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Development of a capacitively coupled plasma as a gas chromatographic detector Huang, Degui 1991

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DEVELOPMENT OF A C A P A C n T V E L Y COUPLED PLASMA AS A GAS CHROMATOGRAPHIC DETECTOR by DEGUI HUANG  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  in THE FACULTY OF GRADUATE STUDIES Department of Chemistry  We accept this thesis as confirming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA June 1991 ©Degui Huang, 1991  In  presenting this  degree at the  thesis  in  University of  partial  fulfilment  of  of  department  this thesis for or  by  his  or  requirements  British Columbia, I agree that the  freely available for reference and study. I further copying  the  representatives.  an advanced  Library shall make it  agree that permission for extensive  scholarly purposes may be her  for  It  is  granted  by the  understood  that  head of copying  my or  publication of this thesis for financial gain shall not be allowed without my written permission.  Department of " The University of British Columbia Vancouver, Canada  Date  DE-6 (2/88)  (9cvh ^  ,fi^J  ii Abstract This work has two objectives: first, to develop an atmospheric pressure radio frequency capacitively coupled plasma (CCP) as a detector for gas chromatography (GC) and, secondly, to understand the excitation process in the CCP from a fundamental point of view. In the process of developing the CCP as a gas chromatographic detector, the design of a CCP torch useful as an atomic emission detector for GC has involved several stages of evolution. Initially, two plasma torches, a cylindrical one with concentric electrodes and a rectangular one with two parallel electrodes, were designed and their performance evaluated. It was found that the parallel plate torch which utilized a rectangular-bore quartz tube had more stable emission signals, which was attributed to less gas flow turbulence. Therefore the rectangular torch was chosen for further experiments. Another consideration of using the rectangular torch was that it was easier to interface it with a capillary column from a gas chromatograph. With a flow of make-up gas consisting of the plasma gas, the outlet flow of resolved analytes from the GC column was entrained into the discharge. This configuration rninimized the dead volume so that the high resolution provided by the capillary column would not be degraded by the plasma torch volume. One problem encountered in the initial work was that arcing occurred occasionally through air between the electrodes which often damaged the plasma torch even though quartz tubes enclosed the electrodes to isolate them from the air. The reason for this is believed to be due to the high voltage from the power supply. In order to make the torch more amendable to testing, a demountable version was designed such that different quartz tube and electrode dimensions could be evaluated. Also, the operating frequency was shifted from 27.12 to 13.56 MHz. One advantage of using the 13.56 MHz RF power supply was that the electrodes did not have to be enclosed in an insulator and could be exposed to air without external arcing around the outside of the  iii torch because the power supply provides lower output voltage. The torch performance was evaluated for the determination of organotin compounds. One difficulty still encountered, however, was heating of the parallel plate electrodes which yielded unstable plasma conditions at times. As a result, a new version of the CCP torch was designed by utilizing water cooled electrodes. With this configuration and the 13.56 MHz power supply, the electrode could contact the discharge tube directly and the plasma was formed without arcing. The efficiency of power transfer to the plasma was increased significantly. Furthermore, the plasma created was extremely stable. Both metal and non-metal elements could be excited sufficiently using the modified configuration and excellent results have since been obtained. The detection limits for F, CI, Br, I, S, and C were determined to be 10 to 78 pg/s. The fulfillment of the second objective was carried out on the modified water-cooled plasma torch as mentioned above. Spatial distributions associated with the CCP which arise as a result of the transverse power coupling geometry were obtained. Some clues as to possible excitation processes in the CCP have been gleaned from the study of the relative intensities of lines for several non-metals. Basically, electron impact is the major excitation mechanism in the CCP. Excitations by the helium or argon metastables may play very a minor role. Charge transfer excitation reactions were found between He2 and CO and +  N . 2  I  iv  Table of Contents  Page  Abstract  ii  Table of Contents  iv  List of tables  viii  List of figures  ix  List of publications arising from this study  xv  Acknowledgements  Chapter 1:  xvi  Introduction  1  1.1  Overview  1  1.2  Comparison of gas chromatographic detectors  3  1.2.1  An ideal detector for gas chromatography  3  1.2.2 Evaluation of a detector for gas chromatography  .4  1.2.3 General characteristics of GC detectors  4  1.3 Survey of atomic spectroscopy for gas chromatographic detection 1.3.1  Coupled gas chromatography-atomic absorption spectrometry  1.3.2 Coupled gas chromatography-inductively coupled plasma  .7 8 9  1.3.3 Coupled gas chromatography-direct current plasma  10  1.3.4 Coupled gas chromatography-microwave induced plasma  11  1.3.4.1 Overview  11  1.3.4.2 Historical development of MLP  11  1.3.4.3 Application of MTP  12  1.3.4.4 Coupling of MIP with G C  13  1.4 Coupled gas chromatography-capacitively coupled plasma (CCP)  16  V  Chapter 2:  1.4.1  Historical development of CCP  16  1.4.2  Principal of plasma operation  23  1.4.3  Application of CCP  26  1.4.4  Coupling of CCP with GC  28  Initial Development of CCP as GC Detector  31  2.1  Introduction  31  2.2  Experimental..  2.3  2.2.1  The CCP GC detector design  32  2.2.2  Equipment and setup  37  2.2.3  Analytical procedure  38  Results and discussion 2.3.1 Description of the CCP detector  2.4  Chapter 3:  ...32  38 :  38  2.3.2 Emission spectra and chromatograms  39  2.3.3  43  Analytical performance..  Conclusion  43  Evaluation of a 13.56 MHz CCP as a Detector for Gas Chromatographic Determination of Organotin Compounds  44  3.1  Introduction  44  3.2  Exr>erimental  46  3.2.1  Power supply  46  3.2.2  Spectrometric system  46  3.2.3  Gas chromatograph  46  3.2.4  Plasma torch  47  3.2.5  Transfer interface  49  3.2.6  Data acquisition  49  vi 3.2.7 3.3  3.4  Chapter 4:  Chemicals  Results and discussion  49 50  3.3.1  Helium plasma background....  50  3.3.2  Spatial emission characteristics  50  3.3.3  Effect of input power  54  3.3.4  Effect of gas flow rate  55  3.3.5  GC-CCP system performance  59  Conclusions  63  Characteristics of an Atmospheric Pressure Parallel Plate Capacitively Coupled Plasma  64  4.1  Introduction  64  4.2  Experimental  4.3  4.4  ....65  4.2.1  Plasma torch assembly  4.2.2  Sample introduction system  66  4.2.3  RF power supply  66  4.2.4  Spectrometer  69  4.2.5  Spectral response of the spectrometer  69  Results  .65  71  4.3.1  Background spectra  4.3.2  Spectra for nonmetals  4.3.3  Spatial distribution of emission  85  4.3.4  Effect of power on emission intensities  90  Discussion 4.4.1  General excitation processes in plasma source  4.4.2  Excitation processes in He CCP  4.4.3  Charge transfer reactions in the CCP  71 ..71  94 94 .95 98  vii 4.5  Chapter 5:  Conclusions  101  Speciation of Halogen and Sulphur Containing Compounds  102  5.1 Introduction  102  5.2 Experimental  102  5.3 Results and discussion  103  5.4 Conclusion  108  Chapter 6:  Future Proposals  109  viii  List of Tables  Page  Table 1  General characteristics of GC detectors  7  Table 2  Experimental facilities and operating conditions  37  Table 3  Detection limits (DL) with GC-CCP  63  Table 4  Compounds used for relative intensity measurements  72  Table 5  Relative intensities of F, CI, Br, I, C, N, and O in the He CCP  80  Table 6  Comparison of detection limits in pg/s (DL)  107  ix  List of Figures  Figure 1  Page  Schematic diagram of the reentrant MIP torch. { Reprinted from B.D.Quimby and J.J. Sullivan, Anal. Chem. 62, 1027 (1990)}  Figure 2  ...15  Schematic diagram of the capacitively coupled plasma described by Mavrodineanu and Hughes. { Reprinted from R. Mavrodineanu and R.C. Hughes, Spectrochim. Acta , 19,1309 (1963)}  Figure 3  17  Schematic diagram of an atmospheric pressure CCP plasma jet {Reprinted from Dong C. Liang, M. Sc. Thesis, Dalhousie Univ. Halifax, N.S. Can.  Figure 4  (1986)}  Cross sectional view of the T-shaped CCP torch developed by Liang and Blades [1, 2]  Figure 5  19  21  Schematic diagram of the Applied Research Laboratories capacitively coupled microwave plasma.{Reprinted from P.W.J.M. Boumans, F J . de Boer, F.J. Dahmen, H. Hoelzel, and A. Meier, SpectrochimActa, 30B, 449 (1975)}  Figure 6  24  Schematic diagram of the experimental system used to test the CCP as a GC detector  33  X Figure 7  Schematic diagram of the interface between the GC column and the plasma torch  Figure 8  Schematic diagram of the CCP torches, (a) Concentric geometry and (b) Parallel plate planar geometry  Figure 9  35  36  Tin emission spectrum obtained using Ar CCP between 270 and 306 nm. RF power: 100 W, frequency: 200 KHz, carrier gas flow rate: 30ml/m  Figure 10  40  Comparison oftinemission spectrum between 270 and 306 nm from Me4Sn and backgroundfromtoluene obtained using Ar CCP RF power: 100 W, frequency: 200 KHz, carrier gas flow rate: 30 ml/m; (a) Spectrum of Me4Sn and (b) Background spectrum detected using the concentric Ar CCP geometry  Figure 11  41  (a) Gas chromatograms for three replicate injections of 150 pg of iodomethane {CH3I} into the GC. (b) Gas chromatogram of 0.1 ul of 0.1 ppm di-iodopropane {(012)312) in hexane detected using the CCP  Figure 12  Schematic diagram of the discharge tube and support structure  Figure 13  Background emission spectrum of the 13.56 MHz helium plasma  42  48  from 200 to 600 nm Input power: 100 W, reflected power 2 W, make-up gas flow rate: 200 ml/m, and carrier gas flow rate: Oml/m  51, 52  XI  Figure 14  (a) Spatial distributions of emission from H1486.13, He 1504.77 nm, and He 1447.15 nm without sample. Input power: 150 W, make-up gas flow rate: 200 ml/m, and carrier gas flow rate: 0 ml/m. (b) Spatial distributions of Sn 284.0 nm and Fe 371.99 nm for a helium CCP with organic sample introduced. Input power: 150 W, make-up gas flow rate: 200 ml/m.  Figure 15  53  Effect of input power on emission intensity for Sn at 284.0 nm. Make-up gas flow rate: 200 ml/m and carrier gas flow rate: 10 ml/m  Figure 16  Effect of make-up gas flow rate on emission intensity for Sn at 284.0 nm. Input power: 100 W and carrier gas flow rate: 10 ml/m  Figure 17  56  57  Effect of carrier gas flow rate on emission intensity for Sn at 284.0 nm. Input power: 100 W and make-up gas flow rate: 200 ml/m  Figure 18  Chromatograms of Me4Sn using GC-CCP. Sn 1284.0 nm, input power: 100 W, make-up gas flow rate: 200 ml/m.  Figure 19  60  Chromatograms of Me3SnCl using GC-CCP. Sn 1284.0 nm, input power: 100 W, make-up gas flow rate: 200 ml/m.  Figure 20  58  Chromatogram of mixture of Me4Sn,  61  Me3SnCl and P4Sn using r  temperature programming. Input power: 150 W, make-up gas flow rate:  Figure 21  200 ml/m and carrier gas flow rate: 10 ml/m  62  Schematic diagram of the water-cooled torch device  67  xii Figure 22  Schematic diagram of the sample introduction system. C: carrier gas, F: gas flow meter, S: volatile sample, T: test tube, V: ice bath vessel, P: plasma torch  Figure 23  68  Instrumental responses of PMTs R955 and R406. Slit width for R955: 10 urn, for R406: 50 um  Figure 24  70  He CCP background with PMT R406 from 300 nm to 1000 nm. RF power: 200 W, make-up gas flow rate: 200 ml/m, carrier gas flow rate: Oml/m  Figure 25  Spectrum of CI from CH2CI2 in He CCP. RF power: 200 W, make-up gas flow rate: 200 ml/m.  Figure 26  75, 76  Spectrum of C6H14 in He CCP. RF power: 250 W, gas flow rate: 200 ml/m.  Figure 27  73,74  77, 78  He (I) 447.15 nm spatial distributions with different sizes of torches vs relative distance. RF power: 200 W, make-up gas flow rate: 200 ml/m, carrier gas flow rate: 0 ml/m.  Figure 28  86  He (I) 447.15 nm spatial distributions with different sizes of torches vs absolute distance. RF power: 200 W, make-up gas flow rate: 200 ml/m, carrier gas flow rate: 0 ml/m.  87  Xlll  Figure 29  He (I) 447.15 nm spatial distributions with RF power. Make-up gas flow rate: 200 ml/m, carrier gas flow rate: 0 ml/m  Figure 30  88  Spatial distributions of selected species of OH, N2, He(I), N2 in He +  CCP, RF power: 200 W, make-up gas flow rate: 200 ml/m, carrier gas flow rate: 0 ml/m.  Figure 31  91  Spatial distributions of several species from carbon tetrachloride in He CCP. RF power: 200 W, make-up gas flow rate: 200 ml/m  Figure 32  92  Relative emission intensity of several species with power from from carbon tetrachloride in He CCP. make-up gasflowrate: 200 ml/m  93  Figure 33  Emission lines of CI (II) in Ar CCP. RF power: 200 W, PMT: R955.... 97  Figure 34  Energy level diagram of He +, H e , He , CO+(B 2:), N + (B I\+), m  2  Ar andAr2 +  m  2  2  2  2  98  m  2  Figure 35  (a) Spectrum of C6H14 in He CCP between 180-300 nm. RF power: 200 W, make-up gasflowrate: 200 ml/m, PMT: R955. (b) Spectrum of He CCP background between 180-300 nm. RF power: 200 W, make-up gasflowrate: 200 ml/m, PMT: R955  Figure 36  99  (a) Spectrum of Ar CCP background between 180-300 nm. RF power: 200 W, make-up gasflowrate: 200 ml/m, PMT: R955.  (b) Spectrum of  C6H14 in Ar CCP between 180-300 nm. RF power: 200 W, make-up gas flow rate: 200 ml/m, PMT: R955  100  xiv  Figure 37  Chromatograms of a multi-element test mixture at carbon channel. Injected amount: 11-23 ng, temperature prograncuning: hold 1 min at 60 °C, then raise to 200 °C at the rate of 30 °C/min  Figure 38  ...104  Chromatograms of a multi-element test mixture at multi-channels. Injected amount: 11-23 ng, temperature programming: hold 1 min at 60 °C, then raise to 200 ° C at the rate of 30 °C/min  105  List of publications arising from this study  Characteristics of an atmospheric pressure parallel plate capacitively coupled plasma, Degui Huang and Michael W. Blades, submitted to Applied Spectroscopy, in press  Evaluation of a 13.56 MHz capacitively coupled plasma as a detector for gas chromatographic determination of organotin compounds, Degui Huang and Michael W. Blades, / . of Analytical Atomic Spectrometry, 6,215 (1991)  Capacitively coupled plasma detector for gas chromatography, D. Huang, D. Liang and M. W. Blades, / . of Analytical Atomic Spectrometry, 4,789 (1989)  xvi Acknowledgements  I would like to thank my wife, Yin Geng, for her love, support and patience during the preparation of this thesis. A special thanks to Mike Blades, my supervisor, for his help and guidance during the years. Thanks as well to all my colleagues in the group for their encouragement and valuable discussion. Financial support from the Petroleum Research fund administrated by the American Chemical Society, the Natural Sciences and Engineering Research Council of Canada, and the University of British Columbia Office of Research Service is greatly acknowledged. Acknowledgement is also made to Professor Mike Fryzuk for the use of the gas chromatograph and Zenon Environmental Labs for the donation of a gas chromatograph.  