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Design and Characterization of a Radio-Frequency-Powered Glow Discharge Source for Double-Focusing Mass Spectrometers Douglas C. Duckworth,’ D. L. Donohue,?and David H. Smith Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 378313375
T. A. Lewis Instrumentation and Controls Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 378313354
R. Kenneth Marcus’ Department of Chemistry, Clemson University, Clemson, South Carolina 29634-1905
A radio-frequency-(rf-) powered glow discharge has been interfaced to a double-focusing mass spectrometer. This type of discharge allows direct analysis of nonconducting, as well as conducting, solids. The rf discharge source and electrical system overcome several problems which have inhibited success in prior efforts. Problems of inadequate rf shielding, maintaining t he necessary dc bias potential on the sample surface, preventing rf modulation of ion energies, and coupling of the accelerating potential to the discharge are resolved. Representative spectra of glass, soil, and brass matrices are presented. Preliminary relative sensitivity factors for conducting and nonconducting matrices show relatively small differences in ion yields across the periodic table. INTRODUCTION Radio-frequency-(rf-) powered glow discharges are reduced pressure atomization and ionization sources that have the unique capability of sputtering nonconducting materials directly without the need for a conducting host to maintain the sputtering ion current. Unlike its more widely used dc counterpart, a wide range of materials, including glasses, ceramics,and refractory materials, as well as metals and alloys, can be atomized directly by sputtering, creating a representative atomic population amenable to various analytical applications. Thus, the rf glow discharge may be thought of as a universal inorganic solids sampling source for elemental analysis. Despite the early work of Coburn and colleagues,13 and Donohue and Harrison$ applications of rf glow discharges have primarily been limited to sputter deposition and etching. Oeschner et al. utilized an rf plasma in the technique of sputtered neutral mass spectrometry (SNMS), which is available as a commercial instrument (INA 3, LeyboldHeraeus Vacuum Products, Export, PA).6 This technique t Present address: International Atomic Energy Agency, A-1400 Vienna, Austria. (1) Coburn, J. W.; Taglauer, E.; Kay, E. J.Appl. Phys. 1974,45,1779. (2) Coburn, J. W.; Kay, E. Appl. Phys. Lett. 1971, 19, 350. (3) Coburn, J. W.; Eckstein, E. W.; Kay, E. J. Appl. Phys. 1975,46, 2828. (4) Donohue, D. L.; Harrison, W. W. Anal. Chem. 1975,47, 152. (5) Geiger, J. F.; Lopnarski, M.; Oeschner, H.; Paulus, H. Mikrochim. Acta ( Wein) 1987, 1, 497.
0003-2700/93/0365-2478$04.00/0
utilizes a 27-MHz plasma as a primary ion source and as a secondary ionization source for sputtered neutrals. The plasma operates at 103-10-4 Torr and employs inductive coupling of the rf power. The plasma serves as the primary ion source in the direct-bombardment mode, as a few hundred volts is applied to the sample surface to induce sputtering with ions extracted from the discharge. Recently, a practical rf-powered glow discharge ion source was developed for quadrupole-based mass spectrometric analyses.6 This ion source, in comparison to Oeschner’s et al., employed the dc self-biaspotential developed on the surfaceto effect sputtering of the sample. The analytical potential of this system was demonstrated for copper alloys as well as a mixture of powdered oxides and a simulated nuclear waste disposal glass. Detection limits at the single ppm level were achieved for both glass and alloy materials. Limiting features of the system were the unit mass resolution of the quadrupole and the presence of polyatomic interferences, with the latter being due to both the enhanced ionization capabilities of the rf glow discharge and the low discharge operating pressures. In an effort to reduce the levels of interfering species formed in the rf-powered glow disqharge,gas-phasecollisionsin the first quadrupole (rf-only made) of a double-quadrupole mass spectrometer were utilized to effect collision-induceddissociation (CID) of polyatomic ions, with subsequent mass filtering performed by the second quadrupole.’ Preferential losses of interfering ions by CID and charge exchangeprocesses were reported, with >90% reduction of polyatomic species, while monatomic analyte species were reduced by only 2Cb 30% (due to elastic scattering within the collision cell). An attractive alternative to removal of isobaric interferences by CID is increased mass-resolving power. A double-sector mass spectrometer affords the mass resolution required to resolve many of the common interferences in GDMS. An additional advantage is the increased transmission and dynamic range that is afforded by sector-based instruments. Previous attempts to interface an rf-powered glow discharge to a sector mass spectrometer, similar to that employed here, met with limited success due to difficulties encountered in meeting two fundamental requirements. The first of these was the need to superimpose the rf potential on the highvoltage accelerating potential, typically 8 kV. The second requirement was proper shielding against rf interference. A high accelerating voltage is desirable for multisector instruments to improve ion extraction efficiency. To do this, the (6) Duckworth, D. C.; Marcus, R. K. A d . Chem. 1989,61,1879. (7) Duckworth, D. C.; Marcus, R. K. Appl. Spectrosc. 1990,44, 649. Q 1993 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 65, NO. 18, SEPTEMBER 15, 1993 Eleclncal BDmn Nitride Feedthmuoh \
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Flgure 1. Schematic of rf glow discharge source and direct insertion probe.
