Glow Discharge Atomic. Spectroscopy

Transcription

1 focal point By Jos( A. C. Broekaert Department of Chemistry University of Dortmund, D Dortmund, Germany Glow Discharge Atomic Spectroscopy INTRODUCTION G low discharge sources (GDSs) have long been used in atomic spectrometry. In particular, they are recognized as sources for atomic emission research in connection with the early work of Paschen ~ with hollow cathodes, which later were studied in detail as the primary sources for atomic absorption work. In its initial stage, glow discharge analytical emission spectrometry also made use mainly of hollow cathodes, which became widespread for the determination of very low absolute concentrations of all elements including most nonmetals. However, for routine analysis of compacted metal samples, the breakthrough first occurred at the end of the sixties with the work of Grimm, 2 who introduced a restricted GDS in which flat samples were directly mounted as a cathode. As an atomic emission device, the source became widespread in industry for bulk and especially for depth-profiling 3 purposes. Also, for atomic absorption and atomic fluorescence work, GDSs proved to be very valuable as atom reservoirs, and the appropriate analytical techniques were developed. At the same time, elemental mass spectrometry became an established method for ultratrace determinations in solids but continuously suffered from the use of unstable sources such as vacuum sparks as well as from severe matrix interferences. Here, GDSs improved the method 4 and became a source of interest for the characterization of working materials used for microelectronics. This contribution discusses the possibilities of the various types of spectroscopies using GDSs in relation to the processes of sample volatilization and signal generation that are taking place in the GDS. At the same time, the need for further research on these processes will be discussed, and conclusions with respect to improvement in the analytical figures of merit will be drawn from knowledge of the different sources. New ways of making use of the analytical information from GDSs will be shown to be possible with advanced types of sources and spectrometers. BACKGROUND GDSs operate in the mbar pressure range. They are characterized by high charge densities and electric fields of up to several kv/mm in the vicinity of the cathode. The current density is low since the entire electrode is normally covered with the discharge, and discharge currents in most cases are below 0.1 A. The sample surface covered by GDSs may be restricted by the electrode dimensions, which leads to an abnormal characteristic with strong increases in the discharge voltage with current. Only when the area covered increases with the current or when the cathode temperature increases with the current will one observe a normal characteristic. Apart from GDSs with flat cathodes, pin geometries and GDSs with hollow cathodes or anodes are used (Fig. 1). The sample is primarily mounted as a cathode, because the latter can then be effectively volatilized by cathodic sputtering, which serves as a typical mechanism for material volatilization in GDSs. The use of a pin-type cathode is often advantageous in terms of the type of material to be analyzed. However, the flat cathode geometry is useful when in-depth analysis is to be made since, in this case, the sample is ablated layer by layer. The use of a hollow cathode 12A Volume 49, Number 7, 1995

2 has the advantage that the cathode temperature can be varied (often by as much as K), allowing compounds to be volatilized selectively, which may be very useful from an analytical point of view. Moreover, the analyte atoms can be kept in the analytical zones for very lon~ periods of time. GDSs have a very energetic cathode fall region near the cathode, but the negative glow is more important for signal generation. The latter results from various mechanisms including electron impact and Penning ionization, as well as radiative recombination and charge transfer processes. The predominance of one of the processes may relate to the rare gas and the analytes, which is a subject for basic studies (see, for example, Ref. 5). The characterization and diagnostics of GDSs are difficult in any case, because of the absence of local thermal equilibrium. In terms of the analytical sources used, therefore, fundamental diagnostics still have to be performed, and these are severely hampered by a lack of suitable methodology. Apart from Doppler linewidth measurements for