of analytes in FAPES may be differentfrom those in GFAAS.57The rf electrode can act as a second surface where analytes can condenseand then vaporize to form a second peak. Smith et al. reported double peaks forAg at high analyte amounts and low rf powers and speculated that theappearance of double peaks was due to the atomization characteristics of Ag inFAPES [24]. Hettipathirana et al. reported double peak for ETAAS absorptionfor Ag when an rf electrode was present inside the furnace and suggested thatwithout a plasma, the electrode acts as a condensation site for atoms and/oroxides which are subsequently re-atomized at higher furnace temperatures asthe electrode is heated radiatively and by convection [60] . Sturgeon et a!.reported the appearance of double peaks for Cd at low rf powers in FAPES [23].In addition, for Cd, Cu, Ni and Be, an early shift in the position of peak maximumwas observed whereas for Fe, Pb, P and Bi, no shift were observed withincreasing rf power up to 75 W [48]. Sturgeon et a!. suggested that the observedshifts were due to increased plasma volume and density which occur withincreasing rf power; electron collision causing more efficient excitation; possiblechanges in the actual observation zone; and reduction of self-absorption in thelarger, hotter plasma [48]. Hettipathirana et a!. reported an increase in peakwidth and peak area and decrease the appearance temperature for Ag withincreasing plasma power from 14 W to 38 W [65]. In addition , for lead adecrease in appearance temperature and presence of a second peak wereobserved with increasing plasma power up to 40 W [60]. In the same report,the authors also mentioned an increase in the peak intensity and the presenceof a second peak for manganese when the plasma power was increased up to40 W. For HA-FANES, Riby at. el. reported two unresolved peaks for Cr, andsuggested that Cr condensation on the electrode was the origin of these twopeaks [61].58In an effort to understand more fully the influence of plasma power on theresponse of analyte signal in FAPES, a study of spatially resolved atomicemission followed by a time resolved atomic absorption and atomic emissionwas undertaken. This chapter presents the results of this study for Ag. Spatialdistribution of the atomic emission signal and the effects of plasma on analyteatomization and vaporization characteristics are discussed. In addition, theeffects of the plasma excitation process on the temporal response of analytesignal during the analyte vaporization and atomization are presented4.2 EXPERIMENTALFor a complete description of the experimental set-up the reader is referred tochapter two of this thesis. This experimental system and an off-axis mountusually known as vertical-mount , for the ri electrode were employed. For theresults presented in this chapter, four replicate measurements were averagedand subjected to a 25-point Savitzky-Golay smoothing procedure [62].Analytical parameters, such as absorbance, peak area, and peak width , werecalculated for each sample deposition. The analyte appearance temperatureswere calculated on the basis of their appearance time and temperature-timeprofile of the graphite furnace. The appearance time was defined as the timetaken to reach the average base line plus three standard deviations of the baseline noise for the emission or the absorption signal. The peak temperature wasdefined as the temperature at which the maximum emission or absorptionoccurs. Prior to analyses all analyte solutions in 1% (vlv) HNO3 were prepared59from 1000 mg L1 stock solution by serial dilution . Silver stock solution wasprepared by dissolving analytical grade AgNO3 (BDH, Toronto, Canada). Nitricacid solutions were prepared by using the analytical grade reagent (BDH,Toronto, Canada).On the inside wall of the furnace a 10 pL aliquot of analyte solution wasdeposited by using an Eppendorf 0.5 - 10 iL micro pipette. Helium ( UnionCarbide, Toronto, Canada ) was used to purge the plasma source work-head.The furnace temperature was set to 500 K for 40 second to ash the sample. Thistime was sufficient to exclude water vapor from inside the plasma source work-head before the ignition of plasma. After the ashing, there was 10 second lagbefore the plasma was