source is almost always floated at the accelerating potential. In the initial rf glow discharge interfacing attempts, applicationof the accelerating potential limited the abilityto shield adequately the rf-powered source. Ketchell utilized an isolation transformer to reference the rf discharge voltage to the accelerating potential! The rf potential, which was superimposed on the 8 kV, was not adequately shielded, resulting in radiated rf noise, and the author reported rf interference with the Hall probes, which monitor magnetic field strength in the mass analyzer. This resulted in unacceptable field fluctuations. Work preliminary to that reported here was attempted in a similar manner, utilizing an isolation transformer and employing modifications in the design of the source and sample insertion assemhly.9 Once again, due to inadequate shielding, the technique waslimitedbyrfnoise, whichseverely affected hothFruadayandDalydetectionsystems. Detection limits of tens of ppm were noted for Sn in a brass alloy. The design and characterization of an rf-powered glow discharge source andelectricalcoupling system for multisector mass spectrometry are presented here.’O The interface, demonstrated on a reverse Nierdohnson geometry (BE) double-focusing mass spectrometer, extends the application of the instrument to insulating materials with little degradation of performance with respect to that attained when conductingsamples wereandyzed. Operating characteristics of the system are presented, with emphases on ion energy considerations and relative sensitivity fadors. EXPERIMENTAL SECTION Mass Spectrometer System. The mass spectrometer (VG9000, Fisons Instruments, Elemental, Cheshire, U.K.) is a double-foeusing glow discharge mass spectrometer of reverse Nier-Johnson geometry. Operating with the provided de glow discharge, mass resolution obtainable is typically 5oo(t7000. Samplesofvariousgeometriesareintrodudviaadirectinsertion probe and vacuum interlock system. Because such sample introduction is desirable for high sample throughput and to maintainvacuum integrity,modificationsforthe rf-poweredglow discharge source include an rf-powered direct insertion prohe (described below). A cryocwling system applied to the dc glow discharge greatly reduces the presence of residual gases in the discharge hy cooling the discharge cell to near liquid nitrogen temperatures, thus condensingout many undesired species. The efficiency of the discharge is improved, and mass spectral interferences are reduced. Cryocwling is incorporated in the present rfglow dischargesourcedesignto take advantageofthese characteristics; this is especially important because interfering species are enhanced in the rf source. The VG9000 utilizes both aFaradaycup and aDalydetectionsystem,whichtogether afford (8) Ketchell. N. Ph.D. Dissertation, University of Manchester, 1989. (9) Durkwonh,D. C.;Marcus, R.K.;Chriitie. W. H.;l)onohue. D L.; Smith, D. H Proceedings of the 38th ASMS Conference on Msps Soearomern, and Allied ToDiea. June Tueson. AZ. 1990: D 393. .(10) ClemknUnivemitv, die&on, SC. United Sta&spa&t07/944.216 pending