Chapter 1 Introduction  1.1 Overview Gas chromatography (GC) is a physical method of separation based on the partitioning of a solute between a gaseous mobile phase and a liquid stationary phase. In a gas chromatograph, a mixture to be separated is vaporized and swept over a relatively large absorbent or adsorbent surface inside a long narrow tube, or column, by a mobile gaseous phase. The different components which have been separated in the column elute at different retention times with the mobile phase and are monitored by a device, or detector, which signals qualitatively or quantitatively the presence of a component eluted from the column. Since the input to the detector is a flow of chemicals - importantly, a flow of separated chemicals, the detector has to be designed to convert this information to a series of continual electrical signals which are time dependent so that each compound can be identified according to its specific retention time and analyzed quantitatively if desired. The analysis for each component must be completed in a few seconds or less. If each compound in an unknown sample is to be identified and quantified , it is required that the detector responds to all the analytes present in the sample and has reasonable sensitivity. Detectors which meet this condition are termed "universal". In many cases, however, only the information about some particular type of analyte, e.g., PCBs in an environmental sample, is needed and the other compounds can be ignored. Therefore instead of a universal detector, a "selective" detector , which only responds to the compounds containing a specific chemical group, a specific atom, or other property, is better because it can simplify the interpretation of a complex chromatogram by providing selective information about each eluting peak. The advantage is apparent when a sample contains very complicated compounds, because not all compounds can always be separated in a GC column even using modern high-resolution capillary columns. "Specific"  2 detectors are those that are so selective that they can distinguish particular structures or elements with a high degree of certainty. For example, flame photometric detector (FPD) is a specific detector for phosphorus or sulphur-containing compounds. A selective detector can become universal in certain conditions. For example, an element-selective detector can monitor all the organic compounds if it is set for carbon analysis because all organic compounds contain carbon. This gives a lot of flexibility for the detection of the gas chromatographic effluent. An atomic emission spectral detector is one such kind of detector. Until recently, atomic emission spectroscopy has not played a significant role in gas chromatographic detection. The reason is that the technique is traditionally used mostly for the determination of inorganic elements which are less often of primary interest to the gas chromatographer, who is usually concerned with organic analytes. Also, at the time of earlier development of GC detectors, atomic emission methods were not readily usable in general laboratories and no commercial detector based on atomic spectroscopy for GC was available until the 1970s. Among the various atomic emission sources, plasmas are playing very important roles. Although the possibility of using a plasma spectral emission device as an element-selective GC detector was explored early in the 1960s, it is only in recent years that interest has increased for this type of detection because of the continuous progress in the availability of power supply hardware and the wide adoption of plasma emission sources. An atmospheric pressure capacitively coupled plasma (CCP), one type of plasma source, has been developed and investigated for a few years in this laboratory. The initial use of this technique was to provide an atomizer for atomic absorption [1]. Later, it was found that the CCP was better suited as an atomic emission source. Several applications of using the atmospheric pressure CCP as emission source have been reported in subsequent publications [2,3]. It is noticed that none of the applications used liquid samples directly. The reason is that, like the microwave induced plasma (MIP), the CCP suffers solvent  3 effects due to its small physical size and low-power consumption. Therefore the solvent in the liquid sample must be removed before the analyte is introduced into the plasma. However, for the G C effluent, the solvent and each analyte are separated prior to detection and the small quantities of analytes have little effect on the plasma. Clearly the coupling of C C P with G C should be of practical use and thus was investigated.  1.2 Comparison o f gas chromatographic detectors  1.2.1 A n ideal detector for gas chromatography A n ideal detector for gas chromatography would have the following characteristics: 1. High sensitivity 2. Long-term and short-term stability and reproducibility 3. Wide linear range 4. Ability to operate in both universal and highly selective modes 5. Wide operating temperature range (room temperature to about 400 °C) 6. Short response time 7. Ease of use 8. Non-destructive of sample 9. Low cost 10. Similar response toward all solutes or alternatively a highly predictable and selective response toward one or more classes of solutes Needless to say, no detector meets all of these criteria, and it seems unlikely that such a detector will ever be designed. However, dozens of detectors have been investigated and used during the development of gas chromatography. Each of them has one or more characteristics as mentioned above. Among the various detectors, thermal conductivity detector (TCD), flame ionization detector (HD), electron capture detector (ECD), and flame photometric detector (FPD) have been the most frequently used.  4  1.2.2 Evaluation of a detector for gas chromatography In practice there are three major response parameters used to characterize the detectors for gas chromatography: sensitivity, selectivity and dynamic range. To be useful in a practical sense, stability and repeatability must be considered also. Sensitivity is the response per amount of sample, that is, the slope of the response /amount curve. However, because of different operating principles, the comparison of sensitivities between different detectors is not easy. Instead, detection limits are used for this purpose. In gas chromatography, detection limit (DL), or minimum detectable level (MDL), is usually defined the equivalent amount of analyte required to produce a peak that is twice the height of the peak-to-peak noise, divided by the full width at half height of the peak in seconds. The unit for DL is grams per second (g/s) or picograms per second (pg/s). Selectivity is a measure of categories of compounds to which the detector will respond. Some detectors respond to almost everything and are considered universal. Others respond only to certain types of compounds and are quite sensitive when the detected compound is present in a complex, non-detected matrix. In gas chromatography, selectivity is usually calculated as the amount of hydrocarbon that gives the same response as the analyte in terms of molecular or elemental weights. Dynamic range, or linear range, is the range of sample concentration for which the detector can provide linear calibration curves. This range is quite large for some detectors, such as FID. For others, such as FPD, the range is small, due to saturation effects at higher concentrations.  1.2.3 General characteristics of GC detectors The flame ionization detector (FID) is one of the most commonly used GC detectors. It has high sensitivity (DL for carbon is 5 pg C /s), wide linear response range ( 10 ) and 7  5 low noise. It is generally rugged, stable and easy to use. For these reasons, it is one of the most popular detectors. The limitation is that FID is only sensitive to compounds which ionize in the flame. For example, in air/H flame, it is sensitive to aliphatic, 2  aromatic, olefinic, and acetylenic compounds. Its response to carbonyl, alcohol, halogenated, and amine compounds is poor or none at all. The thermal conductivity detector (TCD), a very early detector for gas chromatography, still finds wide application and is a non-destructive detector. The advantage of TCD is its simplicity, large linear range ( 10 ), and its general response to almost all compounds both 6  organic and inorganic species. The limitation of TCD is its relatively low sensitivity (400 pg/ml). The detection limits of TCD are generally insufficient for trace analysis. The electron capture detector (ECD) is a selective and high sensitive detector for molecules containing electronegative functional groups such as halogens, peroxides, quinones, and nitro groups. It is especially sensitive for chlorinated compounds (0.1 pg Cl/s). However, despite high sensitivity, ECD has some significant shortcomings. For example, the response of ECD is highly dependent on the molecular structure of the compounds, and for accurate quantitative work, response factors for each component of interest must be determined. This limitation may restrict the analytical utility of the ECD if proper calibration standards are unavailable. In addition, the linear response range for ECD is usually limited to about four orders of magnitude. The flame photoionization detector (FPD) is a specific detector for the determination of sulfur and phosphorus. The combustion chamber is compatible with FID and as such affords the analytical chemist a discriminating ability beneficial to many analyses. The advantage of FPD is its high selectivity. The selectivities of sulfur and phosphorus to hydrocarbon are 10 : 1 and 10 : 1 respectively. Sulfur and phosphorus are linear over a 4  10 and 10 range respectively. 4  3  s  6 Mass spectrometry (MS) and infrared spectroscopy (IR) are increasingly playing important roles in detection for gas chromatography. Despite the high cost, modern laboratories are gradually equipped with GC-MS systems because of their widespread demand. The GC-MS has been widely used for the identification and quantitation o f compounds in various samples, natural, biological, and environmental, etc. One advantage of GC-MS is that structural information can be obtained from the fragmentation patterns in mass spectra and molecular formulas can even be deduced. In addition to the GC-MS system, GC-infrared spectroscopy is a tool to obtain molecular structure information.  Gas chromatography-Fourier transform infrared  spectroscopy (GC-FTIR) provides rapid response. A n advantage of this system is that it is non-destructive, but GC-FTIR suffers from low sensitivity. Other less common detectors are photoionization detector (PID) which detects compounds that are ionized by an ultraviolet ( U V ) light source, thermionic detector (NPD) which is an ionization detector and is generally selective for organic compounds containing nitrogen and phosphorus. Atomic emission detectors (AED) are versatile when used in the detection of G C effluents. A s was mentioned earlier, they can be both universal and selective. The detection limit is in the range of 0.1 pg - 1 ng/s depending on the element analyzed. The selectivity is extremely high for AED. More than 10 is easily achieved with a moderate 5  resolution spectrometer. A very important characteristic of this system is its multi-element ability, that is, several elements can be monitored separately and simultaneously. The fact that the sensitivity is independent of molecular structure makes it possible to calculate the empirical formulas of unknown compounds without standard calibration. To some extent, it can replace some work which is currently done by GC-MS system. However, it still has drawbacks: for some elements the sensitivities are not high enough. For example, it is not competitive with E C D when used to determine chlorinated compounds. The high cost of this system is another disadvantage.  7 Some main features of above detectors are summarized in Table 1.  Table 1. General characteristics of GC detectors  Name  Selective for  DL  ELD  Materials that ionize in air/H flame  5pgC/s  lO'  TCD  Anything but carrier gas  400 pg/ml carrier*  10*  ECD  Gas-phase electrophiles  0.1 pgCl/s  10*  PID  Compounds ionized by UV  2pgC/s  lO'  NPD  N, P containing compounds  0.4 pg N/s 0.2 pg P/s  10* 10*  FPD  P, S containing compounds  20 pg S/s 0.9 pg P/s  103 103  FTIR  Materials absorbing IR light  1000 pg/s  103  MSD  Tunable for any species  10 pg - lOng/s  105  AES  Tunable for element  0.1 pg - 1 ng/s (depending on element)  103  2  Linear dynamic range  *: TCD responses to concentration, rather than massflowrate.  1.3 Survey on atomic spectroscopy for gas chromatographic detection Atomic spectroscopy offers the possibility of selectively detecting a wide range of metals and non-metals. The use of detectors responsive only to selected elements in a multi-component mixture drastically reduces the constraints placed on the chromatography step as only those compounds in the mixture which contain the elements of interest are detected.  8 The first coupling of atomic spectroscopy with GC was reported by McCormack and co-workers [4], who used a microwave induced plasma (MIP), a typical plasma source for atomic emission, as an element-selective detector for gas chromatography. The coupling of atomic absorption spectrometry (AAS) was also reported as early in 1966 [5]. In spite of its inherent advantages as the most selective of the atomic spectroscopic techniques due to the 'lock and key" mechanism [6], AAS suffers two major drawbacks when used for the GC detector. First, it lacks multi-element capability since the AAS usually works in the mode of single element determination at any given time. Secondly, the low temperature in the atomizer for AAS does not allow the non-metallic elements in the sample to be decomposed from the compounds and atomized sufficiently for atomic analysis. For this reason, most publications on the GC-AAS have been limited to the determinations of metals from organo-metallic samples [7-33]. Emission sources, especially plasmas, provide very high temperatures, and therefore a wealth of emission lines can be produced for a particular analyte. Although spectral interferences in plasma are more likely than that in AAS, the flexibility of choosing alternative lines can minimize this problem. The high excitation temperature of plasmas not only favors the excitation of metal atoms, but also makes non-metal atoms detectable. Furthermore, plasma sources often provide wide linear working range and low detection limits. Among the various plasmas used as the GC detectors, inductively coupled plasma (ICP), direct coupled plasma (DCP), and microwave induced plasma (MIP) have been mainly investigated [34 - 45, 97 - 139].  1.3.1 Coupled gas chromatography-atomic absorption spectrometry (GCAAS) The first coupling of GC with atomic absorption spectrometry was made by Kolb era/. [5], who introduced the GC effluent into the nebulization chamber in the AAS with an airacetylene flame via an interface tube to determine tetraalkyllead compounds in gasoline.  9 Later both the flame [7 - 13] and electrothermal atomizers have been involved in the coupled GC-AAS applications. Electrothermal atomizers used in the GC-AAS can be further classified into three main categories: electrothermally heated quartz or ceramic tubes [14 - 20], commercial graphite furnace [21 - 28], and cold vapor mercury analyzers [29 33]. It is not surprising that almost all the analytes detected in the GC-AAS applications have been metals from organo-metallic compounds since the AAS is only a good tool for the determination of metal elements. Alternatively, atomic emission spectroscopy provides the possibility of detecting the non-metal elements in the effluents from a GC. Plasmas, which are composed of hot gaseous matter where free charges dominate their behavior, are ideal emission sources for both metallic and non-metallic elements because of their high temperature. Various plasmas, including ICP, DCP, MIP, have been explored in the past 3 decades as the detectors for GC.  1.3.2 Coupled gas chromatography-inductively coupled plasma (GC-ICP) The fist coupling of GC-ICP was reported by Windsor and Denton in 1978 [34]. They showed the capability of ICP-optical emission spectrometry for the elemental analysis of organic compounds using an all-argon plasma. This capability was then utilized in a GC-ICP coupling for simultaneous multi-elemental analysis of organic and organo-metallic compounds [35]. A successful use of the GC-ICP was demonstrated by Windsor and Denton [36] to find the empirical formulae of various organic compounds from carbon, hydrogen and halogen ratios. Sommer and Ohls [37] used both all-argon and nitrogencooled ICPs for the monitoring lead emission. Fry et al. investigated the emission characteristics of several non-metallic elements in the ICP and used the emission lines in the near infrared range to make fluorine [38] and oxygen-selective [39] detections. More applications of the GC-ICP have been made on the determinations of volatile hydrides [40], alkyllead compounds [41] and yttrium and rare earths [42].  Although it is well known that the ICP has very high gas temperature and excitation temperature (6000- 10000K), which ensures that the compounds in the sample dissociate and excite efficiently, its large physical size and the high capital cost limit its use as a GC detector. The requirement that the ICP works at high flow rate (12-201/min) not only may reduce the sensitivity by dilution, but also results in high operational costs. The analyte coming from a GC column is usually in small quantities.  It is common for a gas  chromatograph to work at 20-40 ml/min with packed columns or less than 10 ml/min with capillary columns, which is significantly lower than the flow rates listed above for ICP sources.  1.3.3 Coupled gas chromatography-direct current plasma (GC-DCP) The discharge in direct current plasma (DCP) is produced between two electrodes or three electrodes in the shape of a Y. There have been relatively few publications on the coupling of DCP with GC [43-49]. One advantage of using the DCP is that it is tolerant to a wide range of gas flow-rates, gas and solvent types. Detection limits in the GC-DCP system are typically in the range of 10 to lO -14  -10  g/s [45, 50]. However, since the plasma  plume is exposed to the atmosphere, interference resulting from the entrainment of the air is significant. Obviously, some elements such as nitrogen, oxygen, hydrogen can not analyzed with this system. In addition, contamination from the electrodes is possible because of the direct contact of electrodes with the discharge. The applications have mostly been limited to the determination of organo-metallic compounds since the detection limits for non-metals in the DCP were poor [45].  1 1  1.3.4 Coupled gas chromatography-microwave induced plasma (GC-MIP)  1.3.4.1 Overview Nowadays an optimal microwave induced plasma (MIP) has low power consumption, lowflow-rates,high excitation temperature, the ability to operate at atmospheric pressure, is easily ignited, and lacks interference from contact with walls. Apart from those desirable aspects, the MIP has a low gas temperature and small tolerance to droplets or molecular species, allowing only small amounts of sample to be introduced without extinguishing the plasma. All the above features make the coupling of MIP with GC natural since the tolerance of only small amounts of sample is compatible with the effluent of a gas chromatograph, and the ability to determine both metals and nonmetals is required for gas chromatography.  