kinetic temperatures 6 and measurements with Langmuir probes, 7 studies on the energetic processes have hardly been done. In this area, input from plasma physics is required in order to make progress. Glow discharges can be generated in gases at reduced pressures by coupling direct-current (dc) or radio-frequency (rf) energy into the working gas. The latter is a result of the different mobilities of electrons and positive ions in the electric fields by which a bias potential near the electrode is created. Therefore, both types of sources can be treated in a similar way with respect to the analyte volatilization and, to a certain extent, with respect to the excitation and ionization phenomena taking place, in all phenomena related to the analytical features of GDSs, basic knowledge of gas discharges is important, as it is described in standard works such as Handbuch der Physik, Band XXI1 ~ and the works of Penning 9 and of Chapman. ~ The latter should be referred to for information on the excitation and ionization phenomena, and the work of Kaminsky ~ for sputtering studies. Material volatilization in GDSs differs completely from the processes in atmospheric-pressure sources. When solids or dry solution residues are to be analyzed by GDS atomic spectrometry, they are transferred on a carrier or directly act as an electrode. Because of the high energies of ions or atoms formed from ions by charge transfer, material of the electrodes can be removed by momentum transfer from the impacting particles to atoms or ions positioned in the solid structure of the samples. Hence, volatilization temperatures of the elements and their compounds are of no relevance for this process, which may exclude matrix effects based on selective evaporation. Of course, for sources where the sample starts to evaporate as a result of high electrode temperatures, selective volatilization occurs and can be desirable in order to selectively remove sample constituents. These processes are difficult to describe by models because of the wide variety of energetic species in the plasma, the structure of the solids, and the importance of chemical reactions occurring in the discharge and at the sample surface. Indeed, the field of reactive sputtering is still almost totally unexplored from the point of view of its mechanisms and certainly from the point of view of its usefulness for spectrochemical analysis. The influence of sample structure and composition on volatilization in GDSs is a field of increasing research interest. The volatilization and the pathways of the volatilized material will greatly depend on the electrode form. Here, phenomena measured with flat cathodes are important in order to determine the penetration of the sputtering process into the sample and its consequences for the lifetime of the discharge and depth profiling. This process will differ considerably from material volatilization and diffusion that oc- APPLIED SPECTROSCOPY 13A

3 focal point curs in hollow cathodes and anodes, where the internal field may result in special electrode erosion patterns, still not fully understood. Accordingly, investigations of the pathways of the volatilized material by probing, by spectroscopic methods as well as by modeling, are important, as shown in the studies of Van Straaten et al. ~2 Excitation and ionization phenomena in glow discharges are more complicated than they are in atmospheric-pressure plasmas, since they are sources that are far removed from local thermal equilibrium. Excited species (including so-called metastables) of the filler gas (A) are formed by A + e --~ A*, A +, A +* (electron impact). Field ionization may play a role in the vicinity of the cathode but only at very high electrical fields (10 7 V/ cm). The processes of relevance for excitation and ionization of a sample analyte S, just as in atmosphericpressure plasma discharges, include S+ e --~ S*, S +* (electron impact), S +A*---->S +* +A (Penning ionization by metastables), S + + e--is* + hv (radiative recombination), S + hv -+ S* (radiation trapping). However, the cross sections for the respective processes differ considerably. This consideration leads, for example, to the fact that a group of slow electrons (responsible for recombination) and fast electrons (responsible for excitation) exists, which leads to the absence of a uniform electron temperature. The latter then also will differ considerably from the neutral-particle temperatures derived from Doppler broadening or approximated by rotational temperatures determined with