ignited. The lag time was followed by the ramping of thefurnace temperature to 2200 K in 5 second for all determinations. For eachdetermination four replicate measurements were carried out. The blankdeterminations were carried out by depositing the same amount of 1% (vlv)HNO3 solution. All determinations were carried out by using the atomicresonance line of Ag (at 328.07 nm).4.3 RESULTS AND DISCUSSION4.3.1. Spatial Effect of Plasma on Analyte EmissionThe spatial distribution of the atomic emission signal was studied by depositingthe analyte sample on the furnace wall and the temporal response at differentradial positions from the furnace was recorded. The results of this experiment,60when 3 ng of Ag is deposited on the furnace wall and the plasma was run at 30W power, are given in Figure 4.1.When the center of the furnace is focused on the monochromator entranceslit , two peaks are observed. The first one is very sharp with an appearancetemperature of 1240 K and a peak temperature of 1275 K. The second peakhas an apparent peak temperature of 1460 K. At the position of 0.25 mm and0.75 mm from the furnace center, two distinct peaks for each position are alsoobserved. For both positions the appearance and peak temperature for the firstpeak are similar to the center position whereas for the second peak theapparent peak temperature is 1455 K and 1495 K for 0.25 mm and 0.75 mmrespectively. The first peak tends to merge into the second one when themonochromator is set to observe at 1.25 mm from the center having theappearance temperature of 1250 K and peak temperature for the first one is1325 K and the apparent temperature for the second peak is 1440 K. The firstpeak appears again having the same appearance temperature, 1240 K, andthe peak temperature of 1250 K, 1275 K and 1275 K for the position of 1.75 mm,2.25 mm and 2.75 mm from the center respectively.These observations suggest that the rf electrode acts as a condensationsite for species vaporized from the surface wall with subsequent “secondsurface” vaporization and excitation. The decrease in the size of the initial peakwith distance from the center shows that the condensation rate decreases withincreasing distance from the center. This decrease also might be because ofexcitation efficiency changes with the distance from the center electrode. The0.0mmAFigure 4.1.A, 4.1.B and 4.1.C (continued on page 62 )610I I I I1 2 3 4Time,SecI I I I5 6 7 8I 0.25mm1B2000-1500—-Q>‘1000-500-0-200015001000500020001500>‘100050000 1 2 3 4 5 6 7 8Time,SecI 0.75mm1C0 1 2 3 4 5 6 7 8Time,Sec20001500>‘11000500020001500‘ 1000Co0)C— 500020001500DLi1000>‘Cl)500062Figure 4.1.D, 4.1.E and 4.1.F (continued on page 63 )Ii .25mm1D0 1 2 3 4 5 6 7 8Time, Sec11.75 mmlE0 1 2 3 4 5 6 7 8Time, Sec12.25 mm)F0 1 2 3 4 5 6 7 8Time, Sec20001 90018002 170016001500140013001200632001 50012.75 mml>.‘U)a)G100050000 1 2 3 4 5 6 7 8Time, SecH0 1 2 3 4 5 6 7 8Time, SecFigure 4.1. Temporal response of the Ag emission signal at an rfpower of 30 W for 3 ng of Ag deposited on the furnacewall when the monochromator is focused at 0.0 mm(A), 0.25 mm ( B ), 0.75 mm ( C), 1.25 mm (D), 1.75 mm(E), 2.25 mm ( F) and 2.75 mm (G ) with respect tothe furnace center ; and the Temperature-Time profilefor the furnace wall (H).64delay in the onset of the second peak is the result of delaying heating of the rfelectrode relative to the furnace wall is somewhat similar to the situationencountered during the radiant heating of the L’vov platform.The process of condensation on the cooler inside surface of the furnace isconsistent with recent studies for GFAAS [63, 64] and for FAPES [60]. L’vov et al.observed double peaks when a platform is present inside the furnace [63]. Tocondense the analyte followed by the re-vaporization in the atomization step toachieve the temporal isothermality, Hocquellet used a “second-surface trap”inside the furnace [64]. Hettipathirana et a!. observed double peaks for Agatomic absorption when the sample was deposited on the furnace wall in thepresence of a co-axial rod in the FAPES system [60].Figure 4.2 provides the spatial distribution of analyte signal in the graphitefurnace at an ri power of 30 W for 3 ng of Ag deposited on the furnace wall. Bothpeak height ( Figure 4.2. A) and peak