1.3.4.2 Historical development of MIP A microwave induced plasma (MIP) is usually an electrodeless discharge generated in a glass or quartz tube having an inner diameter on the order of a few millimeters to a few centimeters. Although the initial MIPs used a single-electrode to couple power to the plasma [51, 52], resonant cavities have proven to be more successful [53 - 58]. The purpose of the resonant cavity is to produce a standing electromagnetic wave at microwave frequency (typically 2450 MHz) to power the plasma. Early cavities used in the MIPs did not transfer the energy efficiently from the magnetron to the plasma. This forced researchers to use either more powerful power supplies or to reduce the cavity pressure to help energize the plasma. In 1976 Beenakker [59 ] designed a cavity efficient enough to couple the energy to the plasma in both argon and helium at atmospheric pressure and at reasonable power (40 - 100 W). In the 1970s, a non-resonant cavity was introduced, in which energy was transferred to the plasma through a surface wave [60 62]. More recently a cavity, termed reentrant [63, 64], has been modified specifically for  12 interfacing with a gas chromatograph which allows end-on viewing of a short plasma. A water-cooled wall discharge tube further improves performance by reducing reaction with the discharge tube walls. Besides argon and helium, nitrogen and air have also been used as support gases for the MIP [65 - 67].  Details on the development of MIP as an analytical source can be  found in a number of excellent reviews [68 - 72].  1.3.4.3 Applications of MIP MIPs experience difficulties when aqueous aerosols are introduced directly into the discharge. The relatively low power density does not allow efficient desolvation, vaporization, dissociation, and atomization of the sample droplets. However, pneumatic nebulization into MIP can be accomplished if an aerosol desolvation system is located after the nebulization chamber to remove the water carrier [73]. With such an arrangement, aqueous nebulization rates can be as high as 1.5 ml/min. Hass and Caruso [74] attempted to resolve the sample introduction problem using a moderate-power resonant cavity operating at 500 W. Both a concentric nebulizer and the glass frit nebulizer were used and compared. The solution uptake rate was controlled by a peristaltic pump at a rate of 1 ml/min and detection limits for most elements were better than those obtained using lower power MIPs. The microwave-induced plasma is a successful but still problematic source for atomic emission spectroscopy. The excitation temperature is generally high enough (3000 - 4000 K) to be able to effectively excite most elements in the periodic table. The MIP also has high sensitivity and a relatively low spectral background. The power supply operating at the frequency of 2450 MHz is commercially available from the medical and oven heating markets. The inexpensive equipment required and economical routine operation ensure extensive investigation and application of this emission source. However, these low power plasmas are significantly affected by the presence of molecular species and have a  13 limited tolerance for the introduction of a foreign substance. To increase operating power means the increasing of power supply size and thus capital cost.  Although the  development of moderate-power MIPs has been made, the results are not as good as those for the ICP and DCP since the stability of the plasma can be degraded even when a relatively small amount of sample material is injected into it. Therefore MIPs have been most successfully applied in situations where small amounts of sample per unit time are fed into the discharge, for example, in gas chromatography (GC) and micro-sample analysis.  1.3.4.4 Coupling of MIP with G C The most successful commercial application of a MEP has been as GC detector. More than one hundred papers have been published in this field. In the first paper on GC-MIP system, McCormack et al. [4] used a 2450 MHz atmospheric pressure argon discharge to detect the elution of halogen and other nonmetal-containing compounds from a GC. The detection limits for most nonmetal elements were found to be in the range of from 10 lO  -9  -12  to  g/s. Bache and Lisk [75, 76] used a helium MIP at low pressure (5-10 torr) and  found that better power coupling and hence better atomization characteristics could be obtained. The determinations of halogens, phosphorus and sulphur were made by using their atomic lines instead of molecular bands [4]. More applications of the reduced pressure He MIP for GC detection were found in the determination of various organic compounds and metal chelates [77 - 91]. Although atmospheric pressure Ar MIP [92 100] have been used for the detection of GC effluent since the first report on GC-MIP, it was not until 1976 that atmospheric pressure He MIP achieved popular recognition. In 1976, the TMoio cavity reported by Beenakker [59] allowed a stable microwave plasma to be produced at atmospheric pressure both for He and Ar. The power transfer was proved more efficient and operation was easier. Combining the ease of operation at atmospheric pressure and the high excitation temperature of the He MIP, the TMoio or Beenakker cavity became more and more popular in the GC-MIP systems [101-129].  14 Plasmas produced by surfatrons are extremely stable over wide ranges of gas flow rate, pressures and operating powers. Moreover, tuning is easily carried out and remains constant. It has been proven that the surfatron is well suited to chromatographic analysis [130- 133]. More recently the reentrant cavity has been reported to the use of GC-MDP system [63, 64]. Since a thin-walled (0.12 mm) and water-cooled discharge tube was used, the erosion of the discharge tube walls was essentially eliminated and thus the tube lifetime was increased. The schematic diagram of the torch is shown in Fig. 1. With the relatively cool discharge tube (about 350 °C), background emission levels were depressed since the low temperature prevented the evaporation of silicon dioxide from the discharge tube walls [64]. A potential use of the MIP detector for a GC is to obtain the inter-element ratios in compounds. If unequivocal inter-element ratios could be determined independent of the sample type, GC-MIP systems would be capable of much identification work currently performed on more expensive GC-mass spectrometers. Many publications have been reported on the determination of inter-element ratios in an attempt to establish empirical formulae [80, 88, 89, 116, 119, 121, 122, 128]. Coupling of the MIP with a GC seems to be its most successful combination. As a GC detector, it provides element selective information which simplifies the interpretation of the chromatograms of some complicated environmental, clinical, petrochemical, and biological samples which are sometimes very difficult for more commonly used G C detectors. Hewlett-Packard has recently begun to market the HP 5921A MIP detector with a slew-scan photodiode array detection system for GC [134].  This development has  stimulated renewed interest in the MIP and continued development can be expected.  15  26 10 8  5 2  1 4 7  9 11  0  20mm Scale  Fig. 1. Reentrant cavity: (1) pedestal, (2) quartz jacket, (3) coupling loop, (4) main cavity body, (5) cavity cover plate, (6) gasket, (7, 8) cooling water inlet and outlet, (9, 10) water plates, (11) O-ring, (12) silica discharge tube, (13) polyimide ferrule, (14) exit chamber, (15, 16) window purge inlet and outlet, (17) sparker wire, (18) window, (19) gas union, (20) threaded collar, (21) column, (22) capillary column fitting, (23) makeup and reagent gas inlet, (24) purge flow outlets. (25) stainless steel plate, (26) standoff, (27) heater block, (28) mounting flange, (29) brass center conductor, (30) PTFE coaxial insulator.  {Reproduced from B.D.Quimby and J . J . Sullivan, Anal. Chem. 62, 1027 (1990)}  16 1.4 Coupled Gas Chromatography-Capacitively coupled plasma (CCP)  1.4.1 Historical development of CCP The principle of capacitive coupling to generate a plasma has been known for many years. In 1941 Cristescu and Grigorovici [135] reported that a CCP discharge would be produced between two circular plates, separated vertically by up to 15 cm. The plates formed a capacitor which was part of a resonant circuit and determined the frequency of the RF generator, typically in the range 60 to 90 MHz. The lower plate had a copper cone with a platinum tip at which a high field strength was produced. By touching the tip with an electrically isolated conductor to generate seed electrons, a discharge was formed at the dp; it was reported to have a temperature of 4000 K at an RF power of 650 W. In 1956, Badarau etal. [136] used a capacitor, consisting of a hollow cylinder and a coaxial electrode, to generate a brush-like discharge for spectrochemical analysis. The discharge was capacitively induced by a rectified RF source at 43 MHz. The calibration curves for lead and barium were found to be linear, and the sensitivities of thirteen elements were compared with those in flames. Another arrangement for capacitive coupling consists of an inductance coil of tubular copper with a coaxial electrode as shown in Fig. 2 [137, 138].  The working gas,  containing nebulized liquid, travels through the coil and emerges from small holes in the electrode tip. The coil is part of the LC loop of the oscillator circuit, but the discharge is sustained essentially by capacitive coupling. A low power (10 - 30 W) capacitively coupled plasma torch was developed for evaluation of an ablation discharge aerosol generator [139]. The configuration of the  17  Fig. 2. Schematic description of the oscillator circuit with the inductance coil showing at the same time the sample supplying procedure. The analytical solution is sprayed through the coil by a pneumatic atomizer operated with the discharge sustaining gas (He). The droplets traverse the coil and exit from the four small holes in the molybdenum discharge tip.  {Reprinted from R. Mavrodineanu and R.C. Hughes, Spectrochim. Acta , 19, 1309 (1963)}  18 plasma is shown in Fig. 3. The electrodes were made of tungsten or molybdenum. The argon flow rate was in the range 0.5 - 2.0 1/min. The upper electrode was not essential. The plasma can be sustained without an upper auxiliary electrode, i.e. a single electrode discharge. However, the grounded auxiliary electrode increased the temperature and the stability of the plasma. Superimposing a DC component on the RF is an alternative to the Cristescu and Grigorovici CCP described above [135]. RF power was applied across two electrodes, and an adjustable DC voltage was applied between them and a third electrode, movable between them [140, 141]. Adjusting the DC current from 0.1 to 1.0 A changed the apparent excitation temperature and also the appearance of the plasma. Applications to spectrochemical analysis were also reported. A capacitively coupled discharge was formed by placing a water cooled, grounded electrode above a helium inductively coupled plasma (ICP) [142]. In comparison to the results of a helium ICP, the plasma stability was enhanced and the power required was reduced significantly. The detection limits of non-metals obtained with this plasma at 500 W are either comparable or superior to those with the helium ICP at 1500 W. The electron number density for this discharge is greater than that for the helium ICP. All the CCPs mentioned above have a direct contact surface with the plasma that causes problems of electrode contamination for spectrochemical analysis. The possibility of using a 13.56 MHz RF capacitively coupled low pressure oxygen plasma for spectrochemical analysis has been investigated by Winslow in 1980 [ 143]. The quartz plasma chamber was cylindrical in shape, with a length of 16 cm and radius of 4 cm. Two RF electrodes consisting of a condenser were located outside the plasma chamber. They were isolated from the plasma gas, so no electrode contamination existed. Capacitive coupling was the only mechanism of energy transfer. Egorova [ 144] used a CCP with two external annular electrodes at each end of a watercooled discharge tube for solution spectrochemical analysis. The plasma was run at  19  Fig. 3.  Schematic diagram of the low power C C P torch  1:Quartz tube, 2: Argon inlet, 3: Connected to RF generator, 4: Center electrode, 5: Plasma, 6: Auxiliary electrode {Reprinted from Dong C. Liang, N.S. Can. (1986)}  M. Sc. Thesis,  Dalhousie Univ. Halifax,  20 atmospheric pressure in argon, and was viewed along its axis. This type of coupling had been used earlier for isotope analysis in plasmas at low pressure [ 145,146]. Two kinds of discharge were found to be useful for spectrochemical analysis. The first was a constricted plasma like an arc channel with high excitation energy. The second was a continuous diffuse luminescence which filled the tube, in which spectra of low energy predominated. Zvyagintsev etal. [ 147, 148] developed an electrodeless capacitive arc (plasmatron) in air at atmospheric pressure. The discharge torch was a dual external annular electrode type similar to Egorova's device. The capacitive arc was run at 150 MHz and a few kilowatts of power with an air flow rate of 30 - 600 Vmin. A U.S. patent [149] described a capacitively coupled plasma jet device. The main difference between this CCP jet and those of Egorova and Zvyagintsev was that one of the annular electrodes was smaller and was placed inside the discharge tube. RF parallel plate capacitively coupled discharges have been widely used in plasma etching caused by RF sputtering at low pressure. The use of it as spectrochemical analysis has only been made in recent years. An atmospheric pressure capacitively coupled radio frequency plasma discharge has been developed by Liang and Blades for atomic absorption [1] and emission analysis [2, 3] of small, discrete sample volumes ( 1 - 1 0 ul). A schematic diagram of the device is provided in Fig. 4. Functionally, the device consists of two parts, the capacitively coupled plasma (CCP) discharge tube and the tantalum strip electrothermal vaporization sample introduction system. This CCP device has been demonstrated as being potentially useful not only for both atomic absorption spectrometry  21  Sample Introduction inlet  Plasma  (end view)  83) Tantalum strip  Stainless steel electrodes (to RF power supply)  Copper rod supports  Quartz  Plasma support • — gas inlet  Water outlet  Water inlet To DC power supply  Fig. 4. Schematic diagram of the capacitively coupled plasma and sampling system {Reprinted from D.C. Liang and M.W. Blades , (1988)}  Anal. Chem., 60, 27  22 (AAS), and atomic emission spectrometry (AES) but also for gas chromatography (GC) and as a spectral lamp for spectroscopic measurements [1 - 3]. The torch design provides for very effective energy transfer from the power supply to the plasma by capacitive coupling. Therefore, the plasma can be generated at atmospheric pressure and in a flexible geometry. As a further development of the CCP technology, an atmospheric pressure furnace capacitively coupled plasma (APF-CCP) has also been developed by Liang and Blades [150,151]. Functionally, the APF-CCP source consists of an electrothermal atomizer (the furnace) and an RF discharge (the CCP). This technology offers simultaneous multielement determinations and better detection limits than those in the graphite furnace AAS. Most recently, Duckworth and Marcus [152] have reported an RF powered glow discharge atomizer/ionization source for soils mass spectrometry (MS). This device was essentially a low pressure (100 - 500 mTorr), RF CCP run at 13.56 MHz and with 5-50 W RF power. Mass spectra were presented for alloy, metal oxide, and glass matrix samples. Afterglows in nitrogen are often called "active nitrogen" (AN). During 1954-1955, Kenty observed that reactions usually observed in AN sources could be duplicated in electrical discharges of argon containing traces of nitrogen at low pressure ( 300 Torr) [153]. In 1980, D'silva, Rice and Fassel used an atmospheric pressure active nitrogen (APAN) discharge for analytical spectroscopy.  The APAN torch consisted of two  concentric fused silica tubes of 1.8 cm and 2.6 cm diameters, and approximately 40 cm in length. The copper foil electrodes were positioned inside the inner and outside the outer tube. An annular afterglow discharge can be formed in the gap of the two quartz tubes at 1800 Hz frequency, 500 W power and 20 kV ac potential [154]. The APAN is essentially a low frequency capacitive discharge. Microwave plasmas can be classified into two groups, microwave induced plasmas (MIPs), discussed in the previous section, and capacitively coupled microwave plasmas (CMPs). In the first type, microwave energy is commonly coupled to the plasma in a  23 discharge tube within a resonant cavity, while in the second type, the microwaves generated from a magnetron are delivered to a coaxial waveguide by a rectangular waveguide, the plasma being formed at the tip of a central electrode. Most of the C M P sources are derived from the designs of Cobine and Wilbur [155] and Schmidt [156], which operate in the frequency range from 500-2450 M H z .  