molecular species such as OH, N2 +, etc., 14A Volume 49, Number 7, 1995 being on the order of <2000 K. Diagnostics of low-pressure discharges used for atomic spectrochemical analysis are still very incomplete. Although much information from, for example, vacuum plasma coating studies exists, it does not seem to have yet reached the field of spectroscopic research. GLOW DISCHARGE OPTICAL SPECTROSCOPY As sources for atomic emission spectrometry the following are all very useful: hollow cathodes; GDSs with flat cathodes, according to Grimm; 2 and sources in which the material is evaporated electrotherreally and excited in a glow discharge, such as the furnace atomic nonthermal emission spectrometry (FANES) source introduced by Falk et al.; ~3 as well as discharges in hollow anodes. ~4 Hollow cathode sources still are of interest, since the long residence times for the analytes lead to very low absolute detection limits. For a long time they have been applied in combined analytical procedures including pre-enrichment and multielement determinations in trace concentrates, and they remain attractive for the determination of trace elements in microsamples, as shown by Chen and Williams. ~5 One also can use graphite electrodes as filters for air sampling, whereby trace analyses in airborne dust with an integrated sampling-analysis procedure become possible. Also the selective evaporation of traces of volatile elements from refractory matrices, such as high-temperature alloys, remains an approach that continues to be attractive for routine laboratory procedures. This method is still useful for the determination of As, Sb, Se, S, R etc., in high-temperature steels down to the ~g/g level (for the literature, see Ref. 16). However, hollow cathodes as atomic emission sources remain special techniques. The use of glow discharges with flat cathodes saw a breakthrough with the work of Grimm 2 in 1968 and was thoroughly investigated in Europe through the work of Laqua and co-workers] v,18 Boumans] 9 Berneron, 2 and others (for the early literature, see Ref. 21). In particular, for the bulk analysis of metal samples, dc glow discharges were optimized with respect to the analytical lines to be used, the detection limits (which in the case of steels are in the Ixg/g range), the matrix influences, and the precision achievable. The possibilities of dc GDSs for bulk analyses, however, are not yet fully explored. Indeed, glow discharges in terms of their operation are very stable sources, as can be shown by noise power spectra. 22 One obtains mainly white noise, hardly any frequency-dependent noise stemming from the vacuum or power supply, and no 1If noise. This characteristic allows the application of Fourier transform atomic emission spectrometry (FTS) for basic studies, as well for the study of the spectra. With such high-resolution systems, one can show the occurrence of selfabsorption for many resonant lines and measure physical linewidths, which mainly m'e in the 1-3 pm range. Further, it can be shown that, down to the p,g/g concentration level, impurities in steel samples can be detected with the use of FTS. Steers and Thorne 23 showed that FTS was very useful for the study of excitation mechanisms of atomic emission lines in the GDS. The strong self-reversal of resonance lines shows that the performance of GDSs can still be much improved by crossexcitation, as reviewed by Leis and Steers 24 and performed, for example, with microwave energy. Of course the power of detection can be improved further by increasing the analyte number densities in the source, possible by jet-assisted sputtering 25,26 or magnetically enhanced sputtering, 27 where in both cases the ablation rates can be increased from about l to 5 rag/rain at the 50-W power level. Here, however, one should be careful since, in the case of resonant atomic spectral lines, this approach leads only to increased self-reversal. The use of magnetic fields, in addition, is much more complex, since it also has conse-

