GLOW DISCHARGE TECHNIQUES IN ANALYTICAL CHEMISTRY W. W. Harrison, C. M. Barshick, J. A. Klingler, P. H. Ratliff, and Y. Mei Department of Chemistry University of Florida Gainesville, FL 32611-2046
The glow discharge (GD) is an old source that is finding many new uses in analytical spectroscopy. Its simplicity of operation, coupled with versatility of application, has led to increasing commercial interest. From an obscure analytical method focusing primarily on metal analysis, the glow discharge has developed into a sophisticated technique suitable for analysis of nonmetals, thin films, semiconductors, insulators, and organic materials. More effort is also being made to understand better the many chemical and physical phenomena that influence the effectiveness of the GD source. The GD is a low-pressure (0.1-10 Torr) plasma composed of two electrodes immersed in a partially ionized noble gas (i). The name arises from the relatively bright central glow originating from excited gas atoms emitting their characteristic optical radiation. A complex combination of atomic and ionic species, from both the discharge gas and the cathode sample, is available for analytical use. Compared with other plasma sources, such as the inductively coupled plasma, the GD is a compact, small-volume source that operates at low wattage. It is inexpensive to build and maintain, and plasma gases are consumed at quite modest rates (cc/min). The deceptively feeble appearance of the GD may account for analysts failing to take it seriously over the years. In actual practice, the atomic collision processes occurring in the GD are sufficiently robust to break many tenaciously bonded species. GDs have been used for years with 0003-2700/90/0362-943A/$02.50/0 © 1990 American Chemical Society
REPORT each of the analytical techniques discussed in this R E P O R T . For the most part, however, such use had been, until more recently, sporadic in nature and centered in academic research laboratories. Growing interest and acceptance have led to the availability of numerous types of GD instrumentation, permitting GD techniques to become more competitive with other established elemental methodologies.
Analytical modes Figure 1 illustrates the primary analytical techniques that now incorporate a GD in their experimental schemes. Although most GD measurements are designed to use only one of these approaches, simultaneous application of more than one technique is possible. As the discharge ablates the cathodic (sample) material into the plasma, elemental species are available for a number of different measurements. The large atom population in the discharge makes sensitive atomic absorption (AA) measurements possible; and be-
cause a fraction of these atoms is excited, the subsequent decay produces atomic emission (AE). In addition, atoms may be stimulated by external sources for atomic fluorescence (AF). Some atoms may even be sufficiently excited to lose an electron; the resultant ions can be sampled by mass spectrometry (MS). For two of these techniques, GDAE and GDMS, the GD is self-sufficient (i.e., atomic excitation and ionization result without any external assistance). For GDAA and GDAF, an external stimulation probe is necessary to measure the atomic absorption or to pump sputtered atoms to an energy level suitable for atomic fluorescence. Although these solid-sample elemental applications make up the bulk of GD work today, the introduction of organic vapors directly into the negative glow for ionization purposes is another important use that promises continued growth. Sputter atomization
The key to the success of the GD is the ease with which it creates an atom reservoir of the sample material directly from the solid state. This step, known as sputtering (2), is depicted in Figure 2. Almost all of the discharge potential is dropped across the cathode fall (dark space), a region very close to the cathode surface. Argon ions are accelerated across this dark space by the cathode fall potential and strike the sample surface, creating a collision cascade with sufficient energy to dislodge one or more surface atoms for each incoming ion. Secondary ions are also released, but the cathode fall potential returns them to the surface. Secondary electrons are accelerated by the same field into the negative glow and help maintain the discharge. The negative-glow region appears as a luminous cloud surrounding the dark space and extending outward toward
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REPORT
MS
Negative glow
Sample cathode
Figure 1. Analytical methods using a GD. I0 is the incident radiation; / is the attenuated radiation.