A  schematic diagram of a commercialized CMP is shown in Fig. 5 [ 157]. The discharge is a torch-like plasma. Recently, Winefordner etal. [158, 159] have developed new torch designs for the C M P , including a high power C M P (1.6 kw). They reported that the high power C M P exhibits a number of advantages over the ICP and most MIPs. For example, it is less sensitive to the introduction of aqueous solutions, has lower detection limits for nonmetals, is less expensive, is safer and is easier to operate. The detection limits for Cd, Mg, Sn, T i , Ni, Fe, A l , Pb, Ca, Cr and Na were in the range from 0.05 - 12 ppb.  1.4.2 Principle of plasma operation Electrical discharges sustained by RF or microwave fields differ significantly from D C discharges in a number of ways. These include the initiation of the discharge and the conditions required to maintain the plasma. The interaction of the RF field, electrons or molecules, ions, and containing walls determine the values of the plasma parameters in a gas which is subjected to an electric field. The breakdown electric field is a function of the ionization potential of the gas, the collision properties of the electrons in the gas, plasma torch dimensions, and the frequency of the applied field. In the case where there are no electrons in the gas during the time aRF  24  Fig. 5. Schematic representation of the A . R . L . capacitively-coupled microwave plasma with nebulizer. 1. Sample solution. 2. Pneumatic nebulizer (concentric type). 3. Fog chamber. 4. Drainage. 5. To exhaust. 6. Aerosol injection tube (PTFE) to "burner." 7. Tuning stub. 8. Coaxial waveguide. 9. Magnetron. 10. Stabilized power supply. 11. Replaceable two-piece burner tip (see text for explanation).  {Reprinted from P.WJ.M. Boumans, F.J. de Boer, F.J. Dahmen, H. Hoelzel, and A. Meier, SpearochinuActa, 30B, 449 (1975)}  25 field is applied, no energy transfer can take place and no discharge can be generated. However, electrons are always present in a gas resulting from cosmic radiation. When an RF or microwave electricfieldis applied across a gas, electrons and charged particles in the volume are accelerated. The accelerating movement of various particles results in large numbers of collisions. Subsequently the gas is getting partially ionized and more electrons are produced and more collisions take place. This procedure repeats until an avalanche occurs. Therefore, the gas is brokendown and a stable plasma is maintained when the energy obtained from the RF power supply is balanced to the energy lost to collisions and other forms. In the discharge the electrons are accelerated much more than the ions due to their much smaller mass. When the direction of thefieldchanges, the electrons oscillate, following the RF frequency within the volume of the gas. If the frequency is too low, the reversal of direction of acceleration does not take place before the electrons strike the wall of the gas container. This is the characteristic that distinguishes an RF or microwave discharge from the low frequency discharge. The electric field required to break down a gas is much higher than that required to maintain the discharge after it has started. Usually a Tesla coil is required to start a plasma at atmospheric pressure. In an ICP discharge, the RF currents flowing in the induction coil generate an alterating magnetic field which induces an eddy current in the plasma. This electron flow is analogous in behavior to the current flow in a short-circuited secondary of a transformer. Therefore, the ICP is a low voltage and high current discharge. In the case of the CCP discharge, the RF potential is much higher than that in the ICP arrangement. If the applied RF potential or the induced potential at the tip of the electrodes is high enough to break down the gases, a self-starting of the plasma becomes possible. While the electrons obtain energy from the field, they also lose energy via collision with gas molecules and atoms. The collisions play a major role in the processes of dissociation, excitation and ionization in a plasma.  26 1.4.3 Application of CCP The applications of the CCP, CMP and A P A N can be found in the areas of A A S [1, 160], A E S [2, 3, 137, 142 - 144], and M S [153]. Some of the applications and results have already been provided in the history section. The detection limits in an argon CCP reported by Egorova [137] for 24 elements were in the range of 0.5-30 ppm and 0.05-0.3 ppm using a pneumatic nebulizer and an ultrasonic nebulizer respectively. Boumans etal. [157] have compared the analytical performance of a C M P to an ICP in emission spectrometry. In 1983, Wunsch etal. [160] used a nitrogen C M P as an atomizer for atomic absorption spectroscopy (AAS). A n excitation temperature of 5600 to 4750 K and a kinetic temperature of 4450 K were reported for their plasma. The sensitivities for Ba, Ca, Co, Cr, Fe, M g , Mo, Ni, Sr, W and Zn were given. Referring to equal optical path lengths the sensitivity in the CMP (and also the ICP ) was not better than in reducing acetylene flames. Emission spectrometric determination of trace elements in aqueous solution by mantle stabilized C M P was reported in 1981. Argon served as the plasma gas. By the addition of nitrogen to a second concentric gas mantle, the excitation source can be stabilized to give better excitation conditions. Detection limits for Be, Mg, Ca, Sr, B , A l , Ga, T l , Si, Pb, Cu, A g , Cd, T i , V , Cr, Mo, Mn, Fe, Co, and Ni were in the range of 0.02-2.0 ppb [161]. The determinations of arsenic , and tungsten by CMP-AES were also reported. In 1976, Zayakina etal. [162] reported an electrodeless high frequency capacitive discharge (EHFCD) as radiation source for A E S of solutions. The detection limits were of the order of 10" g for A l , As, B, Ca, Cd, Co, Cr, Cu, Ga, Fe, In, M g , M n , M o , N i , Pb, 8  Sb, Si, Te, Ti, T l and Zn. The relative standard deviation of individual determinations was 4-8 %. Compared to low pressure [143], an atmospheric pressure RF CCP has some definite advantages for spectrochemical analysis. Atmospheric pressure operation is not only convenient for changing samples but also provides for the possibility of high-yield R F  27 sputtering sampling. Atmospheric pressure plasmas provide a relatively high thermal gas temperature, which should allow more complete dissociation of molecular species. This should reduce the occurrence of chemical interferences in a excitation source.  The CCP  torch developed by Liang and Blades [ 1] in our laboratory can be operated at atmospheric pressure and at very low RF input powers (30 - 600 W) which allows for optimal conditions for atom resonance line absorption measurements. Sample introduction into the plasma is accomplished by using an electrically heated tantalum strip vaporizer. The vaporization and atomization steps are controlled separately. Analyte absorption takes place in the plasma discharge, which is characterized by a long path length (20 cm) and low support gas flow rate (0.21/min), in both of which provide for a relatively long residence time. The optimum RF power for Ag analysis was found to be between 100 and 200 W and the optimum support gas flow rate was found to be 0.6 1/min. The device exhibits linear calibration plots over the concentration range 0-10 ng of silver. The sensitivities were in therangeof 3.5 - 40 pg for Ag, Cd, Cu, Li, and Sb, and are comparable to those obtained with graphite furnace AAS. Further research indicated that the signal to noise ratio and the precision in the CCP emission mode are better than in the absorption mode. As a result, the detection limits of the CCP in AES were lower than those in AAS, and were in therange0.7- 80 pg for Ag, Cd, Cu, Li, and Sb. The interferences of thirteen elements were negligible in the determination of silver. Chloride interference, a notorious interference in GF-AA, cannot be found. The plasma discharge tube and sample introduction device allow for the separate control of vaporization and excitation, giving the CCP better detection limits than the ICP. Further development of the CCP in this laboratory resulted in a combination of the CCP with graphite furnace. As it was termed, atmospheric pressure furnace-capacitively coupled plasma (APF-CCP) [150-151] is a new and very promising source for analytical spectroscopy which combines the high efficiency of atomization in furnaces with the high efficiency of excitation in atmospheric pressure plasmas and provides many advantages  28 over furnace atomization non-thermal excitation source (FANES) [163-165]. The detection limit for A g of 0.3 pg for the APF-CCP is comparable or superior to that for F A N E S and GF-AAS.  1.4.4  Coupling of CCP with GC  Some capacitively coupled plasmas mentioned in the previous sections have already been used for a GC detector. Rice et al. have reported on the determination of halogen and sulphur-containing compounds [166 - 168] using a helium afterglow produced in concentric quartz tubes. Skelton and co-workers used similar system to detect H, S, CI, Br and I [ 169]. The detection limits were in the pg/s range. Uchida and co-workers used a CMP  to detect organotin compounds [170]. Recently Platzer et al. [171] introduced a  commercial GC-atomic selective detector based on a capacitive plasma. Similar to the design by Zvyagintsev [147], the plasma torch consists of two annular electrodes. The discharge tube is water-cooled, thin-walled (0.1 mm),  1 mm  i.d. quartz tube, with  detection limits also in pg/s range. Clearly the capacitive plasma is an attractive alternative for GC atomic-selective detectors. Although the microwave induced plasma has been demonstrated the most useful detector among the various plasma sources for gas chromatography, it still has some shortcomings such as the need to tune microwaves often to get impedance matching and the short lifetime of discharge tubes due to plasma erosion. The atmospheric pressure helium capacitively coupled plasma is a good alternative source which overcomes these problems. The CCP developed in our laboratory has the following characteristics: (1) Low power required to sustain the plasma (2) Low gas consumption for routine analysis (3) Atmospheric pressure operation (4) Self-ignition (5) Ease of achieving impedance matching  29 (6) Good stability (7) Lack of interference from contact with the electrode walls (8) High excitation ability for both metal and non-metal elements (9) Flexible torch configuration (10) Choice of helium, argon and other gases On the other hand, like MIPs, the low-power CCP has low gas temperature and small tolerance to the molecular species and droplets. This makes it impractical for use in the analysis of solution samples directly. However, it is very suitable in the case that small amounts of sample are introduced. Therefore it is apparent that the coupling of CCP with GC is one of its practical combinations. The development of GC-CCP to be discussed in this thesis has undergone 3 steps which are based upon torch configurations. The three different torch structures and their performance will be evaluated separately. In the first step to develop the GC-CCP system, two plasma torches, a cylindrical one with two concentric cylindrical electrodes and a rectangular one with two parallel electrodes, were designed and their performance evaluated.  It was found that the  rectangular torch was more stable than the cylindrical one since gas flow in the latter caused turbulence and the formation of an arc-like plasma was observed.  Therefore the  rectangular torch was chosen for further experiments. Another consideration of using the rectangular torch was that it is easier to interface it with the capillary column from a gas chromatograph. With a flow of make-up gas consisting of the plasma gas, the outlet flow of resolved analytes from the GC column was entrained into the discharge.  Such  configuration minimized the dead volume so that the high resolution of the capillary column would not be degraded by the plasma torch volume. One problem encountered in the initial work was that the arcing occurred in the air between the electrodes which often damaged the plasma torch, even though quartz tubes  30 enclosed to isolate them from the air. The reason was believed to be due to the high voltage from the power supply. In the second step, a new RF power supply was used. This 13.56 MHz RF power supply provided lower output voltage so that the electrodes did not have to be enclosed in insulator and can be exposed to air without external arcing around the outside of the torch. This made torch designing much simpler. At this time, a demountable plasma torch which could be separated from the electrodes was designed. The advantage is apparent, it is easy to change the plasma torch and the performance of different tube sizes could be easily evaluated with the same torch device. The torch performance was evaluated for the determination of organotin compounds. Although the results from the first two steps were promising, there were still problems with the torch structures. For example, the power could not be transferred to the plasma sufficiently and the heating of the electrodes was unpredictable. Therefore further improvement of the torch design was needed. Based on the previous torch structure, a water-cooled torch was designed. With this configuration and the 13.56 MHz power supply, the electrodes could contact the discharge tube directly and the plasma was formed without arcing. The power efficiency transferred to the plasma was increased significantly. Furthermore, the plasma created was extremely stable. Both metal and non-metal elements could be excited sufficiently using this modified configuration and better results have since been obtained.  31 Chapter 2  Initial Development of CCP as G C Detector*  2.1 Introduction The use of atomic emission spectroscopic detectors for gas chromatography (GC) is an important methodology because it can simplify the interpretation of complex chromatograms by providing element-specific information about each eluting peak. In this context, plasma sources are playing a very important role. Since the first paper [4] based on the plasma emission spectroscopy was published, several kinds of plasmas, including microwave induced plasmas (MIP), direct current plasmas (DCP) [44, 45, 172], alternating current plasmas (ACP) [173, 174], and inductively coupled plasmas (ICP) [34] have been utilized as GC detectors. The MIP has been the most successful because a relatively low power is required to sustain the plasma, a relatively small quantity of gas is consumed during routine operation and the detector volume can be kept small. In the initial work using a MIP as a GC detector McCormack et al. [4] used a 2450 MHz, atmospheric pressure argon discharge to detect the elution of halogen and other nonmetal containing compounds. The detection limits for most of the non-metal elements were found to be in the range of 10* to 10 g/s. Bache and Lisk [75, 175 - 177] used a 12  -9  helium MIP at low pressure (5-10 mm Hg) and found that better power coupling and hence better atomization characteristics could be obtained. This situation was improved with the *Part of this chapter was published in /. Anal. Atom. Spectrom^ 4, 789 (1989), Degui Huang, Dong C. Liang and Michael W. Blades.  32 development of the Beenakker [59] TMoio cylindrical resonance cavity which allowed efficient power coupling to a MIP at atmospheric pressure and at a relatively low power level (40-100 W). Although the MIP is an excellent excitation source for element selective GC detection it is constrained by some operational limitations. MIP cavities must be resonant with the driving frequency. According to Beenakker and Boumans [178], coupling of power using a fixed loop as originally described by Beenakker [59] is adequate for a helium plasma but not for an argon plasma. Additionally, discharge conditions in MIP depend both on the inner diameter of the cavity and the dielectrics inside the cavity. MIPs are normally ignited using a tesla discharge. This chapter describes an atmospheric pressure capacitively coupled plasma (CCP) torch which can be used as an element selective, spectroscopic detector for GC. This plasma source is similar in design to a CCP source which has been used for atomic spectroscopy [1,3]. One of the main differences between the configuration described in this paper and that previously described is that the plasma torch was made smaller to reduce peak broadening and to enhance the sensitivity for GC detection.  Using the torch  described in this paper, a stable self-igniting plasma can be sustained over a wide range of operating frequencies, input powers, and at very low carrier gas flow rates. The major advantages of this torch over a MIP is that the discharge can be sustained over a very wide range of conditions; power from 10 to 500 W, frequency from 200 KHz to 30 MHz, and carrier gas flow rates as low as 20 ml/m. In addition, the torch configuration is simple, the plasma is stable, and a separate ignition system is not required.  2.2 Experimental 2.2.1 The C C P G C detector design A schematic diagram of the experimental system is provided in Fig. 6. Details of the  33  —1  POWER SUPPLY Fig. 6. Shematic diagram of the experimental system used to test the C C P as a gas chromatographic detector P: Plasma torch L: Focusing lens M: Monochromator V: PMT h.v. power supply A: Amplifier R: Chart recorder  34 experimental facilities are outlined in Table 2. The outlet of the column was connected to the gas inlet of the CCP through a 1-m length of 1.5-mm i.d. Cu tubing which was maintained at a slightly higher temperature than the GC column using heating tape (Electrothermal Engineering Ltd.), which surrounded the interface tubing. Details of this interface are provided in Fig. 7. Two different plasma discharge geometries have been used. Schematic diagrams for each of these are provided in Fig. 8. Fig. 8(a) shows a concentric design which is made from a single piece of quartz. The electrodes which couple the RF power into the discharge are arranged concentrically in that the inner electrode is housed inside a hollow quartz shaft which is sealed such that there is no electrode contact with the plasma. The outer electrode is simply a stainless steel cylinder which is wrapped around the outside of the quartz tube in the manner shown in Fig. 8(a). When the RF power is applied, the plasma fills the annular space between the electrodes as well as the small space at the end of the torch. The second geometry [Fig. 8(b)] consisted of three rectangular quartz tubes which had dimensions 2 mm by 4 mm. The central tube containing the plasma was sandwiched between two quartz tubes, each of which enclosed a stainless steel electrode of slightly smaller dimensions than the interior dimensions of the quartz tubes. Carrier gas from the GC column was directed into the plasma through one end of the torch. A B5 joint tube served as support but did not act to introduce any extra flow of gas.  