4 quences for the excitation processes. Overall, this approach may result in large signal enhancements for analytical GDSs, as shown by Sacks and co-workers. 28 In atomic emission spectrometry, the use of new detectors such as photodiode array and charge-coupled device (CCD) systems allows the simultaneous recording of many atomic spectral lines. With segmented Echelle spectrometers, as shown in Fig. 2, 29 one can achieve flexible multielement detection including easy background acquisition. However, noise considerations must be assessed in order to make sure that detector noise does not limit the power of detection. Indeed, with a 0.35-m monochromator, the spectral background currents in the case of 15-txm slit widths and a ines/mm grating with a 1P28 photomultiplier were on the order of A, which is already hardly above the dark current ( A) level. The capability to ablate samples layer by layer seems to be especially interesting for industrial applications. Indeed, with GDSs having flat cathodes, penetration rates at the 50- W power level are on the order of 1-3 ~m/min. Accordingly, GDSs now are extensively used for the product control of surface-improved steels, 3 as required, for example, by the automobile industry. Work on the systematic quantization of these depth profiles (Fig. 3) has been performed by Bengtson. 3~ In many cases, however, the fundamental limitations of the depth resolution as a result of the sputtering and redeposition phenomena with their implications for curvature of the burning crater are not reached. In order to make progress in this area, probe measurements and also spatial mapping of the analyte emission across the burning crater and studies on the diffusion of the ablated material in the source must be done. This assumption applies to GDSs with flat cathodes, but even more to discharges with pin geometries, as used, for example, in the study of coatings on wire materials. For solids analysis, dc GDSs have been excellently complemented by the radio-frequency discharges. Indeed, the use of dc discharges is limited to metals, whereas the rf discharges, realized in a usable spectro- APPLIED SPECTROSCOPY 15A

5 focal point metric source by Marcus et al., 32 also are applicable to electrically nonconducting samples, as impressively demonstrated by spectra published for MACOR samples. Here, differ- ent sources including probe sources and even sources for jet-assisted sputtering have been described. Problems arising from sample thickness must be solved (for example, by FIG. 4. Mass spectrum obtained from argon gas sampling glow discharge combined with hydride generation. Solution concentration: 10 i~g/ml As; 20 ma, 0.2 Torr, 760 V; quadrupole mass spectrometer: Balzers? 5 internal standardization) when one is using the source for real sample analysis, and also careful optimization is needed with respect to the burning crater profiles. The latter is certainly the case when the source is applied to the surface characterization of nonconducting materials such as thin-film-coated glasses or surface-improved ceramics as they become more and more important for implants. GDSs also have been proposed for dry solution residue analysis. This implementation is easily performed with hollow cathodes, but in the case of flat cathodes difficulties may occur. Also, the direct introduction of liquids into low-pressure discharges has not been very successful, despite the potential for element-specific detection work. Certainly, creative ideas may change this situation, and once again there will be the need for the appropriate diagnostics to evaluate the feasibility of the various approaches. Much more success has occurred with the use of GDSs for determinations in gases. Although these possibilities were long recognized, this particular approach gained more attention after the success of the atmospheric sampling glow discharge, introduced as a soft ionization method for mass spectrometry by Mc- Luckey et al. 33 This source indeed has proved to be very useful for the detection of ultratraces of organic substances directly in air. Howevel; this GDS also has been shown to be very useful for the atomic emission spectrometric detection of elementary species such as the halogens as well as S and P in vapors, as described by Pereiro et al. 34 The gas sampling GDS can be operated with Ar (1-3 mbar) and He (5-10 mbar), as well as with Ne (2-5 mbar), at the 100-W level and was shown to be capable of breaking down small molecules such as the AsH 3 generated by hydride generation? 5 When the gassampling GDS is interfaced as an inlet to a quadrupole mass spectrometer with the aid of a skimmer, the detection limits indeed enter the ng/ ml range in the hydride-forming so- 16A Volume 49, Number 7, 1995