area (Figure 4.2. B) show that the signalintensity increases with increasing distance from the furnace center andreaches maximum at 1.25 mm from the center. This spatial distribution wasobserved by translating an image of the FAPES source laterally in increments of0.50 mm.Figure 4.2. Spatial response of the Ag emission at an rf power of30 W for 3 ng of Ag deposited on the furnace wall. APeak Height Vs radial distance, B : Peak Area Vsradial distance.6520001 600D. 1 2005 800ct3o 4000A0.0 0.5 1.0 1.5 2.0 2.5Distance From Centre. mm3.0140012001000800ct5ci)4002000B0.0 0.5 1.0 1.5 2.0 2.5Distance From Center, mm3.0664.3.2. Effect of Plasma Power on analyte Emission and AbsorptionThe effect of rf power on the emission signal was studied for 1 ng of Agdeposited on the furnace wall while the center of the furnace is focused on themonochromator entrance slit. The results are provided in Figure 4.3. As seen inthe figure, the emission signal is shifted to earlier time by 248 ms when the rfpower is increased from 30 W to 150 W. The tendency of showing doublepeaks also disappears when the rf power is increased to 125 W.The observed shift of the emission signal with increasing rf power may bedue to an increased evaporation rate of Ag from the furnace wall when theplasma is present [65]. It is also might be because of some plasma-assistedheating of the surface of the furnace wall at higher plasma powers. At higherplasma power, for example 125 W and higher, the rf electrode could becometoo hot to act as a second surface for condensation and as a result, the secondpeak was not observed.200E 1DCo0)C020001500D1000>CO500020001500D1000>‘Co500067Figure 4.3.A , 4.3B and 4.3.C (Continued on page 68 )IP=30W1A0 1 2 3 4 5 6 7 8IP=50w1B0 1 2 3 4 5 6 7 8I P=75w1C0 1 2 3 4 5 6 7 8Time,SecCD>‘Cl)Ca)C3 4Time,SecFigure 4.3. Temporal response of the Ag atomic emission signalfor 1 ng of Ag deposited on the furnace wall at an rfpower of 30 (A), 50 (B), 75 (C), 100 (D), 125 (E) and150 (F) W.Cci>(I)Cci)=68I P=ioowIDk0I I I I I I I I1 2 3 4 5 6 7 8I P=125w2000-1500-1 000-500-0-20001500100050002000-1500-1000-500-0—I-00 1 2 3 4 5 6 7 8CD>.(I)Cci)CI I1 2I I I5 6 7 869As seen in Figure 4.3 , for plasma power 30, 50, 75 and 100 W the emissionsignal changes gradually; but when the plasma power is raised to 125 W, thechange in emission signal is dramatic. The changes in peak area for emissionsignal at different plasma power is shown in Figure 4.4.2000-CD1500--Q>U)Cci)1000-ccici)C0500-Ew0-I I I I I0 20 40 60 80Plasma Power, WI I I I100 120 140 160Figure 4.4: Effect of plasma power on emission signal for 1 ng ofAg deposited on the furnace wall at an rf power of 30,50, 75, 100, 125 and 150 W.70With increasing rf power there might be some loss of analyte due to preatomization loss, changes in the excitation characteristics , or an increase inionization of Ag. The drastic decrease in the analyte signal at the r power of125 W and more is likely because of plasma extinguishes which is consistentwith the observation of He ( I ) line emission at different plasma power (Section3.4.2).Pre-atomization losses in graphite furnace methods are due to the highvolatility of the analyte molecular species and a slow heating rate of thegraphite furnace coupled with a lower atomization temperature. The atomizationlosses may also be significant because of an increase in diffusion withincreasing temperature at high plasma powers. The analyte loss may also bedue to rf sputtering from the furnace wall prior to atomization . However,Sturgeon et a!. reported no measurable decrease in the Ag emission signaleven during an extended 2 minute period of plasma operation prior toatomization step [ 54].The excitation characteristics of analyte could be affected by the presenceof thermionic electrons at high plasma powers . The excitation characteristicscan be affected by decreased collisional excitation rate ( due to changes in theelectron number density ) and/or by increased collisional de-excitation rate (due to low energy thermionic electrons ). Although no changes in the reflectedpower are observed before 3.5 s ( up to 100 W plasma) into the atomizationstep , some temporal characteristics of ionic species such as CO is attributedto the evolution of thermionic electrons. The