the chamber wall, which normally serves as the anode. The glow results from the discharge gas colliding with electrons moving through the region, causing excitation of the fill-gas atoms, normally argon. Relaxation of these atoms causes emission at a wavelength characteristic of the discharge gas (e.g., blue for argon). Because the electric field is dissipated across the dark space, the negative glow is considered essentially a field-free region. Atoms, ions, and electrons all possess relatively low energy. At typical GD pressures, a short mean-free path results, so that most accelerated ions undergo numerous charge exchange collisions before reaching the cathode, causing the aver age ion energy to be much smaller than the discharge voltage. As shown in Fig ure 2, a fast argon ion, Ar^, can collide with a slow argon atom, Arj?, yielding a fast atom and a slow ion. Some of the fast atoms that result from the charge
exchange collisions bombard the cath ode and contribute to the net sputter rate. Sputtered atoms tend to diffuse away from the cathode, although twothirds or more are redeposited on the target cathode by collisions with the dominant argon atoms. Those atoms that escape redeposition diffuse into the negative glow, where they can be excited or ionized by collisions with metastable argon atoms or electrons. Thus, the sputtered sample exists in the discharge as ground state atoms, excited atoms, and ions. Although some polyatomic species also exist in the glow discharge, the sputter-plasma process is primarily atomic in nature. The reactions shown in Figure 2, however, represent a gross oversimpli fication of the plasma chemistry in volved. Many other species of transient stability are formed and may be ob served, particularly by GDMS. The glow discharge chamber is essentially a small chemical reaction cell that can be
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controlled to the analyst's benefit. A facet of GDs that has yet to be exten sively studied for analytical utility is the modification of the plasma chemis try (3) to produce unique chemical en vironments. In addition, pulsed GD op eration (4) also introduces new oppor tunities for time-resolved decoupling of plasma reactions and products. A major strength of the GD is the separation of the sampling step (sput ter atomization) from the subsequent analytical steps of excitation and ion ization. This process can be thought of as an atom generator delivering a steady-state population of sample at oms to analytical measurement sites separated in space and time from the sample. The atoms have no "memory" of their original chemical environment; thus, sample matrix effects are mini mized. Samples must be electrically con ducting for use in dc discharges. Met als, alloys, and even semiconductors work quite well in this mode. Noncon ducting samples must be mixed with a conducting matrix, such as pure copper or silver, and pressed into a suitable sample form. Alternatively, a radio fre quency (rf) discharge can handle non conducting samples directly, eliminat ing the need for matrix modification. Solution analysis is possible, but re quires special handling. Small (μί,) vol umes are placed on a host electrode and the solvent is removed, leaving a thinfilm sample residue that can be sput tered like any other solid. Alternative ly, it may be possible to use flow injec tion methods to introduce directly small volumes of liquid samples. The major use of GDMS, however, is clearly for solids analysis. At a time when many complex procedures are being studied for introduction of solid sam ples into plasmas that are, by nature, more suited for solution injection, GD sputter sampling represents a simple process for conversion of a solid into an atomic vapor. GDMS Much of the early analytical work in GDMS (5, 6) was aimed toward re placement of the spark ion source, a vacuum discharge ionization source that yields broad energy spreads, an erratic ion beam, and generally unreli able quantitative results. The GD ion source has gained popularity in recent years because of its stability, sensitiv ity, and simplicity of operation. At least three commercial instruments are now available, two from VG Microtrace and one from Extrel. Elemental analysis by GDMS offers critical advantages over competing techniques in that it responds to metals
and nonmetals, exhibits high sensitivity, suffers from minimal matrix effects, and provides isotopic information (7). Thus it is not surprising that GDMS has developed into the most prominent of the GD methods. Figure 3 shows a cross section of a typical GDMS source, illustrating the cathode-anode relationship. Although a pin-type source is shown, other geometries, such as a disk or pellet, are equally feasible. A key difference in MS compared with the optical methods is that a physical transfer of sample material out of the source must take place. Analyte ions are extracted by electrostatic lenses and transferred through a mass analyzer for eventual detection. Although ions are formed throughout the source cell, only those created very close to the exit orifice are able to survive the high-collision environment and depart in the charged state. Both magnetic-sector and quadrupole-based instruments have been used for GDMS, and commercial versions of each are available. Typical discharge operation conditions for GDMS are 1-5 mA, 800-1500 V, and 0.2-2 Torr. Sample preparation for metals and alloys usually consists of brief cleaning and/or polishing, although the sputter process itself serves as a self-cleaning mechanism to remove surface impurities. Powder samples are compacted in a special die to form pins or disks. Solutions can be analyzed by evaporation to a residual film on the surface of the host cathode (e.g., glassy carbon or a high-purity metal), followed by sputter ablation of the sample film into the plasma. Reduction of the solution sample into a thin surface residue permits very low detection limits from the transient signal. GDMS has been widely applied to the analysis of metals, alloys, and highpurity materials, offering perhaps the only method that combines the required sub-part-per-billion detection limits with the breadth of coverage necessary for survey analysis. GD mass spectra are much simpler than line-rich optical spectra, making qualitative analysis rapid and straightforward. Elemental ion ratio intensities are used for quantitation, usually with the matrix element acting as an internal standard. Polyatomic mass spectral interferences resulting from chemical combination of the discharge gas and the sample matrix (e.g., FeAr + ) can create problems at low detection limits. Given the capabilities of modern detection systems, however, GDMS ion intensities are linear over a wide range of concentrations, from major components to trace constituents.