Heating tape Copper  fitting  I Plasma torch  Teflon tape Copper tube  Fig. 7. Schematic diagram of the interface between the GC column and the plasma torch  36  End View  O.D.=3mm " I.D.-1.5mm From G C  0. D.=7mm 1. D.=5mm  Support arm  End View  80 mm  From G C  2x4mm Electrodes Support arm  Fig. 8. Schematic diagram of the C C P torches, (a) Concentric geometry and (b) Parallel-plate planar geometry  37 2.2.2 Equipment and setup The equipment and experimental setup employed in this research are summarized in Table 2.  Table 2: Experimental facilities and operating conditions  Gas Chromatograph  Varian 6000 Gas Chromatograph Columns: OV-101 and OV-07 Carrier gas flow rate: 30-80 ml/min  Plasma Power supply  (a) Perkin-Elmer ICP 5500 system consisting of a PlasmaTherm (Kreeson, N.J.), HFP-2500F RF generator, AMN2500E automatic matching network, APCS-3 automatic power (b)  control  system  and  PF2500  torch  box  ENI power systems Inc.(Rochester,N,Y,), Model  HPG-2 RF power supply; frequency 125-375 KHz, output power 0-200 W Spectrometer:  Schoffel-McPherson (Acton, MA) Model 270,0.35 CzernyTumer mount scanning monochromator with 1200 lines/mm holographic grating, reciprocal linear dispersion of 2 run/mm in the first order, entrance and exit slits set to 50 um  Detector electronics  The photocurrent from a Hamamatsu R955 photomultiplier tube was amplified by an amplifier made by the department electron shop. The photomultiplier tube was powered by a McPherson Model EU-42A PMT power supply  Data Acquisition  Digital data acquisition: Zenith computer, Model W-248-82, (512 K, Zenith Electronics Corp.), IBM-AT compatible computer equipped with a RC Electronics (Santa Barbara, CA) Model ISC-16 analog-digital converter running the RC computerscope software package. Analog data recording: Fisher Recordall 5000 chart recorder  38  2.2.3 Analytical procedure The spectra of organic compounds were obtained by using a 200 ml plastic bottle as a sample diluting container, in which the carrier gas was mixed with the injected volatile organic compounds and introduced to the plasma. Using this apparatus a continuous flow of volatile compound could be introduced into the plasma so that emission spectra could be obtained from a scanning monochromator. After selection of an appropriate analysis line, the wavelength was fixed, the bottle was removed, and the GC interface was arranged according to Fig. 7 so that the GC effluent could be introduced to the discharge. Samples were prepared in hexane or toluene solvents. The sample solution was injected using a Hamilton Series 7000 (Reno, Nevada) 1 ul syringe. Using the current configuration, liquid samples larger than 0.2 ml could not used because the discharge was extinguished by the solvent. Although the discharge would automatically re-ignite after the solvent front passed through the discharge, deposition of soot inside the discharge tube could be observed. It is possible that this problem could be alleviated by using a make-up gas such as oxygen to create an oxidizing environment inside the plasma discharge.  2.3 Results and discussion  2.3.1 Description of the C C P detector When operating at 27.12 MHz energy was lost through heat radiation more easily in the concentric torch [Fig. 8(a)] than the planar, parallel plate one [Fig. 8(b)]. This is probably because turbulence is much more serious due to the cylindrical shape of the torch. Also, when the input power was increased, the discharge in the concentric torch became unstable. Both argon and helium gases were used in this work. It was found that the helium plasma was more suitable for nonmetal elements (I, CI, and Br); whereas metal elements (Sn) give much larger responses in argon plasma. It is possible that there are  39 some selective energy transfer processes between excited He and the non-metals which are not operative for Sn. Throughout this work, helium was used when non-metal elements were being detected while argon was for metal elements.  2.3.2 Emission spectra and chromatograms To show that it is feasible to use the CCP to dissociate organic compounds and excite the atoms we have continuously introduced tetramethyltin (Me4Sn) into the concentric plasma. The tin emission spectrum from Me4Sn between 270 and 306 nm is reproduced in Fig. 9. A comparison of tin emission from Me4Sn with the background spectrum from toluene is provided in Fig. 10. The RF input power was 100 W at 200 KHz and the carrier gas flow was 30 ml/m. The presence of Sn I lines indicates that this CCP is an effective dissociation and excitation source. Fig. 11 shows the emission spectrometric gas chromatograms of iodomethane  (CH3I)  [Fig. 11(a)] and l,3-diiodopropane(I(CH2)3l) [Fig. 11(b)] solution in hexane. The planar parallel electrode plasma [Fig. 8(b)] was operated at an input power of 50 W at a frequency of 27.18 MHz and a GC carrier gas flow rate of 30 ml/m. The monochromator was fixed at the iodine 206.2 nm line. The column temperature for C H 3 I and I(CH2)3l were 90 °C and 120 °C respectively. The injector block temperature was maintained 20 °C higher than the oven temperature in each case. The interface transfer line temperature was maintained at 10 °C above the column temperatures. The  CH3I  was injected as the headspace vapor;  whereas I(CH2)3l was injected as a solution. From Fig.9 it can be seen that the solvent (hexane) also gave response because of an increase in background emission. This problem could be overcome with background correction by using polychromator or a multi-channel photodiode array instead of a single channel PMT detector.  40 CO  CO Z3  OJ  ro co CO  3  -ICO  3  o  co  OJ  3  b  CO  3  ro  OJ  OJ  o o  CD CO CD ^ OJ  CD  ro co  CO  3  ro CD  306.0 Wavelength (nm)  Fig. 9. Tin emission spectrum obtained using Ar C C P between 270 and 306 nm Power, 100 W;  frequency, 200 KHz; carrier gas flow-rate, 30 ml/m  41  00 3  2 7 0 . 0 nm  Fig. 10. Comparison of tin emission spectrum between 270 and 306 nm from Me4Sn and background from toluene obtained using Ar CCP Power, 100 W; frequency, 200 KHz; carrier gas flow-rate, 30 ml/m; (a) Spectrum of Me4Sn and (b) Background spectrum detected using the concentric Ar CCP geometry  42  (0  c ® c c o  "55 tn  E ill  40  20  Retention time (s)  (0 c CD  c o (A  E  LU  0  40  80  Retention time (s) Fig. 11. (a) Gas chromatograms for three replicate injections of 150 pg of iodomethane {CHI} into the GC. (b) Gas chromatogram of 0.1 M-l of 0.1 ppm di-iodopropane {l(CH )3l} in hexane detected using the CCP. 3  2  43 2.3.3 Analytical performance The 3a detection limit for the GC determination I in I(CH2)3l was determined to be 8.2xl0" g/s. The value, obtained using MIP emission spectrometry which has been 14  reported previously using the 206.2 nm I line, is 2. lxlO* g/s [ 107]. 11  2.4 Conclusion The discharge design described in this chapter provides for very effective energy transfer from the power supply to the plasma by capacitive coupling. A He or Ar plasma can be generated at atmospheric pressure at any frequency between 0.20 and 27.18 MHz (and probably higher although it has not been tested in this laboratory). Preliminary work using this plasma as an element selective detector for gas chromatography was promising. The advantages of the CCP when compared with the MIP are its relatively simple construction, ease of ignition, flexibility in choice of operating conditions (RP power and frequency) and, as Figs. 3 demonstrates, the flexible geometry. However, there existed several problems in the current experimental setup. First, the plasma torch tube was damaged often by the high voltage form the 27.12 MHz power supply. Secondly, arcing could be produced in the air between the two electrodes. As a result, the electrodes have to be insulated from air. So the electrodes were enclosed in quartz tubes as indicated in Fig. 8. Thirdly, the torch was designed in this chapter to be coupled a packed column and the carrier gas was used for the plasma support gas. In order to couple it with capillary column a make-up gas inlet has to be added for the use of plasma support gas since the optimal flow-rate of capillary column GC is much less than that for plasma support gas. The first and second problems were overcome by using an Adanced Energy, 13.56 MHz power system and a new plasma torch was designed to solve the third problem. The next chapter will discuss the improvement.  44  Chapter 3 Evaluation of a 13.56 MHz Gas  Capacitively Coupled Plasma as a Detector for  Chromatographic Determination of Organotin Compounds  3.1 Introduction In recent years, metal and  non-metal speciation studies have increasingly attracted the  interest of analytical chemists because of the importance of trace elements in toxicology environmental science. One  and  means of obtaining species specific information is through the  use of chromatographic separations. A variety of atomic emission sources has been developed as chromatographic detectors since these sources can provide element specific information about each eluting peak. Plasma sources, in particular the inductively coupled plasma (ICP), direct current plasma (DCP)  and microwave induced plasma (MIP)  have  been extensively investigated for this purpose. The and  coupling of gas chromatography (GC)  with an ICP  was described by Windsor  Denton for simultaneous multi-elemental analysis of organic and organo-metallic  compounds [34]. Microwave induced plasmas have been investigated for many years. The  first coupling of an MIP  with GC  was reported by McCormack et al. in 1965 [4].  Since then many examples using this methodology have been developed including the use of capacitive microwave plasmas (CMP)  [179] and surface wave sustained plasmas  (Surfatron) [62]. Various types of samples have been determined, such as pesticide residues [75], haloforms in drinking water [105], and  organo-metallic compounds [107].  Organotin compounds have been widely used as biocides, catalysts, and polymer stabilizers and  their effects on environment are causing concern [180]. A knowledge of  *Part of this chapter was published in /. Michael W. Blades.  Anal. Atom. Spsctrom.,  5, April (1991), Degui Huang and  45  the concentration, chemical form, and distribution of these compounds provides important information on the origin and transport mechanisms.  Several G C and liquid  chromatography (LC) approaches incorporating plasma-based detection have been developed to determine the organotin compounds. Krull and Panaro [181] used a system whereby the organotin compounds were separated using a high-performance liquid chromatograph (HPLC), followed by a continuous, on-line, hydride generation with a DCP emission spectrometer to detect the effluent. Suyani et al. [182] described the use of helium microwave-induced plasma mass spectrometry for capillary gas chromatographic detection of organotin compounds. Uchida [170] etal. recently reported a capacitively coupled helium microwave plasma as an excitation source for the determination of organotin compounds. They used a capacitively coupled microwave plasma (CMP), in which microwaves were generated using a magnetron and conducted through a coaxial waveguide to the CMP excitation source. A tubular tantalum electrode sample injector was employed for the CMP in order to achieve high sensitivity and a more stable discharge. The analytical performance of this plasma source for the determination of inorganic tin and butyltin was evaluated by interfacing the helium CMP to the front end of a gas chromatograph. The analytical merit compared well with helium MIP systems, although electrode contact with the plasma introduces the possibility of contamination. One of the problems of using the MD? as a GC detector is that materials deposit on the walls of the discharge tube. Noticeable deposits can be found for long-chain hydrocarbons and other oxygen-free compounds. This problem is even more acute with samples containing inorganic compounds prone to forming refractory species. Besner [ 131 ] et al. investigated the effect of dopants on tin emission in a helium MD?. They doped various liquid and gaseous materials to the helium plasma and found sulphur hexaflouride gave the best results.  46 A novel parallel plate capacitively coupled plasma (CCP), which can be used as an emission spectrometric detector for gas chromatography, has been described in a previous chapter. When using this CCP a helium or argon plasma can be generated at atmospheric pressure at frequencies of 0.20 or 27.18 MHz and at carrier gas flow-rates as low as 20 ml/m. This chapter describes the further development of the CCP as a GC detector operated at 13.56 MHz, outlines some of the spectral and operational characteristics, and characterizes its application to the determination of some environmentally important organotin compounds.  3.2 Experimental  3.2.1 Power supply An Advanced Energy Model RPX 600, 13.56 MHz (Fort. Collins, CO, USA), radiofrequency (RF) generator equipped with an Advanced Energy Model ATX-600 automatic impedance matching system was used to supply power to the CCP torch. The ATX-600 tuner was modified to include a 4 - 5 uH inductor in series with output line to improve matching efficiency. When operated at 200 watts forward power the reflected power could be maintained at less than 3 W.  3.2.2 Spectrometric system The monochromator, PMT, current amplifier, and chart recorder used in this chapter were the same as was described in the previous chapter.  3.2.3 Gas chromatograph The gas chromatograph was the same as in the previous chapter except that a SUPELCO Model SPB-1, fused silica capillary column (15 m x 0.53 mm o. d. with a film thickness of 0.50 um) was used rather than a packed column because of its chemical  47 inertness and high column efficiency. For organotin compounds, if a solid column support is insufficiently covered by the stationary liquid phase (e.g., 2-5%), sample adsorption on the exposed siliceous sites becomes significant with polar solutes and peak tailing occurs [183]. The injector block was maintained at 280 °C for all experiments.  3.2.4 Plasma torch Fig. 12 shows a schematic diagram of the CCP and holder used in this chapter. The torch was fabricated from a section of fused silica, rectangular in cross-section, with external dimensions of 6 by 4 mm and internal dimensions of 4 by 2 mm. The total length of the rectangular portion of the torch is 6 cm. One end of the torch, from which emission was observed, was open to the atmosphere and the other end was connected to a T-shaped arrangement of quartz tubes (o.d. 4 mm, i.d. 2 mm) for the introduction of sample and make-up gas (see Fig. 12). The effluent from the GC was introduced into the torch through a quartz capillary which was sealed to the torch at one end. An additional gas inlet allowed the use of a make-up gas to support the plasma. The torch was placed in a vice-like clamping device and 40 mm long 1.8 mm thick wafers of Boron nitride were placed between the torch and the stainless steel electrodes, which were 4 cm in length. These were clamped in place using Delrin and an aluminium holder which was tightened using an adjustable clamp as depicted in Fig. 12. Using this torch mount the CCP discharge could be easily assembled and different sizes of quartz tubing and electrodes could be tested. The torch was operated both with and without the presence of the boron nitride insulator between the quartz and the electrodes. With the former, the intensity of H(I) 686.13 nm at 150 W was almost the same as that at 100 W with the latter. It would appear that 50% power is lost with the former structure. However, without the boron nitride insulator, the electrodes became fairly hot and the Delrin softened which made it difficult to maintain the integrity of the torch and holder. For this reason the torch was operated with  48  Support rod  From gas chromatograph  Plasma torch  Make-up gas inlet Adjustable screw clamp Boron nitride  Top view  Stainless steel  H  Al  §^3 Delrin®  End-on view  Fig. 12. Schematic diagram of the discharge tube and support structure  49  the boron nitride strips for all the experiments described in this chapter. Helium was used for both the carrier and make-up gases.  3.2.5 Transfer interface For the transfer line between the chromatograph and the CCP, the capillary column was enclosed in a copper tube (70 cm x 0.32 cm o. d.), which was wound with a heating tape and enveloped with glass wool and cotton tape. The temperature was controlled by using a Variac rheostat to adjust the voltage to the heating tape. The transfer line was maintained at a temperature of 280 °C for all experiments in this chapter.  3.2.6 Data acquisition Except when indicated otherwise, the working conditions were as follows: the RF power supply operated at forward power 150 W with the reflected power 2 W in automatching mode. The make-up helium gas flow rate was 200 ml/m. The monochromator wavelength setting for Sn(I) 284.0 nm was made by using a tin hollow cathode lamp. After the sample was injected, the gas chromatographic column was maintained at the initial temperature. The solvent began to elute at the retention time of 0.61 minutes. As soon as the solvent was eluted, the RF power was applied; the plasma self-ignited and the chromatograph was operated in isothermal or temperature program mode and data were collected on the chart recorder.  