6 lution or are at the 10-pg/s level, as demonstrated by the magnitude of the ion signals obtained (Fig. 4). This observation shows that GDSs may become very useful for atomic emission or mass spectrometric element-specific detection in chromatography as well. This possibility also was demonstrated by studies on the introduction of vapors of halogenated hydrocarbons into a Grimmtype GDS. With a helium discharge, the chlorine signals could be easily detected around the inlet orifice, and, without much optimization, detection limits of 10 ng/s in atomic emission work could be obtained, which are not very much above the values of helium microwave discharges. As is shown by the spatial mapping of the filler gas and analyte lines across the discharge diameter (Fig. 5), with the careful selection of the observation zone, and eventually side-on observation, further improvement in the figures of merit of this source is likely. Because of the multielement capabilities of any atomic emission spectrometric technique, element ratios and accordingly empirical formula can be easily determined, which of course is extremely useful, for example, in the identification of halogenated hydrocarbons after their gas chromatographic separation. By the use of rf GDSs, the construction of the source in the case of reactive gases may be simplified and also the excitation efficiencies may be improved. This capability also applies to the type of electrode geometry, the filler gases used, etc. Accordingly, it can be expected that GDSs may become even more useful chromatographic detectors than are atmospheric-plasma sources, especially since their gas consumption is low and their coupling to mass spectrometry is easy. In general, the spectral features of GDSs in optical atomic emission may profit from the pulsing of the sources and the performance of atomic emission detection in the afterglow, where, as a result of the long lifetimes of the excited species (for the argon metastables at l 1 ev, the lifetimes are in the 0.1-s range), high line intensities but also low spectral background intensities can be expected. GDSs are also very useful atom reservoirs for atomic absorption and atomic fluorescence work. This observation relates to the fact that the analyte atom densities in GDSs are high (up to 0.1-1% of the total atom number densities to be compared, to below 10-4% in atmospheric-pressure plasma discharges such as the ICP) and that their ground-state population is high as well. The use of GDSs as atom reservoirs for atomic absorption work goes back to the 1970s, to the early work of Lowe, who described the use of jet-assisted sputtering cells for the direct analysis of alloys by atomic absorption spectrometry. Further developments in this area can be expected since atomic absorption instrumentation, through the use of diode lasers, might become more simple. However, with respect to the processes occurring in jet-assisted sputtering and especially in view of ablation and possible redeposition, the mechanisms are not fully understood. 25,26 For instance, element-specific redeposition was found when brass samples were sputtered with jet-assisted GDSs, demonstrating the need for further research. GDSs are also useful as atom reservoirs for atomic fluorescence and laser-enhanced ionization work. Indeed, apart from their high groundstate atom densities, GDSs, as the result of low foreign atom densities, might also have lower risks for quenching the excited states than is the case for devices operated at atmospheric pressure. With GDSs, detection limits in laser-excited atomic fluorescence spectroscopy were at APPLIED SPECTROSCOPY 17A

7 focal point the picogram-level. 36 Similar advantages may be found for laser-enhanced ionization work, where GDSs also may benefit from the absence of rf fields. The latter occur in the case of atmospheric-pressure plasmas such as ICPs, MIPs, etc., and may make the signal acquisition difficult. Optogalvanic studies in the case of hollow cathode sources have been reported in the literature (see, for example, Ref. 37). The hollow cathode geometry again has advantages through its high excitation efficiencies as a result of the high analyte residence times. The laserbased techniques, of course, are especially useful for diagnostic studies of GDSs, as shown by atomic absorption using diode lasers for probing excited species densities. 