10 decades of dynamic range. Major, minor, and trace species can be analyzed in the same analysis. Glow Discharge Source. An expanded view of the glow discharge source and direct insertion probe assembly is given in Figure 1. The source is designed to interface directly to the VG9ooo without modification of the existing dc source interface. Interchanging the dc and rf sources is thus a straightforward process. Complete conversion from rf to de mode requires approximately 45 min, including evacuation of the vacuum chamber, switching direct insertion probe assemblies, and recoofiguration of the electrical system. The rf source can also be dc powered, requiring only a change in power connections. The rfsource (2.7 em 0.d. X 5.6 cm) consists of two sections, a boron nitride portion and a stainless steel dischargecell, which are bolted together. Thedischargeis maintainedwithin thesteel cell (2 cm i.d. X 1.3 em). A 6.4-mm Pyrex wiudow is mounted in thesideofthesteelcell,providinganopticalportforalimited viewofthedischarge. Thisportdoesnotseem toadverselyaffect thesymmetryofthedischarge. Argon,purified througha heated gettering system, is introduced through a controlled leak into the top of the steel discharge chamber, and ions are extracted through the exit slit assembly; by design, these features are identical to the de source. The accelerating potential (8 kV) is applied directly to the steel discharge cell-exit slit assembly. The boron nitride section servesthree functions. First, it serves as insulation between the grounded prohe body and the steel dischargecell. Second,it provides high thermal conductivityfor the attachment of the cryocooling system; the liquid nitrogen cold finger iitssnuglyaround the horonnitride. Third,the boron nitride also houses an wring, which forms a gas seal around the ceramic extension of the probe tip. This seal results in an intermediate pressure (1-2 Torr) as discharge gas is introduced into the cell (i.e., the glow discharge source assembly shown is housed inside a high-vacuum region (--l(r Torr during operation)). Actual discharge pressures could not be measured directly and were referencedtothe main chamher preeaureduring operation. Relative measurements of the discharge pressures were made with a ‘thermocouple probe” which consisted of a thermocouple (Model DV-GM,TeledyneHastings-Raydist,Hamp ton, VA) mounted onto a 1.27-cm-diameter probe and inserted into the source, analogous to the direct insertion probe, Approximate operating pressures were 0.8-1.5 Torr; measurement precision was 10% . A portion of the direct insertion probe, partidy inserted into the source assembly, is shown in Figure 1. The basic design of the prohe has been presented elsewhere.” It consists of stainleas steel tubing (1.27 em diameter X 58.42 an long) which houses the center conductor of a RG-214 cable. The insulating cover and thecableshieldingareremovedfromthiscenter conduetor,which couplesthe rfpower through the probe to the sample. The coaxial shielding is extended to the prohe body to provide isolation for the rf power throughout the length of the probe. RF power is coupled to the sample holder assembly through a customized electrical feedthrough(SpecialPart No. 601B4476,Insulator Sed, Inc., Hayward, CA), which is soft silver-soldered to the probe
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(11)Duckworth, D.C.; Marms, R.K.J . A d . At. Spectrosc. 1992,7, 711.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 18. SEPTEMBER 15, 1993
GENERATOR
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Fbun 2. Schematic of rl glow discharge electrical interface.