3.2.7 Chemicals Ferrocene (98%), tetramethyltin (Me Sn, 99%), trimethyltin chloride (Me SnCl, 4  3  99%,) were purchased from Aldrich Chemical Company, Inc. Dichloromethane (CH2CI2, 99.9%), from BDH and tetrapropyltin (purity unknown), was obtained from Alpha Inorganics. All the chemicals were used without further purification.  50 3.3 Results and discussion  3.3.1 Helium plasma background A wavelength scan of the background emission from the 13.56 MHz atmospheric pressure helium CCP between 200 and 600 nm is reproduced in Fig. 13. The identification and assignment of the molecular bands were made using reference [184]. The most prominent features are those originating from OB.(A V-^GU), N H ^ n - X ! " " ) , 2  3  N O ( A I - X n ) , N ( C n - B n ) ,and N ( B Z u - X I ) , similar to those observed in 2  +  2  3  2  3  u  +  g  2  2  +  2  +  g  an MIP [185]. However, one of the differences is that a Q branch with 308.9 nm (0,0), 2  and 282.90 nm (1,0) for OH ( A I - X ^ ) was found more intense for the CCP compared 2  +  with the Rj branch at 306.36 nm(0,0) and 281.13 nm (1,0) for the MIP [185]. This may be related to the difference in gas temperature and suggests that the gas temperature in the CCP is probably lower than that for an MIP. However, further experiments must be completed to verify this suggestion.  The background features observed at different  positions (vertically and axially to the discharge tube) did not change significantly. From this observation, it can be concluded that the OH, NH, NO, N , and N .molecular bands +  2  2  arise from the impurities in the helium gas supply. Estes etal. [107] used a 5 A° molecular sieve immersed in liquid nitrogen to find if the molecular bands in a MIP were via back diffusion of air. Their results confirmed that no air entrainment occurred and the bands resulted primarily from the impurities.  3.3.2 Spatial emission characteristics The spatial emission characteristics for He(I) at 447.15 nm, He(I) at 504.77 nm, and H(I) at 486.13 nm, were measured for the helium CCP and are shown in Fig. 14. The measurements were made by forming an end-on image of the plasma at the entrance slit of the monochromator and translating the image across the entrance slit by moving the torch  200  250  300  350  400  W A V E L E N G T H (nm)  Fig. 13(a). Background emission spectrum of the 13.56 MHz helium plasma from 200 to 400 nm. power: 100 W , reflected power 2 W , make-up gasflowrate: 200 ml/m, carrier gasflowra  (B) CM  CU  X  400  450  500  550  W A V E L E N G T H (nm)  600 Ul  Fig. 13(b). Background emission spectrum of the 13.56 MHz helium plasma from 400 to 600 nm. power: 100 W , reflected power 2 W , make-up gas flow rate: 200 ml/m, carrier gas flow  53  Distance from left side wall (mm)  Distance from left wall (mm)  Fig. 1 4 . (a) Spatial distribution of emission from: A , H (l) 486.13 nm; B, He (I) 504.77 nm, and C, He (I) 447.15 nm without sample. Input power 150 W; make-up gas flow-rate, 200 ml/m and carrier gas flow-rate, 0 ml/m. (b) Spatial distribution of: A , Sn 284.0 nm; and Fe 371.99 nm for a helium CCP with organic sample introduced. Input power, 150 W; and make-up gas flow-rate, 200 ml/m  54 assembly. As can be seen from Fig. 14(a), the spatial distribution for all three species shows a maximum near the walls and minimum at the center. The spatial distributions of emission from Fe at 371.99 nm and Sn at 284.0 nm, introduced as as organic compounds, were also measured. For these lines, a relative maximum is observed at the center, as shown in Fig. 14(b). The Sn spatial distribution was obtained using the output from the gas chromatograph whereas the Fe signal was collected with continuous introduction of sublimed ferrocene in the headspace of a sampling vial. A similar distribution has been observed in a surface wave plasma (Surfatron) [187, 188], in which cylindrical plasma tubes were used. Richard et al. explained that if one-step excitation through electron collision with an atom in the ground level is assumed, the distribution of emission is dependent on the distribution of the total electric field intensity Ex and the electron density {n(r)J through the relation:  nj(r) = An(r)E*(r)  (1)  where nj(r) is the population density of the excited atoms in level j , A is a constant independent of position and k is a value dependent on the plasma medium and excited state parameter, and can be determined from theory, r refers to the radial distance. The magnitude of both the electric field and the electron density are spatially dependent. The electric field intensity is higher at the wallsjwhereas the electron density decreases near the walls as a result of recombination losses. Although the appropriate measurements have not been made at this time it is possible that the spatial distributions observed for the CCP are similar in origin to those for the surfatron.  3.3.3 Effect of input power In order to study the effect of changes in RF input power several injections of 20 ng of Sn (0.1 ul of solution of 100 ppm Me3SnCl in CH2CI2 solvent) were made into the gas  55 chromatograph and the emission intensity for Sn was measured. The intensity was not dynamically corrected for background; however, it was found that the background was relatively constant for these experiments. The peak intensity for Sn(I) 284.0 nm as function of input power is shown in Fig. 15. As stated earlier, about 50% power is lost with the electrodes isolated from the discharge tube but this structure prevented the electrodes from becoming too hot and produced more stable plasma. From Fig. 15 it can be seen that the intensity of Sn increased almost linearly with an increase in RF power. It is probable that the total applied RF power is not all delivered to the plasma since there are losses in the output inductance, the dielectric materials and the electrodes themselves. Therefore the actual power consumed in the discharge is less than is indicated in Fig. 15. As a result of the heating of the electrodes it was difficult to operate at powers higher than about 400 W since the heat would soften Delrin insulators. For these reasons, a torch with water cooled copper electrodes has been developed and future work will be carried out with this new design.  3.3.4 Effect of gas flow rate The effect of helium make-up gas flow-rate on the Sn peak intensity is shown in Fig. 16. The Sn signal increased with the gas flow-rate from 50 to 350 ml/m reaching a maximum between 350 and 400 ml/m after which a decrease was observed. At flow-rates greater than about 250 ml/m turbulence could be observed in the gas flow through the torch, resulting in instability of the emission signals. Therefore, a make-up gas flow-rate of 200 ml/m was used for a further experiments. The response of Sn emission intensity as a function of gas chromatograph carrier gas flow-rate over the range 8-20 ml/m is depicted in Fig. 17. Although an increase in carrier gas flow-rate causes an increase in the Sn emission intensity, the chromatographic resolution degrades at flow-rates higher than about 10 ml/m. Therefore a 10 ml/m carrier gas flow-rate was used for the further studies.  56  50 |  1  1  - i  1  1  1  1  r  500 Power (W)  Fig. 15. Effect of input power on emission intensity for Sn at 284.0 nm Make-up gas flow rate : 200 ml/m and carrier gas flow rate: 10 ml/m  57  30  I  0  >  1  100  '  1  200  1  1  300  1  1  400  '  1  500  r  600  Make-up gas flow-rate (ml/m)  Fig. 16. Effect of make-up gas flow rate on emission intensity for Sn at 284.0 nm Input power: 100 W and carrier ga? flow rate: 10 ml/m  58  70 i  i i i i i i i i i i i i i i i i ii  Carrier gas flow-rate  (ml/m)  Fig. 17. Effect of gas flow rate on emission intensity for Sn at 284.0 nm Input power: 100 W , and make-up gas flow rate:200 ml/m  59  3.3.5 G C - C C P system performance The stock solutions (1000 ppm of Sn for each compound) were prepared by dissolving the relevant organotin compounds in dichloromethane and working solutions were prepared by appropriate dilution with the same solvent. Fig. 18 and Fig. 19 show the chromatograms of Me Sn andMe3SnCl, in which 10 ppm Sn solution from Me4Sn and 4  20 ppm solution from Me SnCl were used and different volumes of the solution were 3  injected. The gas chromatograph was operated in the isothermal mode; 45 °C for Me4Sn and 70 °C for Me3SnCl. Fig. 20 is typical chromatogram of a mixture of Me Sn , 4  Me3SnCl, and Pr4Sn. A 0.1 ul aliquot of 10 ppm Sn of each compound, that is 1 ng Sn, was injected. The signal was collected at 284.0 nm without background correction. The RF voltage was switched on after the solvent was eluted. The chromatograph was used in the temperature program mode, i.e., it was maintained at 45 °C for 1 minute, then raised to 260 °C at the rate of 50 °C/m and then held at this temperature for another 2 minutes. As can be seen in Fig. 20 the sensitivity of the peak signal for Me Sn is 2.8 times better than 4  that for Me SnCl. Serious tailing is exhibited by Pr Sn, which is believed to be due to 3  4  condensation of Pr Sn on the walls of the discharge tube. Since the heating tape could not 4  be brought close to the electrodes because of possible discharge between the electrodes and the heating tape, a "cold gap" existed in this region. Therefore, condensation of PtjSn, which has high boiling point (222 °C) is a strong possibility. Another reason for tailing may be from the deposition of tin oxide on the walls. Doping with some reagent materials can minimize this problem [ 12]. Table 3 lists the detection limits for Me Sn, MesSnCl Pr Sn. For these values, the 4  4  definition of minimum detectable level suggested by Sullivan [189] has been used. This is the mass of analyte required to produce a peak twice the height of the peak-to-peak noise, divided by the full width at half height of the peak in seconds. Chromatographers usually (but not always) measure peak-to-peak base line variation, which is considered to be 6a, as  8ng  4ng  3ng 2ng I ng  T  T  \  1.0 0.5 0 1.0 0.5 0 1.0 0.5 0 1.0 0.5 ~0 LO o!5 6 1  T  1  T  Retention time (minutes)  Fig. 18. Chromatograms of  Me4Sn  CTl  o  using the GC-CCP. Sn I 284.0 nm, input power: 100 W , make  16 ng I  12 ng  8ng 4ng  -|  1.5  1  1  1.0 0 . 5  n——i 1— n — i 1.5 0 1.5 1.0 0 . 5 0 1'  ^  1  1  1.0 0 . 5  r ~i 0 1.5  1  1  1.0 0 . 5  1  0  Retention time (minutes)  Fig. 19. Chromatograms of Me SnCI using the GC-CCP. Sn I 284.0 nm, input power: 100 W, make-up gas flow rate: 200 ml/m. 3  Me Sn 4  Me SnCI 3  i  4  i  2  I  TTo RFON\ RF OFF  Retention time (minutes)  Fig. 20. Chromatogram of mixture of Me4Sn, Me3SnCI and P 4Sn using temperature programming, Input power: 150 w, make-up gas flow rate: 200 ml/m and carrier gas flow rate: 10 ml/m. r  63 a measure of the noise [63]. For this report the peak-to-peak noise measurement was averaged over a time of period of 30 seconds.  Table 3: Detection limits (DL) Solute  DL(ng/s)  Me4Sn  0.079  Me SnCl  0.190  Pr4Sn.  0.168  3  3.4 Conclusions The parallel plate capacitively coupled plasma torch is a potential alternative to the MIP or CMP as a detector for GC for the determination of organotin compounds. The CCP can be operated at an RF input power of up to 400 W and make-up gas flow-rates ranging from 50 to 400 ml/m. The current system is an improvement over that in the previous chapter in that a demountable torch structure has been used and an automatic impedance matcher has been coupled to a 600 W, 13.56 MHz RF oscillator. Table 3 indicates that detection limits for organotin are superior to those obtained using a CMP [170] but inferior to those obtained using a MIP [182]. However, the CCP in this chapter was still at an early stage of development and there was certainly scope for improvement and further exploration into torch geometries, operating frequencies, power sources and sample introduction strategies. Using the present configuration, there is some power dissipation in the RF electrodes which leads to power loss and heating of the electrodes. A torch incorporating water cooled copper electrodes was designed to overcome this problem and the results will be provided in next chapter.  64  Chapter 4 Characteristics of an Atmospheric Pressure Parallel Plate Capacitively Coupled Plasma*  4.1 Introduction  Previously, work on the parallel plate GC detector focused on the determination of metals and some selected non-metals using ultraviolet and visible emission lines. A number of studies have shown that the near-infrared spectral region provides an attractive alternative for the detection of nonmetals such as F, CI, Br, I, C, H, O, N, S, P. Fry and co-workers [189-196] used an argon inductively coupled plasma (Ar ICP) to study the near-infrared spectra of F, CI, Br, O, N, C, H. The near-infrared spectra in He ICP at both atmospheric [197-200] and reduced pressure [201] have also been well characterized. In addition to the ICP, various other plasma sources such as the microwave induced plasma (MIP) [202-204], the hollow cathode discharge [205, 206], and flowing afterglow discharge [166-169] have been used for the near-infrared determination of nonmetallic elements. One of the objects of this chapter is to characterize the emission of nonmetallic elements in the near-infrared region from the CCP and the use of it in GC-CCP system appears in the next chapter. The last two chapters have described two stages of evolution for the design of a CCP torch useful as an atomic emission detector for gas chromatography. Initially, both parallel plate and concentric geometries were evaluated. It was found that a parallel plate torch  *Part of this chapter has been submitted to Appl.  Spectrosc., and is in press, D. Huang and M.W. Blades.  65  which utilized a rectangular-bore quartz tube had more stable emission signals which was attributed to less gas flow turbulence. In order to make the torch more amenable to testing a demountable version was designed [Chapter 3] such that different quartz tube and electrode dimensions could be tested. Also, the operating frequency was shifted from 27.12 to 13.56 MHz. One difficulty still encountered was heating of the parallel plate electrodes which yielded unstable plasma conditions at times. As a result, the present version of the CCP torch utilizes water cooled electrodes. For this chapter the fundamental and analytical characteristics of this 13.56 MHz, parallel plate CCP are further explored. The effect of changing the dimensions of the torch on emission intensities is examined, spectral characteristics in the near-infrared are reported on, and the question of excitation mechanisms is addressed.  4.2 Experimental  4.2.1 Plasma torch assembly A schematic diagram of the CCP torch assembly is provided in Fig. 21. The torches used were fabricated from fused silica. Four different sizes of quartz tubes were evaluated with the following inside dimensions - (a) a 1 mm capillary tube with round cross section, (b) a 2 X 4 mm tube with rectangular cross section (as used in previous work), (c) a 4 X 4 mm tube with square cross section and (d) a 6 X 6 mm tube with square cross section. The wall thickness for the square and rectangular cross section tubes was 1 mm and 0.2 mm for the round cross section tube. Each torch had two inlets as depicted in Fig. 21 one of which was a capillary inlet for introducing sample and the other a side inlet for adjusting the total plasma gas flow rate and composition. The torches were placed in a 'vice-like' clamping device, fashioned from aluminium and Delrin®, which enabled the  Support Rod Water out  Water in  Plasma Torch  FromGC  Make-up Gas Inlet Water out  I  Water in  Adjustable Screw Clamp Top View  End View  Fig. 21 Schematic diagram of the water-cooled torch device  67 torch to be firmly held between water-cooled copper electrodes. This differed from the torch assembly previously described (Fig. 12) in that the RF electrodes were in direct contact with the torch for the present version whereas in the previous torch, a layer of boron nitride was placed between the quartz tube and the RF electrodes. Using the device depicted in Fig. 21a variety of quartz tube sizes could be evaluated since changing from one size tubing to the next could be accomplished by simply loosening the clamp and replacing the quartz tube. To obtain the spatial distribution of emission intensities the entire torch assembly was mounted on micrometer-adjusted transverse and vertical translation stages (Ealing Scientific Ltd., Pointe Claire-Dorval, Quebec, Canada). The position was adjusted manually.  4.2.2 Sample introduction system The sample introduction system for the measurement of non-metallic element emission is shown in Fig. 22. Volatile organic compounds containing the test elements were brought into the tube (T) which was maintained at 0 c-C using an ice bath, and introduced into the discharge through continuous headspace sampling. The sample (S) uptake rate was controlled by using a Matheson (Matheson Gas Products, Edmonton, Canada) model 601 flowmeter (F) which was placed before the sample introduction system.  The  headspace sample was introduced into the torch through a quartz capillary tube which was sealed to the torch at one end. The additional gas inlet allowed the use of a make-up gas.  4.2.3 R F power supply The power supply used to generate the plasma was an Advanced Energy (Fort Collins, CO, U.S.A.) Model RFX-600 RF generator operating at 13.56 MHz and an Advanced Energy Model ATX-600 automatic impedance matching system. The maximum output power from this combination was 600 W. An adjustable 1 - 10 uH inductor was placed in series with the ATX-600 to facilitate impedance matching.  