study of CO emission behavior as71a function of rf power shows that CO intensity decreases drastically at around1450 K of the furnace temperature (Section 3.4.1 ) and it is suggested thatthermionic electrons might recombine with CO and responsible for thedepression of CO emission intensity. Furthermore, the He-excitationtemperature measurement for FAPES by Sturgeon at el. [50] and Ar and He-excitation temperature measurement for HC-FANES by Falk et a!. [53] show thatthe thermionic electrons do not affect the excitation temperature at higherfurnace temperatures; however, they can influence the electron density of theplasma.For plasma spectrometric methods, analyte ionization may also besignificant. As the plasma power increases, it not only increases analyteexcitation, and, hence, increases the emission but also increases analyteionization. For HC-FANES, Falk at el. reported an increase in the emissionintensity for Ni , Cr, Cu, Fe , Co and Al when the discharge current is increasedfrom 20 to 60 mA [53]. However , the power coupled to the HC-FANES source isonly 18 W at a discharge current of 60 mA. For HA-FANES, Harnly at el.reported a constant emission intensity for Cu and Cd above a threshold current(50 and 20 mA for Cu and Cd respectively) when the discharge current waschanged from 10 to 80 mA [61]. FANES ( with the excitation temperatureranging from 1000 - 3000 K ) is considered to be a relatively low ionizationplasma source compared with a high temperature plasma source like the ICP(with the excitation temperature ranging from 4000 - 7000 K ) [18]. For the rfplasma sources in FAPES, the excitation temperature is ranging from 3000 -5000 K depending on the ri power and the thermometric species used for themeasurement [45 1. Hettipathirana at el. reported Ag ionization in FAPES at an72rf power of 40 W [60] . They found that the integrated emission intensity for Agis decreased in the presence of Na interferent compared to that withoutinterterent and suggested that there is an ionization suppression of Ag when Nais present during the atomization. It is most likely that the analyte ionization maybe significant at higher rf powers, and, hence, a source for the suppression ofatomic emission signal intensity in FAPES.A study of time resolved atomic absorption and emission were taken underthe same conditions at different plasma power to understand the atomicpopulation inside the furnace. The results of this experiment , when 1 ng of Agwas deposited on the furnace wall and the plasma was run at different ( 0 W,30 W, 50 W, 75 W, 100 W, 125 W and 150 W) power, are given in Figure 4.5.As seen in Figure 4.5, in the absence of a plasma, two peaks are observed foratomic absorption ( Fig. 4.5.A). The appearance temperature for the first smallpeak is 1243 K and that for the second large one is 1532 K. At higher plasmapower (75 W and more ) the atomic absorption signal tends to show a singlepeak instead of double peaks ( Fig. 4.5.D, 4.5.E, 4.5.F and 4.5.G ). Theappearance temperature of this peak is approximately the same as the smallpeak observed without a plasma. This observed single absorption peak athigher plasma power supports the idea that, with increasing the rf power, the rfelectrode becomes too hot to act as a second surface for condensation. Figure4.5 also shows that the response time for absorption is longer than that for theemission at higher plasma power, for example, 50 W or more. The emissionsignal ended 400 , 88 , 208 , 296 and 1408 ms earlier than that of absorptionfor the plasma power of 50, 75, 100, 125 and 150 W respectively.731.00.8D0.6>‘: 0.4G)0.2C’)0.01.0• 0.80.6>‘0.4a).E 0.2C,)0.01.0-0.8-D0.6->0.4-Ca)- 0.2-Cl)0.0-AIP=oowI“. ! \%_.I I I I I I I I I0 1 2 3 4 5 6 7 80 1 2 3 4 5 6 7 82000m315001000 CDC’)500C2000m315001000 CDCl)5009-C0 1 2 3 4 5 6 7 8Time,SecFigure 4.5.A, 4.5.B and 45.C ( Continued on Page 74 )741.0• 0.8D0.6>‘• 0.4a)- 0.2U)0.00 1 2 3 4 5 6 7 81.00.8D0.6>.0.4ci)00.02000m15001000 CD50002000m15001000 CDCo500C02000m15001000 CDDCO500C0Figure 4.5.D, 4.5.E and 4..5.F ( Continued on Page 75 )0 1 2 3 4 5 6 7 81.00.8D0.6>.• 0.4ci)- 0.2Ci)0.0Abs.