body to form a vacuum seal. Glass tubing insulates the center conductor from the probe body, which is at ground potential. It is important to isolate all grounded portions of the probe assembly from the relatively high pressure region in the source toeliminatearcing betweenthesteelcelland the groundedprobe. For this reason the feedthrough is designed to allow the o-ring seal to be formed around the0.762-em-diameteraluminumoxide insulatorextendingfrom the electricalfeedthrough. This removes all grounded surfaces of the probe from the region of high discharge pressure. The shoulder of the electrical feedthrough allows reproducible positioning of the probe within the source by butting against the boron nitride when the probe is fully inserted. In prior applications a steel cap was used to protect the sample holder assembly from sputtering." The discharge cell assembly serves this function here. Metal samples (0.476 cm diameter X 0.83 cm) were press-fit intoa copper sampleholder positionedon theendoftheelectrical feedthrough. Small samples and nonconductors, which frequently have small sample heights, were elevated to the appropriate position by means of a copper stand. Samples were mountedonto thestand withsilver paint andpositionedtoexpose only the sample to the sputtering region, leaving the silver paint and copper stand protected outside the sputtering region. The probe was inserted through a 1.91-em ball valve assembly, equipped with a vacuum interlock, into the source -35 cm away from the interlock. To ease sample introduction, particularly with those mounted with silver paint, the probe was axially positioned by contact between the ceramic extension of the feedthrough and the o-ring before the sample approached the confining orifices of the source. A variety of sample sizes could he accommodated by changingthe diameter of the orifices at the interface of the boron nitride and stainless steel cell. RF Glow Discharge Electrical Interface. The electrical interface, designed to allow coupling of the rf glow discharge to the VG9000, is illustrated in Figure 2. This design satisfies a varietyof stringent requirements resultingfrom the need to float the plasma at the acceleratingpotential of the mass spectrometer. These requirementsincludethe need to couplethe dc accelerating potential with the rf glow discharge voltage and the need to provide complete shielding of the rf electronics to reduce noise to acceptable levels. Thus, three areas of the electrical system had to be accommodated: (1)the rf power network, (2) the dc power network (acceleratingvoltage),and (3) thegroundnetwork. (Due to the rf and high dc voltages employed, care should be taken to ensure that all circuitry is properly insulated and shielded.) The first requirement for the maintenance of an rf-powered glow discharge is that the rf power be efficiently coupled to one of the electrodes. In theory it does not matter which electrode is rf-driven since cathode identity is established on the hasis of relative target impedances, which are a function of the relative electrodesurfaceareasincontactwith thedischarge. Asinusoidal rf potential is supplied by a 13.56-MHz generator and is impedance-matchedby an automatic matching network (Model RF5S and AM5, respectively,RF Power Products, Inc., Marltou, NJ). The rf power is coupled acros9 a 0.1-pF (10-kV) hlocking capacitor to thesample. In rf glow dischargesystems,acapacitor is always necessary for establishing the dc bias on the surface of a conducting sample (this is often accomplished through the capacitance of the matching network). For nonconductors, the impedanceof the sample itself is sufficient to maintain a dc bias; for conductors, the absence of a hlocking capacitor results in
equilibration of the developing de bias through the external circuitry. The blocking capacitor (10-kV maximum voltage rating) indicatedinFigure2 servestoprovideisolationoftherf generator and matching components from the high-voltage accelerating potential. The accelerating potential is coupled directly to the stainless steel dischargecell and the exit slits only and is applied to the sample surface through the conductance of the discharge. In this manner, the sample, which is electrically floating and biased relative to the counter electrode, floats to the potential of the counter electrode (6 kV in most instances) minus the dc self-bias potential. Thus the generator, matching network, and probe are at ground potential, while the sample floats at the accelerating potential. A choke is also incorporated into the electrical design to isolate the rf from ground while allowing a path to ground for high voltages, protecting the rf generator and electronics should the capacitor fail The ground network is also made complete with the isolation of the high poteutial from the rf circuitry. Use of isolation transformers to reference the rf potential to the accelerating voltage does not allow for the complete shielding of all the rfcarryingcahles,particularlyaroundtheprobe hodywhich isnear grounded surfaces of the vacuum chamber. With electrical isolation employed, all rf-carrying cables and the rf probe are coaxially shielded, reducing the radiated noise considerably. An rf filter between the high-voltage direct current power supply and the ion source is shown in Figure 2. This filter was necessitated by rf interference with the accelerating potential. This interference adversely affected ion energy distributions as discussed in the next section.