V  Fig. 22 Schematic diagram of the sample introduction system. C: carrier gas, F: gasflowmeter, S: volatile sample, T: test tube, V: ice bath vessel, F.plasma torch  69  4.2.4 Spectrometer A 40 mm diameter quartz lens with a focal length of 90 mm was used to image the plasma with unit magnification onto the entrance slit of a Schoffel-McPherson (Acton, MA, U.S.A.) Model 270, 0.35-m monochromator.  The grating installed into this  monochromator had 1200 groves per mm and was blazed at 500 nm. The entrance and exit slits were set at 50 um. Two different photomultiplier tubes were used for spectral data acquisition. A Hamamatsu (Middlesex, NJ. U.S.A.) Model R955 for the UV and visible and and a Hamamatsu Model R406 for the visible and near infrared. The tubes were biased at -1000 V using a Schoffel-McPherson Model EU-42A PMT power supply. The output of the PMT was amplified using a Keithley (Cleveland, OH) Model 301 Electrometer Operational Amplifier powered using a Keithley Model 3012 Power Supply and was recorded on a chart recorder.  4.2.5 Spectral response of the spectrometer To obtain corrected relative intensities for the principal atomic emission lines, an Electro-Optics Associates (Palo Alto, CA) Model L-10 quartz-iodine, tungsten filament standard lamp supplied with an Electro-Optics Associates Model P-101 power source [207] was used to establish the spectral response of the instrument. The relative spectral response of the spectral system using the two PMTs is reproduced in Fig. 23.  70  300  200  400  600 800 Wavelength (nm)  1000  Fig. 23 Instrumental responses of PMTs R955 and R406 A R955,  •  R406(X10)  1200  71 4.3 Results  4.3.1 Background spectra Fig. 24 is the background spectrum collected from a He CCP using an R406 PMT over the wavelength range 200 nm to 1000 nm. In addition to helium atomic lines, strong emission from hydrogen and oxygen atomic species were also found in He CCP background. The most prominent lines were those of atomic hydrogen at 656.272 and 656.265 nm (unresolved) and atomic oxygen at 777.194, 777.417 and 777.539 nm (unresolved), 844.636, 926.084 and 926.601 nm. At low powers nitrogen atomic lines were very weak but could be easily identified above background when at high RF powers. Nitrogen atomic lines at 744.229, 746.831, 821.634, 824.239, 859.400, 868.028, 868.340, 870.325, 871.170, and 871.883 nm could be identified at an operating power of 300 W. Hydrogen, oxygen and nitrogen were probably present as impurities in the He gas or from entrainment and diffusion of laboratory air. No effort was made to eliminate these sources for N,0, and H.  4.3.2 Spectra for nonmetals One of the objectives of this study was to evaluate the effectiveness of the parallel plate CCP for exciting the non-metallic elements F, CI, Br, I, C, N, and O since one potential application for the CCP is as a detector for gas chromatography. The samples used for the intensity measurements for each of these elements are listed in Table 4. The emission lines of F, CI, Br, I, C, H, and O in near-infrared (NIR) were found to be quite intense in the CCP. Fig. 25 is a typical CI spectrum from CH C1 in the He CCP from 300 to 1000 nm. 2  2  A hydrocarbon spectrum obtained using CeRu is provided in Fig. 26 for background comparison. In the latter, CH emission (431.42 nm) was much stronger than that from C H 2 C I 2 and carbon atomic lines in near-infrared range were more intense than from CH C1 . 2  2  72 Table 4. Compounds used for relative intensity measurements  Element  Compound  Formula  R.F. Power  F  2-Iodo-1,1,1 -trifluoroethane CF CH I  250 W  CI  Carbon tetrachloride  CCI4  200 W  Br  2-Bromo-2-methylpropane  (CH ) CBr  I  2-Iodo-1,1,1 -trifluoroethane CF CH I  250 W  C  Hexane  250 W  N  Impurities in helium gas  250 W  O  Impurities in helium gas  200 W  s  Carbon disulfide  3  2  3  3  3  QH14  cs  2  2  200 W  200 W  Fig. 24(a). He CCP background with PMT R406 from 300 to 600 nm RF power 200 W, make-op gas flow rate: 200 ml/m, carrier gas flow rate: 0 ml/m  He  OI  He He  HI  -lei  OI  600  700  800  900  Fig. 24(b). He CCP background with PMT R406 from 600 to 1000 nm RF power 200 W, make-up gas flow rate: 200 ml/m, carrier gas flow rate: 0 ml/m  1000  He I  He I  He I \ N  2  OH HI N  Ji 300  iHel  2  400  300  Fig. 25(a). Spectrum of CI from CH2CI2 in He CCP between 300 and 600 nm RF power, 200 W, make-up gas flow rate: 200 ml/m  6C  H I He  He I  o  He I  CI I  CI I  CI  CI I CI  CI I OI  CI I CI  CI I CI  CI I  CI I  in  UJ  700  J—*  800  JUL  U L 900  Fig. 25(b). Spectrum of CI from CH2CI2 in He CCP between 600 and 1000 nm RF power; 200 W, make-up gasflowrate: 200 ml/m  1000  He I  HI \  He  He I  He I  01  /  OI  NI  ll 600  V  700  CI CI I / C ,  N I  800  CI  CI  1  900  100  Fig. 26(b). Spectrum of C0H14 in He C C P between 600 and 1000 nm RF power, 200 W, make-up gasflowrate: 200 ml/m  co  79 Corrected relative intensities for the principal emission lines of F, CI, Br, I, C, N, and O are presented in Table 5. The table lists the spectrally corrected relative intensities for the CCP at the RF powers noted in Table 4, the corrected relative intensities for a He-MD? [202] operated at 100 W forward power, and the (non-corrected) intensities for a hollow cathode plasma [206]. From Table 5 it can be seen that the relative intensities of nonmetallic element emission lines, most of which are in near-infrared range, in the He-CCP are different from those in the other sources. It is true that the strongest emission lines for CI, Br, C, O and S are 739.869 nm, 912.115 nm, 889.762 nm, 940.573 nm 777.194 nm and 921.29 nm respectively, which are the same as in MIP [202]. However, the relative intensity of analyte ion lines (compared to the neutral atomic lines) in the He CCP are much weaker than those in He-MIP [202] or the hollow cathode plasma [206]. The prominent emission lines for the non-metals in near infrared range make the CCP an attractive atomic selective detector for GC since many line choices are available and spectra in near infrared range are relatively free from molecular interference since most molecular emission appears in UV or visible range (e.g., N N , NH, and OH). Overall the carbon emission lines in +  2>  2  the near-infrared range were very weak; the strongest line for carbon was the C(I) line at 193.091 nm. One problem with carbon originating from the sample is that it reacts with oxygen (entrained from the atmosphere and present as an impurity in the plasma gas) to produce CO. This was confirmed by the presence of strong C O molecular band emission +  found in the spectra coming from any carbon containing sample excited in He-CCP. It was also noted that that O(I) emission lines from He-CCP background were reduced substantially when carbon containing samples were introduced into the discharge.  80 Table 5 Relative intensities of F, CI, Br, I, C, N, and O in the He CCP Wavelength  Relative intensity  (nm) This work  MIP [19]  Hollow cathode plasma[23]*  FLUORINE 623.965  8.0  91  634.851  7.2  14  641.365  3.2  31  677.398  2.0  24  683.462  4.5  27  685.603  26.9  100  687.022  4.1  21  690.248  15.4  61  690.982  5.1  23  696.636  2.1  13  703.747  7.4  64.1  93  712.719  3.3  34.7  60  720.236  4.5  20.4  26  731.102  5.2  39.9  57  733.196  38.1  100.0  55  739.869  100.0  742.565  21.1  71.7  29  748.272  7.4  26.1  11  755.224  21.4  60.6  21  757.338  17.2  52.5  17  775.470  3.8  86.3  23  890.092  35.3  15.4  902.54  51.7  4.9  99  CHLORINE 660  479.455(H)  0.06  481.006(H)  0.06  77  0.04  53  481.947(H)  o  0  100  497.622  0.06  542.352(H)  0.03  22  544.342(H)  0.01  14  544.425(H)  0.02  725.652  2.9  7.5  29  741.411  0.45  2.4  25  754.707  2.5  6.8  17  767.242  0.04  0.4  771.759  0.42  2.3  15  774.497  1.8  4.8  10  1.4  7  782.136 783.075  0.01  0.4  787.822  0.25  1.8  789.928  0.12  5  791.509 ^  0.04  6  793.385  0.24  797.695  0.05  799.780  0.05  808.667  1.7  819.442  0.18  820.021  0.37  821.204  1.43  822.174  1.3  833.331  2.3  16.4  10  837.954  26.3  91.6  60  842.825  1.6  11.1  846.732  0.08  857.524  2.1  12.2  858.597  0.72  33.0  868.626  1.0  891.292  0.87  4.3  894.806  32.3  42.2  903.898  2.5  4.8  904.543  2.4  7.2  906.966  2.0  4.0  907.317  1.1  8.0  3.7  34 22  912.115  100.0  100.0  919.173  12.6  25.6  919.749  0.46  928.886  0.93  939.386  0.65  945.210  1.2  7.4  948.696  1.1  2.7  958.480  12.6  15.4  959.222  14.2  18.2  970.244  6.8  12.1  9.2  BROMINE 635.073  1.4  641.032  0.64  658.217  1.2  663.612  5.3  700.519  2.7  716.210  0.30  726.045  0.72  734.851  20  3.5  15  11.7  7.9  24  751.296  0.73  2.3  780.302  1.8  3.9  8  793.868  0.30  0.8  5  798.994  0.70  1.5  802.654  0.21  0.3  813.152  2.4  3.7  815.400  0.51  1.1  827.244  39.4  78.0  54  833.470  7.2  11.6  10  834.370  9.8  13.4  7  838.404  0.01  0.2  847.745  18.3  19.6  851.338  0.08  0.3  855.773  0.85  1.2  856.628  0.30  1.2  863.866  28.6  40.0  6  869.853  9.0  11.1  879.347  0.31  0.6  881.996  1.2  2.1  882.522  42.1  48.2 0.4  888.898 889.762  100.0  100.0  893.240  3.6  2.6  894.939  0.79  1.0  896.400  0.86  1.8  916.606  2.8  12.0  917.363  2.3  7.0  917.816  27.7  23.0  932.086  2.1  9.4  979.848  0.96  4.2  989.640  1.9  7.7  926.542  CARBON 833.515  57.8  13.2  906.143  40.9  16.1  907.828  14.2  6.4  908.851  25.7  9.2  909.483  80.1  33.4  911.180  28.8  12.5  940.573  100.0  55.6  960.303  25.3  4.7  962.080  53.6  14.0  965.844  83.1  20.6  OXYGEN 777.194  100.0  100.0  777.417  78.4  79.5  844.636  84.7  39.6  926.277  16.0  8.8  926.601  20.5  11.2  NITROGEN 739.864  6.0  742.364  4.0  4.6  744.229  8.8  8.9  746.831  17.0  14.6  818.487  6.9  17.5  818.802  12.9  17.8  820.036  2.7  3.6  821.634  27.0  7.7  822.239  11.8  18.7  856.774  23.1  9.0  859.400  17.5  19.6  862.924  38.9  47.6  865.589  6.8  10.2  868.028  100.0  100.0  868.340  66.5  58.2  868.615  32.8  24.1  870.325  27.0  22.6  871.170  32.3  26.3  871.883  22.7  22,6  938.680  8.1  24.5  939.279  19.7  42.8  SULPHUR 868.05  5.8  869.47  5.2  0.12  921.29  100.0  100.0  922.81  76.0  73.2  923.75  46.9  44.4  *: The relative intensity is not instrumental corrected. (II): donates ion lines, otherwise are neutral lines if nothing is specified.  85  4.3.3 Spatial distributions of emission The axially viewed spatial distribution of emission from He(I) for the different size discharge tubes described in the experimental section is presented in Figs. 27 and 28. In Fig. 27, He (I) 447.15 nm emission is plotted as a function of relative distance, d/a, where d is the distance from the left hand wall of the discharge tube and a is the inside width of the tube. When plotted in this manner the emission is normalized to the width of the tube. The spatial distribution with the absolute distance for the first 1.5 mm for the various torch sizes is provided in Fig. 28. There are several important features in those two figures. First, for all the tube sizes the maximum emission occurs near to the walls and is a minimum at the center of the tube. This is most evident for the wider 4 X 4 and 6 X 6 mm tubes which exhibit very little emission from 60% of the width near the center of the tube. A second feature is that the maximum intensity increases with the decrease of the torch tube width except for the 1 mm i.d. capillary tube. The latter observation is not all that surprising since the tube height is much smaller than that for the other torch tubes. A third observation is evident in Fig. 28; the maximum in emission intensity appears at the same position regardless of which tube size is being used to sustain the discharge. The position of maximum intensity is approximately 0.2 mm from the inside edge of the tube wall. In addition, the position of maximum emission does not change with the applied power (Fig. 29) or plasma gas flow rate with the current experimental configuration. It seems clear that the reason for emission maxima near the walls of the discharge is that most of the power is dissipated in this zone. The pressure is too high for the existence of a cathode dark space and negative glow region similar to that found in DC glow discharges. Rather, the RF field is attenuated exponentially as it propagates into the plasma. The cool walls of the torch act as a third body for non-radiative ion-electron recombination and so  86  Fig. 27. He(I) 447.15 nm spatial distributions with different sizes of torches RF power: 200 W, make-up gas flow rate: 200 ml/m, carrier gas flow rate: 0 ml/m '  O—  1mm, — - • — 2 X 4 m m , 4 X 4 m m ,  6X6mm  87  100  I — — i — ' — i — — i — • — i — « — i — i — i — i — i — ' — i — » — i — 1  1  r  Distance from the left wall (mm)  Fig. 28. He(I) 447.15nm spatial distributions with different sizes of torches RF power: 200 W, make-up gasflowrate: 200 ml/m, carrier gas flow rate: 0 ml/m —O—  lmmi.d., — —  2X4 mm, - - - - - - -  4X4 mm,  — 6 X 6  mm  120  I  1  1  1  1  r  Fig. 29 He (I) 447.15 nm spatial distributions with power 2X4 mm rectangular tube, make-up gas flow rate: 200 ml/m O—  0—  50 W, — 1 0 0 200 W,  W, — - « — 250 W  150 W,  89 the emission intensity is very low immediately adjacent to the torch wall. This spatial distribution is quite advantageous for analytical spectroscopy since the background is minimized in the zone where analyte is introduced along the torch axis. The spatial distribution of emission intensity for He(I) {388.865 nm}, N2 {391.44 +  nm}, OH {308.9 nm}, and N2 {337.13 nm} at an RF power of 200 W and plasma gas flow rate of 200 ml/m is provided in Fig. 30. All the measurements were background corrected. These data indicate that the axially viewed spatial distribution of emission intensity is different for different species in the CCP discharge. The N - C n u - B n g 3  3  2  emission band has behavior similar to He(I), however, there is more emission intensity at the center of the tube relative to the edge for this species. For the OH - A £ - X n band, 2  +  2  maximum emission is observed at the center of the tube rather than the edge as with the other species. It should be noted that the spatial emission structure for He(I) 388.865 nm, H(I) 486.133 nm, 0(1) 777.147 nm, N N  + 2  + 2  (B Iu -X Xg ) 391.44 nm and other He, H, O, 2  +  2  +  (B Zu - X Z ) lines had the same distributions as the He (I) 447.15 nm line in Fig. 2  +  2  +  g  30. The spatial distributions of emission intensity for carbon and chlorine species introduced into the discharge as CCI4 are provided in Fig. 31 which plots the spatial profiles for C(I) {193.09 nm}, CC1 {277.76 nm}, C1(I) {937.954 nm}, Cl(II) {479.455 nm}, C O {218.98 nm} and background species He(I) {728.135 nm} and O(I) {777.147 +  nm}. All the measurements were background corrected. It should be pointed out that the intensities of different species in Fig. 31 are not relative to each other and were normalized for the comparison. It can be seen that Cl(II), He(I) and O(I) have very similar spatial emission distributions as He(I) exhibiting a minimum at the center of the discharge and maxima near the walls of the torch. On the other hand C O , CC1, C1(I) and C(I) have +  much more emission near the center of the discharge although there is some evidence for a maximum near the torch wall for CO and C(I). +  90  It appears the species with higher excitation energies (> 14 eV) have sharp spatial distributions with emission maxima near the the wall of the torch (e.g. He(I) 388.865 nm). Species with lower excitation energies (<10 eV) have a maximum in the center. Most metals and some easily excited molecular bands, e.g. OH ( A £ - X n ) (4-5 eV) show 2  +  2  this behavior. Species with moderate excitation energies have a spatial emission distribution between these two situations (e.g. N (C^u-B-TIg)). The distribution for 2  C(I) 193.091 is more complicated which may result from the convolution of the analyte density distribution and the excitation species distribution because the sample was introduced into the plasma via a central capillary quartz tube.  4.3.4 Effect of Power on Emission Intensities Fig. 32 is a graph of the response of emission from C(I) {247.856 nm}, CC1 {277.76 nm}, C1(I) {894.806 nm}, Cl(II) {479.455 and 481.006 nm} to changes in forward power. To obtain this data, CCI4 was introduced into the discharge at a constant flow rate. The observation position was set to the position at which the He(I) 447.15 line had the maximum emission near to the left wall. Some trends can be seen from the plot. In general the emission for all species except CC1 increased with an increase in forward power. Emission from the CC1 band increased with power below 225 W, exhibited a maximum at 225 W, then decreased at powers above 225 W. The emission intensity of C1(I) increased sharply with an increase in power below 50 W with slower rate of increase above 50 W. The emission intensity of CI (II) exhibited an almost linear increase as a function of power.  91  0.00  0.50  1.00  1.50  2.00  Distance from the left wall (mm)  Fig. 30. Spatial distributions of selected species of OH, N , He, Nz+ in He CCP 2  RF power: 200 W, make-up gas flow rate: 200 ml/m, carrier gas flow rate: 0 ml/m OH 308.9 nm,  *—  N 337.13 nm, 2  O  He 388.865 nm,  *f"  N 391.44 nm +  2  92  Fig. 31. Spatial distributions of several species from carbon tetrachloride in He CCP R F power: 200 W , make-up gas flow rate:200 m l / m —*— — C —  C(I) 193.091 nm, Off) 777.147 nm,  • — CC1277.76 nm, • — —O— ClOD 479.455 nm.  