— EmissionP=1 25w0 1 2 3 4 5 6 7 8Time, Sec75De 0.6>0.4Cci)C; 0.2Figure 4.5.2000mB15000D1000 CDDCl,500C0Temporal response of the Ag emission andabsorption signal at 328.07 nm for 1 ng of Agdeposited on the furnace wall at an rf power of 0 W(A,only absorption), 30 W (B), 50 W (C), 75 W (D), 100W (E), 125 W (F) and 150 W (G) . The Temperature-Time profile for the furnace wall is also shown onFig..4.5.H1G0 1 2 3 4 5 6 7 81Dccici)02ci)I—ccici)C)CUCIDI-I-H20001 80016001400120010000 1 2 3 4Time, Sec5 6 7 876This observation suggests that at higher plasma power there are some non-excited atoms present in the system . The only possible reason of not excitingthese atoms is the absence of plasma which might be the result of plasmashut-off due to the changes of power coupling efficiency between the loadimpedance and out put impedance of the rf oscillator (Section 3.4.2)Figure 4.6 is a plot of the ratio of absorption and emission as a function oftime based on the calculation from Figure 4.5. As seen in the figure, withincreasing rf power, the ratio of absorption and emission increases. Thisincrease in absorption-emission ratio indicates that the total fraction of atomsexcited from the ground state atoms in the plasma has decreased, during theatomization cycle, with the increase in rf power ( except at 75 W plasma ).When the system is operated at 30 W plasma power ( Fig. 4.6.A) , the rate ofnon-excited atom is less in comparison of higher plasma power. Thiscalculation again supports the idea that the plasma shuts-off with the increasein operating plasma power.Absorption/EmissionRatioAbsorption/EmissionRatioAbsorption/EmissionRatioCo_L00’o010010(Ti00101CD0II0II0I-ciS-ci0100780(Ua:00)0)EwC0a0U)-a0(Ua:C0U)U)uJC010U)-a0CUa:C0U)C’)SwC0a-0U)Figure 4.6: Ratio offor 1 ngpower ofabsorption and emission as a function of timeof Ag deposited on the furnace wall at plasma30, 50, 75, 100, 125 and 150 W.P=125 WE15-10-0 1 2 3 4 5 6 7 8P=150 WF0 1 2 3 4 5 6 7 8Time, Sec79Another important observation is that, not only the emission signal but alsothe absorption signal shows a decrease in the peak area with increasingplasma power which is presented in Figure 4.7. Unlike emission there is nodrastic decrease in analyte absorption signal. The gradual decrease inabsorption signal is likely because of the loss of analyte population due to preatomization loss or an increase in ionization loss.Figure 4.7: Effect of plasma power on absorption signal for 1 ng ofAg deposited on the furnace wall at plasma power of 0,30, 50, 75, 100, 125 and 150 W.0 40 80 120Plasma Power, W160804.4. SUMMARYThe spatial effect of the plasma on Ag analyte emission was studied for the328.07 nm Ag resonance line at an rf power of 30 W. The highest emission wasobserved at 1.25 mm from the center of the furnace. The temporal response ofthe analyte emission signal in FAPES was also studied for Ag using both atomicabsorption and atomic emission spectroscopy while the sample was depositedon the furnace wall.With lower rf power, the co-axial rod is cooler than the furnace wall resultinga temperature lag between the co-axial rod and the furnace wall . As a resultthe co-axial rod acts as a condensation site for atoms and oxides thatsubsequently vaporize ( second - surface vaporization ) at higher furnacetemperatures. At higher rf power, the co-axial rod becomes hot enough toprevent any condensation, results in the appearance of single peak . The studyof ri power effect on analyte also shows a maximum response of Ag emission at30 W rI power which then decreases with increasing rf power. The absorptionsignal also decreases with increasing if power but the rate is much slower thanthat for emission . This decrease in signal might be because of some loss ofanalyte population due to pre-atomization loss, changes in the excitationcharacteristics , or an increase in ionization of Ag. The drastic decrease in theanalyte emission signal at the if power of 125 W and higher is likely because ofplasma shut-off.81The calculation of the ratio of absorption and emission shows that there is aportion of atoms that remain un-excited . The amount of un-excited atomsincreases with increasing rf power suggesting a plasma shut-off at higher rfpower due to the change of power coupling efficiency between the loadimpedance and output impedance of the rf oscillator.The emission signal is shifted to earlier time when the