RESULTS AND DISCUSSION DC Self-Bias Potential. The first step in solids analysis by GDMS is the sputter atomization of the sample, producing an atomic population representative of the sample constituents. To maintain sputtering, a constant dc bias potential of sufficient magnitude must be sustained on the sample surface. Theprobleminsputteringelectricallynonconductive materials is that positive charge accumulation at the sample surface tends to neutralize any negative bias applied. It was due to the need for a constant bias potential that Wehner proposed the application of a high-frequency potential to an electrode backing a nonconducting target.Iz He correctly reasoned that the positive charge accumulated at the target surface during the negative half-cycle would be neutralized by plasma electrons during the positive half-cycle, resulting in a constant negative bias on the target surface. The same fundamental phenomenon is invoked here, but in this case it must be superimposed on the high-voltage accelerating potentialof the mass spectrometer. The acceleratingvoltage must also be efficiently coupled to the sample surface to maintain a stable discharge. Figure 3 illustrates the dc bias measured between the exit slit and sample electrode as a function of power for a range of common operating pressures @-kV nominal accelerating voltage). These voltages were measured directly via a highimpedance voltage probe with an estimated precision of fl V. The linear dependence on discharge power was noted previously, utilizing a discharge design which is similar in operating principle but referenced to ground potential.1a These authors demonstrated the same linear dependence on discharge power and noted that this result was unexpected due to the fact that the relationship between peak-to-peak rf voltage and the rf power is nonlinear. It is suspected that, at the relatively low powers employed here (10-55 W), the peak-to-peak rf voltage and rf power relationship might be (12) Weber, G. K. Adu. Electron. Electmn Phys. 1966. 7,239. (13) Winchestm, M. R.: Marcus, R K.Spectrochirn. Acto 1991,46B,
483.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
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more linear than at higher powers. Kohler et al.14 and Vossen and ONeill15 have also noted a linear relationship between rf voltage and the magnitude of the dc self-bias voltage. The dc bias illustrated in Figure 3 does not demonstrate a large dependence on discharge pressure, a may be expected. Apparently the interaction of the plasma with the surface of the anode is maximized due to the confined geometry of the discharge (12.5 cm3, as compared to that previously utilized for quadrupole GDMS (>200cm3)e). The dc self-biasvoltage (vdc) in a capacitively coupled rf discharge is given by
v,, = V&,
- C,)/(C, + C,) where Vfl is the excitation potential and C, and C, are the time-averaged capacitances of the sheaths at the sample and wall, respectively.14 With the present confiied source,changes in pressure do not greatly affect the relative cathode-to-anode surface area ratio as defined by surfaces in contact with the plasma. In unconfined discharges, C, decreases with increasing pressure as the plasma interacts with less of the discharge chamber surface, resulting in a more positive dc self-bias voltage. This is consistent with the observations of Winchester and Marcus,13 who observed an inverse relationship between the magnitude of the bias potential and discharge pressure. It is likely that a more dramatic relationship between dc bias and discharge pressure would be observed in this application if a less confined discharge source was utilized. Likewise, a greater dc bias should result from a less confined geometry since higher C, values should be observed. Even though the dc biases were less than those expected in a less confined discharge, they were sufficient to promote sputtering. The average sputtering energy is expected to be reduced to about 25-30% of the applied cathode voltage due to collisions in the cathode sheath, leading to primary ion energies of about 90-110 eV. Figure 4 illustrates the magnitude of dc bias sustained on the sample surface a a function of power (corrected for reflected power) and mass spectrometer accelerating voltage. The response is approximately linear with respect to power. The magnitude of the dc bias is approximately the same for each accelerating potential employed (within measurement errors). This illustrates that the accelerating voltage is being efficiently coupled to the floating electrode without altering the basic plasma operating characteristics and means that mass spectrometric performance can be improved by operating at higher accelerating voltages without compromising the sputter atomization efficiency of the ion source. (14) Kohler, K.; Coburn, J. W.; Horne, D. E.; Kay, E.; Keller, J. H. J . Appl. Phys. 1986,57,69. (16) Vossen, J. L.; ONeill, J. J., Jr. RCA Rev. 1968, (Dec), 566.
6100 6 1 2 0 6 1 4 0 6160 6180 6 2 0 0 6 2 2 0
Accelerating Potential (V) Flguro 5. rf and dc ion energy distributions for Ar2+, Cu+, and Pb+, demonstrating the effect of rf modulation.