C1(I) 837.954 nm, •— * — He(D 728.135 nm  CO* 218.98 I  93  100  200  300  Power (W)  Fig. 32. Relative intensity of several species with power from carbon tetrachloride in He C C P Make-up gas flow rate: 200 ml/m C - C l 277.76 nm. Cl(II) 479.455 nm.  * —  C1(D 894.806 nm. Cl(Et) 481.006 nm  C(I) 247.856 nm,  400  4.4 Discussion  4.4.1  General excitation processes in plasma source  In plasma sources there are a number of possible reactions which can contribute to the excitation of analyte species. To represent the reactions which are related to the excitation process we denote the analyte by A, electron by e. Superscripts m, *, +, and ++ refer to m etas table, excited, singly and doubly ionized species, respectively. (a) Electron collisions: The most simple and most frequent excitation process is through direct collisions between electrons and analytes:  A + e - A * +e  (1)  A +e - A * +e  (2)  A + e -»• A * + 2e  (3)  A + e - A * + 2e  (4)  +  +  +  +  + +  (b) Charge transfer: Other kinds of collisional excitation that could occur between two species are so-called charge transfer reactions involving either the atomic or molecular ion [208-210]:  He + A - He + A * +  He  + 2  (5)  +  + A -+ 2He + A * +  (6)  The condition for the occurrence is that the sum of the ionization and excitation energies of the analyte atom should be almost equal to the He or He2 ionization energy [211]. (c) Penning ionization and excitation: The rare gases all possess metastable levels (atoms in the lowest triplet state) which can also participate in excitation:  95 H e + A - He + A + e  (7)  H e + A - > He + A  (8)  m  +  m  +  He + A  +e  He + A*  m  (9)  m + A - He + A * +  H e  + +  He  m 2  (10)  +  + A - 2He + A*  He m + A 2  +  (11)  - 2He + A * +  (12)  Reactions (7) and (8) are the well known Penning ionization processes. These reactions can occur if the energy of the particles before the collision is greater than the first or second ionization energy of the analyte A; the difference between the energy of the metastable atom and the ionization energy of A is carried off as kinetic energy by the electron. Reactions (9) to (12) involve direct excitation by metastables. They are relatively less probable since a more rigorous condition applies , i.e, the excitation energy of the analyte atom should be approximately equal to the metastable energy [211]. (d) Radiative processes: The low energy electrons are effective in recombination yielding excitation of neutral atoms and ions:  A + e -*• A* + ho (continuum)  (13)  A  (14)  +  + +  +e  A * + hu (continuum) +  All the above excitation reactions could occur in the CCP. But some of them may be dominant and some of them might not be significant.  4 . 4 . 2 Excitation processes in He CCP Excitation processes in a CCP may be different from those in MIP. It is now widely accepted that the metastables play a very important role in a MIP. Direct excitation (processes 1-4) of the analyte atoms and ions by electron collisions is not dominant [212-  96  214,101]. A likely excitation process involves a sequence of steps beginning with impact by metastables leading to Penning ionization of the analyte (processes 7 and 8), followed by ion recombination with low-energy electrons (processes 13 and 14) to produce excited analyte atoms [212,214,101] or by collision with metastables ions (processes 10 and 12) [215]. Bauer and Skogerboe [215] carefully studied the excitation mechanism in MIP and concluded that for the ion lines of nonmetals, the excitation mechanism is mainly through Penning ionization and excitation (processes 10 and 12). There is some evidence that electron collisional excitation might be the main excitation and ionization process in a He-CCP. An inspection of Table 5 reveals that neutral atomic lines are more prominent in He-CCP compared with a He-MIP. For example for ionic chlorine, in the CCP it was found that Cl(II) emission lines were very weak compared to C1(I) lines. The data in Table 5 highlight the fact that this feature is different from that found for the MIP. The most likely explanation is that electron collisions play a more important role in the CCP compared with MIP and, concurrently, Penning ionization is not as significant. In agreement with this postulate it was found that Cl(II) emission lines were found in an Ar-CCP (Fig. 33); this in spite of the fact that neither the Ar metastable (11.76 eV) nor the A r 2 (10.20 eV) is energetic enough to ionize CI (LP. = 12.97 eV) through a m  Penning process. It is also possible that the electron energy distribution between the two sources is quite different. The high frequency excitation used for the MIP (2450 MHz) means that electrons can be accelerated during one cycle of the applied field before a significant number of electron-heavy particle or electron-electron collisions take place. As a result it is quite possible that the high energy tail of the electron energy distribution is overpopulated relative to a Maxwellian distribution [216]. The excess high energy electron density could be quite effective in exciting high energy atom and ion levels and ionizing neutral non-metals. However, for the He-CCP operating at 13.56 MHz there is sufficient time for many collisions during a complete cycle of the applied field such that the electron energy distribution should be Maxwellian.  Fig. 33. Emission lines of C1(H) in Ar CCP, RF power. 200 W, PMT: R955  98  4.4.3 Charge transfer reactions in the CCP An interesting observation is that the C O first negative band (B L-X*L) has been +  2  found to be very strong in He-CCP but is not found in an Ar-CCP. A typical spectrum of CCI4 and background from 180-300 nm in a He-CCP and Ar-CCP are shown in Figs 35 and 36. The total energy required to excite CO ground to CO (B Z) is about 19.7 eV +  2  [217] which falls in the range of H e , values 18.3-20.5 eV. Fig. 34 is a energy level +  2  diagram of H e , H e , H e , CO (B I), N - B I u , A r +  m  2  m  +  2  2  +  2  2  +  + 2  and A r . From Fig. 34 it m  2  is also possible that He (2 S, 2 S) reacts with CO to form C O excitation state. m  1  3  +  However, The most probable reaction in the CCP is the charge transfer reaction, since it is favoured at higher pressures and the vibrational intensity distribution is consistent with this reaction [218,219]. Additional evidence for charge transfer reaction is suggested by the fact that the N  + 2  B E -X I 2  +  u  2  band was found in He-CCP but not in Ar-CCP. Once  + g  again the upper energy level (B Lu ), 18.8 eV, is close to the ionization energy of He . 2  +  2  24 -  He m  22 -  - 2S "2 S  CO  1  ^20  3  >-  O  B I 2  B ^ 2  cr 18 UJ UJ  16 He?  14  X Z 2  Ar,  12 10  Ar  L  m P  Fig. 34. Energy level diagram {Redrawn from A.P. D'silva, G.W. Rich and V.A. Fassel, Appl. Spectrosc. 34, 578 (1980)}  C0+B2S -X2E (0,0)  200  250  300  Wavelength (nm) Fig. 35.  (a) Spectrum of C&HM in He CCP between 180-300 nm, RF power: 200 W (b) Spectrum He CCP background between 180-300 nm, RF power 200 W  200 Fig. 36.  250  300  200  25'0  3(5c  (a) Spectrum Ar CCP background between 180-300 nm, RF power 200 W, make-up gas flow rate: 200 ml/m, PMT: R955  -  (b) Spectrum of C^Hu in Ar CCP between 180-300 nm, RF power. 200 W, make-up gas flow rate: 200 ml/m, PMT: R955.  °  101 From Fig. 34 it is also possible that the spectra of N  + 2  and C O result from the +  metastables of helium through Perming ionizations and excitations. Although both the charge transfer and Penning mechanisms could occur at the same time, we believe that the Penning process contributes less than the charge transfer process to the N  + 2  and C O  +  spectra. However, further experiments must be done before such a conclusion can be reached.  4.5 Conclusions The atmospheric pressure parallel plate capacitively coupled plasma is a relatively new spectroscopic source for analytical atomic spectroscopy.  The use of water cooled  electrodes enables the plasma to be used for long periods of time with relatively little degradation in the quartz torch and provides quite good signal stability. For non-metals there are a large number of emission lines located in the near-infrared which can be utilized in conjunction with the CCP. There is interesting spatial emission structure associated with the CCP which arises as a result of the transverse power coupling geometry. The electromagnetic field is attenuated during passage through the plasma such that at the center of the discharge there is very little emission from support gas lines. Some clues as to possible excitation processes in the CCP can be gleaned from a study of the relative intensities of lines for several non-metals. However, a detailed discussion of excitation mechanisms in this plasma source must await measurement of temperatures and number densities of electrons and other plasma species.  102 Chapter 5 Speciation o f Halogenated and Sulphur-Containing Compounds  5.1 Introduction  In chapter 4, a water-cooled CCP torch, which was a further modification of the adjustable configuration, was described. As was demonstrated in the chapter, the He  CCP  was sufficient to excite carbon, hydrogen, oxygen, fluorine, chlorine, bromine and sulphur. Although metals and more non-metals have not been tested, it is believed that the helium CCP has the ability to excite almost all the elements in the periodic table. For most non-metallic elements a large number of intense, non-resonant emission lines are available in near-infrared range. Although there is a problem with the spectroscopic measurement in the near-infrared region because of the lack of sensitive, low-noise photogalvanic devices, it is still beneficial to use the emission lines in the region because the background is low and "neat", that is, there is little spectral overlap from molecular emission such that spectral interference is insignificant. This chapter will discuss the use of the water-cooled torch for gas chromatographic speciation of carbon, fluorine, chlorine, bromine, iodine and sulphur containing compounds. Detection limits for these elements are tabulated.  5.2 Experimental  The gas chromatograph used in this chapter was a Hewlett-Packard model 5840A with a 15-m, 0.53-mm i.d. J & W DB-5  fused silica capillary column. To minimize the dead  volume, the outlet of the capillary column was placed in the plasma torch directly. About 10-cm coating from the outlet was removed by burning the column in the flame of a Bunsen burner with a flow of air sweeping the column. The transfer interface was arranged similar to that described in chapter 3.  103 Other experimental facilities were the same as in section 3.2 except for that a nearinfrared sensitive PMT, R406, was used to monitor the emission lines between 600 - 1000 nm.  5.3 Results and discussion  When applying element-selective detection to chromatographic problems, two properties determine the usefulness of the method - sensitivity and selectivity. For sensitivity subpicogram detection limits for most elements can be reached. A selectivity with value of 10 or better is not difficult to get. The selectivity property is demonstrated 4  in Fig. 37 and Fig. 38 which show the results of the separation of a mixture of seven halogenated and sulphur-containing compounds. The gas chromatograph was operated in the mode of temperature program: hold at 60 °C for 1 minute, then raise to 200 °C at the rate of 30 °C/m and hold at 200 °C for 2 minutes. The chromatograms in Fig. 37 and the upper chromatographic trace in Fig. 38 give the responses of the carbon channel for which the monochromator to the carbon line was set 190.10 nm and a R955 PMT was used. The chromatograms very much resemble what would be the flame ionization detector (FID) signal for the same mixture of the compounds, with a notable exception that its response is proportional to the amount of carbon present in the eluted compounds, rather than the number of CH groups as in FID. The upper chromatogram in Fig. 37 demonstrates the presence of a "solvent effect". The large amount of the solvent (pentane) results in a wide and tailing peak, which made the compounds whose retention times are close to the solvent unresolved. However, this problem can be eliminated by changing the operating mode of the CCP. The lower chromatogram in Fig. 37 was obtained by turning off the RF power during the time at which the solvent eluted. Fortunately, if the detected elements do not include the elements which are one of the components of the solvent, the "solvent effect" is insignificant. As  104  C 193.09 nm  1. 2. 3. 4. 5. 6. 7.  F CCH I CH CH CH Br CICH2CH2CI CH CHBrCH Br m-BrC H CF o-CIC H CH CH S0 CH CH 3  2  3  2  2  3  2  6  4  6  4  3  3  3  2  —1—  4.0 Fig. 37.  3  3  r 3.5  1  1  3.0  2.5  1  ——1  2.0  1.5  1 1.0  <  0.5  1  0.0  <- TIME (min)  Chromatograms of a multi-element test mixture at carbon channel, injected amount: 11-23 ng, temperature programming: hold 1 min at 60 °C, then raise to 200 °C at the rate of 30 °C/min  05 C 193.09 nm  CI 912.12 nm  6  1. F CCH I 2. C H C H C H B r  -A.  3  2  3  2  2  3. CICH CH CI 4. CH CHBrCH Br 5. m-BrC H CF3 6. o-CIC H CH 7. C H S 0 C H C H 2  2  3  Br 827.24 nm  2  6  6  3  F 739.87 nm  J\ I 905.83 nm  S 921.29 nm  4.0  Fig. 38.  3.5  3.0  2.5  2.0  1.5  1.0  0.5  0.0  «_ TIME (min)  Chromatograms of a multi-element test mixture at multi-channel  3  4  4  3  2  3  106 shown in Fig. 38, the chromatograms were obtained by repeatedly injecting the mixture but monitoring at different channels without turning off the RF power. Except for carbon, other channels do not demonstrate the "solvent effect" because the solvent, pentane, only contains carbon and hydrogen. Fig. 38 also demonstrates the high selectivity of the GC-CCP system. For example, peaks 3 and 6, which are chlorinated compounds, show up in carbon and chlorine channels and not in other channels. The brominated compounds, 2, 4 and 5 give signals in carbon and bromine channels. Compound 5 also shows up in the fluorine channel because it contains fluorine. Compound 1, which is composed of carbon, hydrogen, iodine and fluorine, give rise to signals of the carbon, fluorine and iodine channels. Only compound 7, ethyl methanesufonate, shows up in the sulphur channel. Primary experiments have suggested that the relative response depends on the elements selected, and is independent of the structures of compounds. This property, which can not be obtained from the most commonly used GC detectors, such as FID, makes it possible to generate approximate molecular formulas of the eluted compounds without standards. Using the definition in section 1.2.2, the detection limits were determined and the results were compared to those from GC-MIP [63,107] in Table 6. With the exception of carbon, the detection limits are comparable with those obtained from GC-MIP systems. The fact that carbon has much lower sensitivity in the GC-CCP may result from the reaction of carbon with oxygen. The relatively lower gas temperature in the He CCP (compared to the He MD?) and the excitation parameters may favor the formation of carbon monoxide. As was mentioned in section 4.3.2, two pieces of evidence were found in the He CCP confirming the reaction. One piece of evidence was the presence of a strong C O emission +  band. The other was that with the increase of introduction of carbon-containing sample, the emission of oxygen in the background was decreased.  107  Table 6  Comparison of Detection Limits (DL) in pg/s.  element wavelength this work  Estes(MIP)  a  QuimbydVlIP)  13  Tanabe(MIP)  c  (nm) F  739.87  10  685.6  CI  912.12  827.24  905.83  39  32  34  75  38  40  21 66  545.4 193.09  43  17  206.2 921.29  11  36  478.6 I  40  16  479.5 Br  20  52 78  247.9  7.2  234  0.5  53  2.7  » S.A. Estes, P.C. Uden and R.M. Barnes, Anal. Chem., 53, 1829 (1981) (without background correction) b  c  B.D. Quimby and JJ. Sullivan, Anal. Chem., 62,1027 (1990) (with background correction) ICTanabe, H. Haraguchi, K. Fuwa, Spectrochim. Acta, 36B, 633 (1981) (without background correction) Reference uses Is instead of peak to peak (6s) to measure noise for DL, and their numbers have been c  adjusted accordingly for comparison.  ^  108 5.4 Conclusion The present torch configuration can excite F, CI, Br, I, O, H, N and S sufficiently. Primary results for F, CI, Br, I, and S in the GC-CCP show that the system is comparable with GC-MIP. Future studies should focus on the improvement of sensitivity for carbon and more quantitative experiments should be carried out to determine detection limits and selectivities. More work needs to be done to develop the system as a commercial instrument.  109 Chapter 6 Future proposals  Future work to development the capacitively coupled plasma as a gas chromatographic detector may focus on: (a) Spectrometer: So far only a monochromator with single exit slit has been used. The limit is that only one element (at fixed wavelength) can be monitored at one time. There are many ways to detect multielements simultaneously. One of them is through use of an interferometer. Since compounds are separated in gas chromatograph and the components of organic compounds are quite simple, it is not necessary to use very high-resolution spectrometer. An interferometer is an inexpensive spectrometer which uses an optical interfering filter to select specific wavelengths and can be built as multi-channels. Another way to detect multielements simultaneously is to use a monochromator and photodiode array. As the photodiode array monitor a range of wavelength at the same time, it is possible that several elements emissions in that range are detected at the same time. (b) Data acquisition: Computer-controlled systems are the most efficient way to get data. 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