Negative glow
Dark space
Figure 2. Atomization, excitation, and ionization processes typically occurring in a GD. M is the sample species.
The advantages of GDMS have resulted in its becoming a routine technique that turns out reliable data on a daily basis in commercial laboratories. Quantitative analysis of such difficult elements as B, C, Si, P, and S in NBS 1164 steel samples yielded good results (8). The surface-active nature of glow discharge ablation also permits the analysis of thin films and coatings. Si doped into a GaAs wafer can be resolved from different layers at concentrations of 1,10, and 20 ppm with resolution of about 100 nm (9). Although redeposition phenomena somewhat limit the sharpness of layer resolution, GDMS can be used effectively for such samples. Routine use of GDMS is often not reflected in the literature, leading to an underassessment of its real utili-
ty. Several recent reviews describe various applications of GDMS (10-12). GDAE GD emission techniques (13) offer simpler instrumentation and a less expensive approach to solids elemental analysis than does GDMS. Major tradeoffs, however, include poorer detection limits and reduced elemental coverage. Although compact commercial GDAE instruments (Jobin-Yvon, ARL/Fisons, and Spectrumat) have enjoyed popularity in Europe in recent years, their use in the United States has been limited. Two commonly used versions of GD emission sources are illustrated in Figure 4. The planar cathode, or Grimm source (14), shown in Figure 4a, is used
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Glass housing
Anode
same as for other GD techniques. Metal or alloy disks or compacted conducting samples are most often used. Solution samples have been examined by deposition onto graphite, aluminum, or copper cathodes. Quantitative analysis is typically performed with a calibration curve prepared from standard reference materials, a process requiring considerably more time than the preparation of a corresponding solutionbased curve. Conversely, time is saved in the direct analysis of solids, and the inert atmosphere of a GD cell reduces the spectral interferences sometimes found in flame atomizers. Although GDAA is not widely used at this time, the recent availability of commercial instrumentation, coupled with the increasing number of reports from a few laboratories, may stimulate additional interest. Sputter atomization offers special advantages for element analysis in solid samples that present problems for other types of AA measurements. For example, Zr in steel can be determined at 0.005-0.19% levels, and La in high-purity Zn can be determined at concentrations of 0.0080.07% (19).
Negative glow
Quartz window Gas in
Cathode sample
Cooling water
vacuum seal
High voltage
Figure 5. Sputter atomization cell for GDAA.
For analysis of metals and alloys, the hollow cathode is either machined from the bulk metal, or the sample (e.g., chips or powder) is placed in the hollow cavity of a pure material such as graphite. Solutions can be analyzed by evaporating the sample to a residual film in the hollow cathode. Detection limits generally range from 0.1 to 10 ppm. Current research includes coupling of the HCD with microwave sources and development of microcavities to improve sensitivity.
gas jets directed onto the sample surface, which reduces redeposition by sweeping sputtered atoms into the negative glow and results in significant sensitivity enhancements. Typical operating conditions are 4-10 Torr, 60200 mA, and 300-800 V. Detection limits for solid samples are generally in the low part-per-million range. Sample preparation is much the
GDAA
The GD is particularly appropriate for AA because the overwhelming majority of the sputtered species is neutral atoms. This ability to produce a steadystate population directly from a solid matrix has obvious advantages that led Russell and Walsh to suggest use of the GD in the early development of AA (18). Despite this attractive incentive, little concerted effort took place to capitalize on the idea until the introduction of commercial instrumentation. Figure 5 shows a sputter atomization cell for AA measurements. Compared with GDMS and GDAE sources, a longer pathlength is necessary to enhance absorbance, which is integrated across a broad volume of the cell. Greatest atom density is near the cathode, with a gradient extending to the cell walls. Like other GD sources, a flow-through gas system is used. Analyte Corporation's commercial instrument features
GDAF
Because the GD creates a cloud of sputtered atoms in a low-pressure, rare-gas environment that is an excellent quenching medium, it is an effective source for atomic fluorescence. In addition, the inert gas diluent provides few chemical interferences and the elemental absorption lines are relatively narrow. Although GDAF has not been
Detector Sample discharge (Grimm lamp) Photomultiplier (reference)
Sample radiation Sample Resonance fluorescence Photomultiplier
Figure 6. Resonance detection instrument for GDAF. (Adapted with permission from Reference 22). ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990 · 947 A
REPORT used as much as other GD techniques, some interesting applications have been reported (20,21). The use of a laser to cause fluores cence from the GD atom reservoir is a natural consideration in the experi mental approach to GDAF. The small background noise from the GD leads to projected detection limits at the attogram level. Although there are no "typical" GDAF operation conditions, given the scarcity of reports in this area, GD parameters would normally be set to generate a high atom popula tion, more in line with GDAA than the low currents associated with GDMS. The use of a pulsed laser as an excita tion source in GDAF makes a comple mentary pulsed GD advantageous in terms of efficient sample utilization and background noise reduction. A less conventional GDAF system (22) is shown in Figure 6. In this case, the instrumentation takes advantage of two different GDs serving comple mentary purposes. The analytical Grimm discharge produces atomic emission from the elements in the sam ple. This radiation is directed into the second discharge, a resonance detector with a cathode composed of the analyte element. Radiation from the sample strikes the resonance discharge, caus ing fluorescence of the analyte element only. By rotating successive pure cath odes into the resonance discharge, multielement AF is possible without dispersion optics.