rf power is increasedfrom 30 to 150 W by 248 ms, whereas the absorption signal remains virtuallythe same. This early appearance of Ag emission is likely due to an increasedevaporation rate from the furnace wall. The temporal response of both atomicabsorption and atomic emission signals shows similar leading and fallingedges . These results suggest that the temporal response of Ag in FAPES isdetermined by the atomization and vaporization characteristics of the analyterather than by the excitation characteristics.82CHAPTER 5CONCLUSIONSThe 13.56 MHz helium plasma used for Furnace Atomization Plasma ExcitationSpectrometry ( FAPES) has been further characterized as a spectrochemicalsource for elemental analysis. The main objective of this study was tocharacterize and examine the stability of the radio frequency ( rf ) heliumplasma source, over a wide power range, during the atomization cycle. An efforthas been made to more fully understand the influence of higher plasma power,up to 150 W, on the response of analyte signal in FAPES.One of the persistent spectral features observed for FAPES is emission fromCOP. The temporal response of CO at different radio frequency (ri) power,during the atomization step, shows complex emission characteristics which islikely due to a combination of factors including recombination of ionic species(CO and/or He2 ) with thermionic electrons and a change in excitationcharacteristics of the plasma. Initially, the CO intensity increases withincreasing the rf power mainly due to the enhanced release of carbon from therf electrode and furnace wall in the presence of plasma.Like CO , the temporal response measurements for He (I) line also showscomplex emission characteristics during the atomization step. The emissionintensity for the He (I) line shows an initial increase with rf power and withfurnace wall temperature, followed by a drastic decrease with increasingfurnace wall temperature. For higher rf power this drastic decrease happensearlier in time than the lower one. This decrease in emission intensity83indicates the shut-off of plasma at higher temperature as a result of changingof power coupling efficiency between the load impedance and outputimpedance of the radio frequency oscillator.The study of reflected power as a function of temperature shows a significantchange in reflected power at higher furnace temperature. The reflected powerlevel rises to as much as 88 W for the forward power of 100 W. The magnitudeof the reflected power level does not depend absolutely on the furnace walltemperature but on the temperature of center rf electrode which heated from thefurnace wall by radiation and by contact with the plasma. Thermionic emissionof electrons increases exponentially with furnace wall temperature. Thecalculated value for the number of electrons per cm2 of the graphite surface is3.46x106,8.43x107 and 1.62x109 for 1300 K, 1400 K and 1500 K of the furnacewall temperature respectively. This large number of thermionic electrons fromthe graphite surface is likely to be responsible for the alteration of theimpedance matching and, hence, causes an increase in reflected power.The spatial distribution of analyte in the plasma was studied by measuringAg analyte emission at different radial positions in the furnace at an rf power of30 W. The result shows an increase in analyte concentration from the centertoward the wall, reaching a maximum at 1 .25 mm from the center followed bydecrease again .The temporal response of the analyte emission signal inFAPES has also been studied for Ag using both atomic absorption and atomicemission spectroscopy while the sample was deposited on the furnace wall . Inthe presence of plasma, inside the furnace, the rf electrode is heatedsignificantly relative to that without a plasma but no significant change in the84furnace wall temperature is observed. However, during atomization, thetemperature of the rf electrode lags relative to that of the furnace wall. Thisdifference in time-temperature characteristics between the graphite furnace andthe rf electrode cause condensation of analytes on the rf electrode andsubsequent re-vaporization and, as a result, two peaks in the temporal emissionprofile are observed. The relative intensities and