Ion Kinetic Energies. Ion kinetic energy studies were performed by setting a narrow energy window with the fiial slit and the electrostatic energy analyzer and focusing the magnet to maximize the ion signal. Ion energy profiles were recorded at a fixed magnetic field as the accelerating voltage was scanned across the region of ion transmission. The ion energy profiles of Ar2+and Cu+ (Figure 5) illustrate a saddleshaped energy distribution commonly observed in rf GDMS. This distribution has been previously noted for both ions in both diode and electrodeless discharge^'"^ and has been attributed to rf modulation of the plasma potential, which directly affects ion energies. RF modulation is a timedependent broadening of the ion energy distribution and is largely a function of the rf frequency and anode sheath thickness.26 The degree of modulation has previously been attributed to the time required for an ion to traverse the plasma sheath. At low rf frequencies ions are able to traverse the sheath in a single period, exitingwith energiesrepresenting the instantaneous plasma potential. Conversely,ions formed in higher frequency plasmas are more monoenergetic and (16) Benoit-Cattin, P.; Bernard, L. C. J . Appl. Phys. 1968,39, 5723. (17) Okamoto,Y.; Tamagawa, H. J . Phys. SOC.Jpn. 1969,27,270. (18)Okamoto, Y.; Tamagawa, H. J. Phys. SOC.Jpn. 1970,29,187. (19) Ero, J. Nucl. Znstrum. 1958, 3, 303. (20) Cook, C. J.; Heinz, 0.;Lorenta, D. C.; Peterson, J. R. Reu. Sci. Znstrum. 1962,33, 649. (21) Tsuchimoto, T. Jpn. J. Appl. Phys. 1966,5,327. (22) Benoit-Cattin, P.; Blanc, D.; Bordenave-Monteaquieu,A.; Dagnac, R.; Vacquie, S. Nucl. Znstrum. Methods 1966,43,349. (23) Williams, J. F. Reu. Sci. Znstrum. 1966, 37, 1206. (24) Benoit-Cattin, P.; Bernard, L. C.; Bordenave-Montesquieu, A. Entropie 1967, U18U,29. (26) Kohler, K.; Horne, D. E.; Coburn, J. W. J. Appl. Phys. 1985,58, 3350.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993 Full Scale Dalleclion
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represent the average plasma potential because the ions require several rf periods to traverse the sheath. Due to lower ion mobilities, heavier ions (Pb+,Figure 5) do not respond to the high frequency to the same extent as lighter ions and therefore have a smaller energy spread than lower mlz ions. We have identified the source of energy modulation to be rf interference with the dc acceleratingpotential. The energy spread exhibited in Figure 5 has been reduced to that of dc glow-discharge-generatedions through appropriate isolation of the dc high-voltage power supply from the rf excitation potential. Inductive and capacitive components were introduced in series with the high-voltagedc power supply to filter rf interference (as illustrated in Figure 1). The effect of this filter is demonstrated in Figure 6 for Cu ions generated from both dc- and rf-powered glow discharges. The rf ion energy profile is equivalent to the dc profile at fwhm (AE= 11.5 eV). Some symmetry is lost in the rf profile, possibly due to incomplete rf shielding. A corresponding small decrease (-25% a t 5% peak height) in mass-resolving power is observed. Spectral Characteristics. The rf glow discharge mass spectrum of tin (0.88% ) in NIST SRM 1103 Free-Cutting Brass is shown in Figure 7a,b. This single-scanmass spectrum was acquired on the Daly detector (multiplier) with an integration time of 200 mS. This spectrum compares favorably to the performance that is routinely observed for conductingspecies. The analytical performance of the rf glow discharge source is similar to that of the dc-powered glow discharge. Resolution (mlhm,using 5% peak heights and 100%beam intensity) is 2000. Based on the signal intensity of 1Wn (31.7 ppm) and 3u of the background noise (Figure 7b), the demonstrated detection limit is 15 ppb. The actual detection limit is thought to be slightly lower as the signal was reduced to monitor all isotopes without tripping the high ion current interlock of the Daly detection system. It should also be noted that the background noise is 19x lower abundance than BSi and are also subject to multiple polyatomic interferences. Interferences with 160,24Mg,2 7 A 4 and s2Swere also noted and easily resolved at ml Am = 4000. For trace analysis of silicon-basedmaterials, high resolving powers and good vacuum practice thus seem warranted. The observed signal intensity for silicon is unexpectedly low, partly due to the low sample content, low sputter yields for silicon, and low operating power (34 W), but