Gas in
Sample injection
Gas in
Anode
Graphite electrode
Window Negative glow
Graphite furnace (hollow cathode)
Water-cooled chamber Gas out
Current source
Figure 7. GD cell used as a furnace atomization nonthermal excitation source with electrothermal atomization.
Quadrupole mass analyzer
FANES Conventional uses of GDs take advan tage of the sputter atomization charac teristics to convert a solid to a vapor. The combination of an alternative at omization source, with the GD used as an excitation or ionization source, has drawn attention to furnace atomiza tion nonthermal excitation spectrome try, or FANES (23). Figure 7 shows a typical FANES source in which a graphite cylinder is used both for ther mal atomization of solution residues and as the hollow cathode of a GD for excitation of the sample. A high cur rent is applied to the graphite cylinder, producing a pulse of sample atoms and a subsequent emission pulse signal as the atoms interact with the GD. The similarities to electrothermal atomiza tion AA are obvious (24). Small (5-50 ML) samples are pipet ted onto the cathode, dried, and ashed at atmospheric pressure while the sys tem is flushed with argon. Typical dis charge conditions are 1-5 Torr, 20-30 mA discharge current, and up to 500 A atomization current, producing tem peratures near 3000 °C. The transient emission signal persists for 0.3-1.0 s,
Electrostatic lenses
λ L To trap and forepump
LC flow
Negative glow Cathode
Ion repelier Anode
Figure 8. GD used as an ionization cell for the effluent from a liquid chromatograph.
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and detection limits range from 0.04 pg for Ag to 800 pg for Se. There is also a complementary technique called FINES (furnace ionization nonthermal excitation spectrometry) in which ionic emission is measured. Direct mass spectrometric measurement of such ions has recently been demonstrated (25). Organic GD applications Of increasing interest is the use of a GD as an ionization source for organic com pounds. Although one might assume that the energetics of the discharge would reduce organic vapors to the ele mental state, eliminating any molecu lar information, this does not appear to be true under certain conditions. For example, direct atmospheric sampling of organic compounds has been devel oped to detect trace effluents from ex plosives (26). The simple experimental arrangement features a sampled air stream that flows past two parallel plates between which a GD is pro duced. Ionization of the air is moni tored by extraction of the ions into a mass spectrometer. When 2,4,6-trinitrotoluene solutions of known concen trations were used, detection limits of 1-2 pptr were calculated and a linear dynamic range of 6 orders of magni tude was reported. A commercial application of GD ion ization of organic compounds is found in detectors used in a number of ther mospray LC/MS instruments. As shown in Figure 8, the effluent from a heated capillary forms a supersonic jet that is injected into a GD for ionization before extraction into a quadrupole mass analyzer and ion detector. Elec tron filament ionizers can exhibit short lifetimes when oxidizing components are used, but a GD arrangement has a virtually unlimited lifetime, and the ionization is comparable in sensitivity and sample fragmentation to that ob tained with an electron gun. Typically, the discharge current is ~ 1 mA. Outlook for the future Although the GD has yet to find accep tance to the degree merited by its ana lytical potential, this situation may be changing. A perusal of the programs of recent analytical chemistry confer ences finds numerous symposia devot ed to GD techniques. Leading analyti cal groups in academia, government, and industry are applying various GD methodologies. Instrument manufac turers are responding to and often con tributing to this interest by producing not only instrumentation but also tech nical reports that detail the striking ca pabilities of GD methods. We appear to be reaching a point at
which the GD will have a fair test of its real value to the analytical community. The combination of interest from lead ing research groups, coupled with ef fective commercial instrumentation, will provide a competitive comparison of these techniques with the stable of other analytical methods. It appears from this vantage point that the GD will be extremely visible in the 1990s. References (1) Chapman, B. Glow Discharge Process es; John Wiley and Sons: New York, 1980. (2) Westwood, W. D. Prog. Surg. Sci. 1976, 7,71. (3) Mei, Y.; Harrison, W. W. Spectrochim. Acta, Part B, in press. (4) Klingler, J. Α.; Savickas, P. J.; Harrison, W. W. J. Am. Soc. Mass Spectrom. 1990, 1,138. (5) Magee, C. W.; Harrison, W. W. Anal. Chem. 1974,46,461. (6) Bruhn, C. G.; Bentz, B. L.; Harrison, W. W. Anal. Chem. 1978,50,373. (7) Harrison, W. W.; Hess, K. R.; Marcus, R. K.; King, F. L. Anal. Chem. 1986, 58, 341 A (8) Jakubowski, N.; Steuwer, D.; Vieth, W. Fresenius Z. Anal. Chem. 1988,337,145. (9) Hall, D. J.; Sanderson, Ν. Ε. Surf. In terface Anal. 1988, //, 40. (10) King, F. L.; Harrison, W. W. Mass Spec. Rev. 1990,9,285. (11) Harrison, W. W.; Bentz, B. L. Prog. Analvt. Spectrosc. 1988,11, 53.