shapes of these two peaks aregreatly affected by rf power. Analyte condensation on the ri electrode is severeat lower ri powers but at higher plasma power, for example at 125 W, the rfelectrode becomes too hot to act as a second surface for condensation and, asa result , the second peak is not observed.The effect of the heating rate of the graphite furnace on the temporalresponse of the emission signals in FAPES is not known yet. The heating rate ofa furnace depends mainly on the furnace mass and the rate of initial currentsupply to the furnace. The effect of heating rate on emission signal could bestudied by using furnaces with different mass or by controlling the rate of currentsupply to the furnace. For this study this was not undertaken mainly due toinstrumental limitations and shortage of time. However, it is quite possible thatthe heating rate of the graphite furnace affects the time-temperaturecharacteristics of the rf electrode relative to the furnace wall, and hence,analyte condensation and re-vaporization characteristics from the rf electrode.Furthermore, results obtained from temporal emission of Ag show an early shiftin peak appearance which is likely due to an increased evaporation rate ofanalyte from the furnace wall . The similarities in peak shapes observed for bothatomic emission and absorption signals show that the temporal emission85response in FAPES is determined by atomization and vaporizationcharacteristics of the analyte rather than by excitation characteristics.An effort has been taken to fully understand the effect of wider rf power onthe analyte emission signal for Ag. Results show a decrease in integratedemission intensity at rf powers higher than 30 W . This decrease in emissionintensity may be due to pre-atomization loss of analyte, a change in excitationcharacteristics, and I or an increase in ionization of Ag at higher radio frequencypowers. Furthermore, the shape of peaks shows that the residence time forexcited Ag atoms is shorter than that for ground state atoms at rf power 50 Wand more. This observation suggests that at higher rf powers some of theground state atoms do not become excited in the system due to quenching ofthe plasma . The probable reason for plasma extinguishing is a change inpower coupling efficiency between the load impedance and output impedancedue to rapid change of temperature and/or rapid change in thermionic electrondensity.Most of the results described in this thesis show that at higher rf power andhigher furnace temperature, thermionic electrons affect the plasma impedance,and the rf power is dissipated by the plasma. The major source of thethermionic electrons are the inner surface of the graphite furnace and the rfelectrode . Since the reflected power changes late into the atomization step,application of higher rf power becomes difficult. The rf power supply andmatching network employed for this study were designed for semiconductorplasma processing work. Therefore, the reasonable remedy for the problem ofvariable ri power dissipation by the plasma would be to modify the matching86network to deliver a constant rf power during the high temperature ramp of theatomization step to prevent the extinguishing of the plasma. Furthermore, theelectrical characteristics of this atmospheric pressure plasma source are not yetwell understand. To understand the plasma extinguishing, a comprehensivestudy of the current-voltage characteristics of the plasma should be carried out.A smaller plasma source workhead having reduced void volume than thepresent one would allow faster cooling of the graphite furnace and the furnacemount. Difficult and time consuming sample deposition on the inner wall of thefurnace can also be avoided by using an auto sampler instead of manualinjection. In addition, the use of a higher heating rates for the graphite furnaceduring the atomization step would be beneficial to improve the analyticalcharacteristics.This thesis presents a study of the effect of radio frequency power on thehelium plasma source in FAPES as a spectrometric source for chemicalanalysis. This study provides an useful understanding of physical phenomenonoccurring in the plasma at a fundamental level. 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