the complete explanation is not yet available. Low signal intensities were not observed with the conducting (brass) sample. RF sources developed previously for a quadrupole system have demonstrated ion currents of lO-'"lO-11 A for similar Si concentrations in nonconducting matrices.6 It is thought that the power may not be efficiently coupled to the sample in spite of the 375-V dc self-bias potential on the backing electrode. There may also be a power loss through the excess inductance of the blocking capacitor (as indicated by heating). When nonconductors are analyzed, the contribution from resistive impedance is minimal (as opposed to capacitive impedance), and consequently, an increase in the rf current through the blocking capacitor results (at constant power). Greater inductive losses in power may be observed at the higher current. This power loss could be manifested by a reduction in ionization and a consequential loss in primary ion current. Capacitors with less inductance (i.e., true parallel plate) are being evaluated. Relative elemental sensitivity factors for elements derived from NIST SRM 1412 Multicomponent Glass are plotted in Table 11. These RSF values were determined relative to %Si and span a very small range, less than a factor of 4. As with dc GDMS analyses, such a small RSF range allows standardless semiquantitative analysis using only the ion beam ratios. Where greater accuracy is required, user-generated RSFs may be applied. A challengingapplication for rf GDMS is the direct analysis of soils. The technique has possible benefits in environmental analysis where such materials can be difficult and timeconsuming to dissolve for analysis by solution-based techniques. Another advantage of rf GDMS is that it is a bulk solids technique as opposed to others such as laser ablation and secondary ion mass spectrometry, where bulk sampling is more difficult. Figure 9 shows a low-resolution mass spectrum of lead and bismuth in NIST SRM 4355 Environmental Radioactivity, Peruvian Soil, and represents an accumulation of three scans with a 1000-mS integration constant. Sample preparation, consisting of pressing the sample into an electrode, is simple and time-efficient. The latter feature, requiring only a few minutes, is particularly desirable in that laboratories performing environmental analysis of such materials usually require high sample throughput. Bismuth and 2wPb, with isotopic concentrations of 12 (noncertified) and 1.77 ppm respectively, indicate the
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
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present limits of detectability for nonconducting samples. The correct isotopic pattern for lead indicates that no interferences are present. Although this is a high-mass element, the noted absence of interferences from hydrocarbons, oxides, and carbides is encouraging. Such interferences were expected due to the complex composition of soil and the presence of over 50 minor elements in the standard. Some interferences have been noted at lower masses, but most were easily mass-resolved. The present success with this matrix is encouraging with respect to environmental applications of rf GDMS.
CONCLUSIONS Clearly, the full potential of the rf GDMS technique on a sector-based instrument has not yet been realized. RF interference on ion energy distributions limits the resolution to -4000 (55% peak height). Ion currents from nonconducting materials are not as high as expected from previous studies, but the promise of the technique is evident. Further rf noise filtering should decrease the ion energy distributions, and further development of the source in terms of coupling rf power to nonconductingsamples will improve sensitivity. At present, the system is capable of analyzing conducting materiale directly with single ppb sensitivity. Nonconductors, such as glass and soils, can be analyzed at the ppm level. Similar relative ion yields for conductors and nonconductors are encouraging. The potential for direct bulk elemental analysis of soils without involved sample preparation is particularly promising; such a technique would increase sample throughput for environmental analyses where heavy sample loads are common.
ACKNOWLEDGMENT Research sponsored by the United States Department of Energy, Office of Basic Energy Sciences, under Contract DEAC05-MOR21400with Martin Marietta Energy Systems,Inc. R.K.M. gratefully acknowledges support from FI Elemental. RECEIVED for review January 22, 1993. Accepted May 28, 1993.