(12) Harrison, W. W. In Inorganic Mass Spectrometry; Adams, F.; Gijbels, R.; Van Grieken, R., Eds.; John Wiley & Sons: New York, 1988; Vol. 95, Chapter 3. (13) Broekaert, J.A.C. J. Anal. At. Spec trom. 1987,2, 537. (14) Grimm, W. Spectrochim. Acta, Part Β 1968,23B, 443. (15) Bengtson, Α.; Lundholm, M. J. Anal. At. Spectrom. 1988,3,879. (16) Brenner, I. B.; Laqua, K.; Dvorachek, J. J. Anal. At. Spectrom. 1987,2,623. (17) Improved Hollow Cathode Lamps for Atomic Spectroscopy; Caroli, S., Ed.; El lis Horwood Limited: Chichester, West Sussex, England, 1985. (18) Russell, B. J.; Walsh, A. Spectrochim. Acta 1959,15,883. (19) Ohls, K. Fresenius Z. Anal. Chem. 1987 327 111 (20) Smith,' B. W; Womack, J. B.; Omenetto, N.; Winefordner, J. D. Appl. Spec trosc. 1989,43,873. (21) Smith, B. W.; Omenetto, N.; Wine fordner, J. D. Spectrochim. Acta, Part Β 1984,39B, 1389. (22) Bubert, H. Spectrochim. Acta, Part Β 1984,39B, 1377. (23) Falk, H.; Hoffman, E.; Ludke, Ch. Spectrochim. Acta, Part Β1981,36B, 767. (24) Falk, H.; Hoffman, E.; Ludke, Ch. Spectrochim. Acta, Part Β 1984,39B, 283. (25) Ratliff, P. H.; Harrison, W. W. Proceedings of the 38th ASMS Conference on Mass Spectrometry and Allied Top ics; Tucson, AZ, June 3-8,1990. (26) McLuckey, S. Α.; Glish, G. L.; Asano, K. G.; Grant, B. G. Anal. Chem. 1988,60, 2220.
IV. IV. Harrison (second from left) is professor of chemistry and dean of the College of Liberal Arts and Sciences at the University of Florida. He earned a B.A. degree from Southern Illinois University and a Ph.D. in analytical chemistry from the University of Illinois. His research concerns the use of GDs in MS and atomic spectroscopy. Chris Barshick (left) received his B.A. degree from Washington and Jefferson College and a Ph.D. in analytical chemistry from the University of Virginia. His research involves the use of analytical probes such as lasers and ion/atom beams to study GD sputter-redeposition processes. Harrison's remaining co-authors are all Ph.D. candidates at the University of Florida. Yuan Mei (third from left) earned her B.S. degree in chemistry from Beijing University. Her research centers on manipulating the chemistry of the GD plasma by introducing modifiers such as gettering reagents. Jeffrey Klingler (second from right) earned his B.A. degree in chemistry from the University of Southern Indiana. His research interests include studies of time-resolved pulsed dc and rf plasmas, both for analytical applications and as tools for investigating fundamental processes in the GD. Philip Ratliff (right) earned his B.A. degree from East Tennessee State Univer sity. His research involves thin-film ablation and the use of electrothermal atomization in GDMS. ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990 · 949 A