38 GLOW DISCHARGE MASS SPECTROMETRY GDSs have found interest in analytical elemental mass spectrometry for solids, as an alternative to the high-vacuum spark sources used originally. Indeed, the power of detection achieved was high, which made this expensive method very popular for microelectronics materials and other fields where extreme trace determinations in solids (down to the ng/g level) were required. The method, however, suffered from low precision (RSDs at the 30% level) and also from severe matrix effects, resulting in so-called relative sensitivity factors (RSFs), which differ significantly from unity. Here, GDSs in both respects brought progress, and their use enjoyed a breakthrough when reliable vacuum equipment with high gas displacement capacity as well as high-quality but relatively cheap quadrupole-based mass spectrometers became available. The combination of GDSs with sectorfield mass spectrometers became commercially available in the mid- 1980s, and these instruments became of great analytical use in the microelectronics industries. Despite their analytical use, however, many basic aspects of the method still require further elucidation in order to im- 18A Volume 49, Number 7, 1995 prove the quantification, as discussed in the literature (see, for example, Ref. 39). However, not only expensive sector-field instruments but also analytically useful quadrupoles have been coupled to GDSs. In the case of the Grimm-type GDS, this has been done by Jakubowski et al. 4 in an instrument which has been shown to be very useful for the bulk analysis of metals, 4~ for the characterization of coatings and multilayers, 42 and for dry solution residue analysis. 43 Indeed, in the analysis of steels, detection limits of 1-10 ng/g were found. 41 As ablation rates due to the low currents required may be down to the nm/s level, the characterization of electrically conducting layers on metals and multilayers was quite possible down to thicknesses as low as the nanometer level. 42 The latter was possible even with ceramic materials such as SiC, for which the electrical conductance was still sufficient to make an analysis without need for the use of special measures such as grid electrodes. In the case of dry solution residue analysis, mass spectrometric techniques allow the detection of very low absolute amounts of elements on a metal carrier, provided that high amounts of salt matrices are not present. This possibility has been clearly shown by the detection of femtogram amounts of noble metals fixed on a copper carrier plate by cementation. 43 Such techniques allow for very low absolute detection limits similar to those in advanced X-ray fluorescence techniques such as total reflection X-ray spectrometry but also for the low Z elements. As in quadrupole mass spectrometry, spectral interferences may lead to serious limitations of the method; the use of gases other than argon such as neon in a number of cases was found to be helpful. 44 With GDSs the use of more advanced types of mass spectrometers is a field of current research. This observation applies to ion traps, as proposed by Blades and co-workers 45 for laser sources, but also to time-of-flight (TOF) and Fourier transform (FT) mass spectrometry, 46 in which coupling to GDSs needs to be evaluated in more detail with respect to the possible analytical features in each case. Also for GDS mass spectrometry, radio-frequency sources are very useful, in principle, since they enable the direct analysis of nonconductors. This capability is very useful for the analysis of ceramics or for the characterization of coating layers on electrically nonconductive substrates, the latter with a high power of detection. Here, analytical comparison with other methods such as sputtered neutral mass spectrometry (SNMS), as introduced by Oechsner et al., 47 certainly will be required with respect to the analytical problems to be solved. Further work on GDS mass spectrometry also will be related to the nature of the spectra, where the impurity levels with respect to molecular species can be improved further with the aid of collision cells. This issue has to be eventually balanced with reactive sputtering, which in a number of cases could be advantageous for sample ablation. Accordingly, the use of GDSs as radiation sources or as atom reservoirs in optical atomic spectrometry, as well as their improvement as ion sources for mass spectrometry, can be regarded as a challenging field of research. The state of the art is treated in its very different aspects in a textbook edited by Marcus. 48 It must be emphasized, however, that at present diagnostic information for GDSs is much less available than for atmospheric-pressure operated plasma sources. This situation is likely to be changed since the coupling of GDSs with mass spectrometry is easy and the rapid development of sources straightforward. 1. E Paschen, Ann. Phys. 50, 901 (1916). 2. W. Grimm, Spectrochim. Acta 23B, 443 (1968). 3. M. E. Waitlevertch and J. K. Hurwitz, Appl. Spectrosc. 30, 510 (1976) 4. Inorganic Mass Spectrometry, E Adams, R. Gijbels, and R. Van Grieken, Eds. (John Wiley and Sons, New York, 1988). 5. E. B. M. Steers and E Leis, J. Anal. At. Spectrom. 4, 199 (1989). 6. N. P. Ferreira, H. G. C. Human, and L. R.

8 P Butlel, Spectrochim. Acta 35, 287 (1980). 7. D. Fang and R. K. Marcus, Spectrochim. Acta 46B, 983 (1991). 8. G. Francis, "The Glow Discharge at Low Pressure", in Handbuch der Physik XXII (Springer Verlag, Berlin, 1956). 9. E M. Penning, Electrical Discharges in Gases, (Philips Technical Library, Eindhoven, 1957). 10. B. Chapman, Glow Discharge Processes (John Wiley and Sons, New York, 1980). I I. M. Kaminsky, Atomic and Ionic Impact Phenomena on Metal Sut'[~tces (Springer Verlag, Berlin, 1965). 12. M. van Straaten, A. Vertes, and R. Gijbels, Anal. Chem. 64, 1855 (1992). 13. H. Falk, E. Hoffmann, and Ch. Ltidke, Spectrochim. Acta 39B, 283 (1984). 14. P. G. Riby and J. M. Harnly, J. Anal. At. Spectrom. 8, 845 (1993). 15. E Chen and J. C. Williarns, Anal. Chem. 62, 489 (1990). 16. hnproved Hollow Cathode Lamps for Atomic Specttvscopy, S. Caroli, Ed. (E. Horwood, Chichestel, 1985). 17. M. Dogan, K. Laqua, and H. Massmann, Spectrochim. Acta 26B, 631 ( 1971 ). 18. M. Dogan, K. Laqua, and H. Massmann, Spectrochim. Acta 27B, 65 (1972). 19. E W. J. M. Boumans, Anal. Chem. 44, 1219 (1972). 20. R. Berneron, Spectrochim. Acta 33B, 665 (1978). 21. J. A. C. Broekaert, J. Anat. At. Spectrom. 2, 537 (1987). 22. J. A. C. Broekaert, C. A. Monnig, K. R. B,'ushwylel; and G. M. Hieftje, Spectrochim. Acta 45B, 769 (1990). 23. E. B. M. Steers and A. E Thorne, J. Anal. At. Spectrom. 8, 309 (1993). 24. E Leis and E. B. M. Steers, Spectrochim. Acta 49B, 289 (1994). 25. E R. Banks and M. W. Blades, Spectrochim. Acta 47B, 1287 (1992). 26. J. A. C. Broekaert, T. Brickel, K. R. Brushwyle~, and G. M. Hieftje, Spectrochim. Acta 47B, 131 (1992). 27. M. J. Heintz, K. Mifflin, J. A. C. Broekaert, and G. M. Hieftje, Appl. Spectrosc. 49, 241 (I 995). 28. L. McCaig, N. Sesi, and R. Sacks, Appl. Spectrosc. 44, 1176 (1990). 29. J. A. C. Broekaert, K. R. Brushwylet, and G. M. Hieftje, unpublished work. 30. K. H. Koch, M. Kretschmel, and D. Grtinenberg, Mikrochim. Acta li, 225 (1983). 31. A. Bengtson, Spectrochim. Acta 49B, 411 (1994). 32. M. R. Winchester, C. Lazik, and R. K. Marcus, Spectrochim. Acta 46B, 483 (1991). 33. S. A. McLuckey, G. L. Glish, K. G. Asano, and B. G. Grant, Anal. Chem. 60, 222O (1988). 34. R. Pereiro, T. K. Starn, and G. M. Hieftje, Appl. Spectrosc. 49, 616 (1995). 35. J. A. C. Broekaert, R. Pereiro, T. K. Starn, and G. M. Hieftje, Spectrochim. Acta 48B, 1307 (1994). 36. M. Glick, B. W. Smith, and J. D. Winefordnet, Anal. Chem. 62, 157 (1990). 37. Z. Zhu and E. H. Piepmeiel; Spectrochim. Acta 49B, 1775 (1994). 38. N. I. Uzelac and E Leis, Spectrochim. Acta 47B 877 (1992). 39. W. W. Harrison and B. L. Bentz, Prog. Anal. At. Spectrosc. 11, 53 (1988). 40. N. Jakubowski, D. Sttiwel, and G. T61g, Int. J. Mass Spectrom. Ion Proc. 71, 183 (1986). 4[. N. Jakubowski, D. StfDwer, and W. Vieth, Anal. Chem. 59, 1825 (1988). 42. N. Jakubowski and D. Sttiwer, J. Anal. At. Spectrom. 7, 951 (1992). 43. N. Jakubowski, D. StiJwel; and G. T61g, Spectrochim. Acta 46B, 155 (1991). 44. N. Jakubowski and D. Sttiwer, Fresenius' Z. Anal. Chem. 335, 680 (1989). 45. C. G. Gill, B. Daigle, and M. W. Blades, Spectrochim. Acta 46B, 1237 (1991). 46. C. M. Barshick and J. R. Eylel; J. Am. Soc. Mass Spectrom. 4, 387 (1993). 47. K. H. Mtiller and H. Oechsner, Mikrochim. Acta [Wien] Suppl. 10, 51 (1983). 48. Glow Discharge Spectl~Tscopies, R. K. Marcus, Ed. (Plenum Press, New York, 1993). 49. J. A. C. Broekaert, T. K. Starn, L. Wright, and G. M. Hieftje, unpublished work. Guest Editor: Jos6 A. C. Broekaert Jos6 A. C. Broekaert earned his Ph.D. degree in Chemistry from the University of Gent (Belgium) in After an Alexander-von Humboldt-Fellowship in 1977 he joined the Institut f'tir Spektrochemie und Angewandte Spektroskopie (ISAS) in Dortmund (Germany). Since 1983, he has lectured at the University of Antwerp (UIA) (Belguim), and since 1991 he has held a position as Professor of Inorganic Chemistry/Analytical Chemistry at the University of Dortmund. Professor Broekaert's group's research centers on the study and use of plasma atomic spectrometry for chemical analysis, with special reference to source development, material analysis, and speciation work. Over 130 publications on these and related topics have appeared in the literature. APPLIED SPECTROSCOPY 19A