rf.Powered Elemental Analysis $Pow ered glow discharge techniques allow bulk or dep th-resolved analysis of materials rangingfrom precious metals to specialty glasses and ceramics
R. Kenneth Marcus Tina R. Harville Yuan Mei Charles R. Shick, Jr. Clemson University
S
ince their resurgence in the analytical community in the 1980s, glow discharge (GD) techniques for elemental analysis have experienced rapid growth (1,2).The increased interest in GD techniques, initially in the academic world and later among instrument manufacturers, can be attributed largely to their performance in the analysis of solid samples. Both bulk and depth-resolved elemental composition of metals, alloys, and semiconductors can be readily determined. Direct solids analysis alleviates the tedious sample dissolution and digestion procedures required for bulk analysis by solution-based analysis methods such as flame atomic absorption and ICP spectrometries. The ability to provide accurate depth-resolved analyses extends the range of applications to thin and thick metallic film systems. One of the newest and most exciting areas of GD development is the coupling of radio frequency (@-powered GD sources with various analytical spectrometries (3).In this arrangement, analyses are not limited to conductive samples. Electrical insulators such as glasses, ceramics, and geological materials-some of the most difficult samples to dissolve for solution-based methods of elemental analysis-can readily be examined. Previously, these samples were mixed and compacted in conductive (metal) powders to form conductive sample disks (4) and then analyzed by direct current (dc) GD spectrometries. This matrix-modification methodology has been used for many years in arc and spark emission spectrometry and MS. Although relatively
902 A Analytical Chemistry, Vol. 66, No. 18, September 15, 1994
straightforward in implementation,this a p proach still poses a number of challenges in terms of sample preparation (grinding, drymg, and mixing) and the introduction of concomitant residual gases that affect source stability and analyticalprecision. These matrix modification steps are not required for analysis by rf-GD techniques. The primary, distinct advantage of rf-GD over traditional spectrochemical sources is its inherent capability for the direct solids analysis of both conductive and nonconductive samples. In this Report, we will review the operational principles as well as the latest developmentsin rf-GD techniques. Because much of the initial work in the area has focused on the fundamental exploration and instrumental development of these techniques (5-8), we have chosen to focus on applicationsand to present some of the most recent results obtained on commercial instrumentationfor both atomic emission spectrometry (AES) and MS. Operational principles The atomization/excitation/ionization processes in rf-GD are maintained through two electrodes (one of which is the analytical sample) immersed in an inert gas such as argon. Unlike dc discharges in which both electrodes need to be electrically conducting to sustain the required flow of current (9, IO),one of the electrodes in an rf-GD cell can be an electrical insulator. The fact that a plasma can be established at the surface of an insulating electrode/sample is intrinsic to rf operation and is also the key point of interest in the introduction of the rf powering scheme in elemental analysis. 0003-2700/94/0366-902A/$04.50/0
01994 American Chemical Society
across the Solids Spectrum The application of a high-voltage pulse (e.g., the rf voltage) to an insulating surface can be considered analogous to the charging of a capacitor. When a high negative voltage (-Vs) is applied to the insulator, the surface potential drops initially to -Ifs. If positively charged species produced by the ionization of the plasma gas are near the cathode, the negative surface potential will become neutralized as a function of time. The time scale of this process (z = - 1ps) is such that the application of voltage pulses at frequencies on the order of 2 1 MHz results in a pseudocontinuous plasma. Key to the application of the rf technique is sustaining the GD plasma through a process called “self-biasing.”Consider the 2-kV peak-to-peak square wave potential (V,) applied to an insulating surface (creating a GD plasma) and the resultant potential on the cathode (VJ as shown in Figures l a and lb, respectively. In the initial half-cycle, the voltage on the sample cathode goes to -1 kV and then begins positive charging to about -0.7 kV. During the next half-cycle, the applied voltage is switched 2 kV to the positive, which produces a surface potential of +1.3 kV. The positively charged surface during this fraction of the cycle causes the electrons in the plasma to accelerate toward the cathode (sample). The greater mobility of the plasma electrons relative to the much heavier positive ions results in a faster neutralization of the surface charge during this second half-cycle such that the electrode surface potential approaches zero much more rapidly than during the previous halfcycle; it reaches +0.7 kV as
--
the next cycle begins. As the polarity of the voltage is switched, the surface voltage will reach -1.3 kV (instead of -1.0 kv), offsetting the voltage by -0.3 kV. As successive cycles proceed, the surface voltage on the insulator sample will be further offset in the negative direction until it reaches an equilibrium level at which the electrode virtually bears a dc-like negative voltage, called the dc bias. Because it is governed by the same principles as the dc discharge, this dc bias voltage induces the sputtering of the cathode (sample) material, causing the
sample to atomize. Sputtered atoms (predominantly) are then available to diffuse into the negative glow region for subsequent excitation and ionization. Detection of analyte species by atomic absorption and emission, MS, and a variety of laserbased methods is possible. Thus far, the exploration of rf-GD sources for analytical uses has focused primarily on the development of rf-GD-AESand rf-GDMS. AES and MS source designs
$GD-AES. Because the vast majority of cases in which direct solids analysis is
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likely to be applied are in metallurgical production facilities,the design of a practical rf-GD-AESsystem must be amenable to the rapid analysis of metal disks or ingots with diameters >1in. Elemental analysis in metals production facilities has been dominated by arc and spark source AES for more than 50 years. In many cases, these techniques have not been supplanted by liquid sample analysis methods (e.g., flame atomic absorption or inductively coupled plasma [ICP] AES) simply because. sample dissolution is not a viable option in terms of speed. Therefore, a practical rf-GD-AES source must be able to accept nominally flat samples directly without appreciable machining and must offer rapid plasma stabilization/analysis times. We have used an approach (6) termed external sample mount geometry (Figure 2). This design, which is related to the traditional Grimm-type GD source, permits direct analysis of nominally flat diskshaped samples with diameters > 5 mm. It ensures that no machining of the sample is required because the sample is held externally to the source/vacuum chamber at ambient pressures. The sample is held against a poly(tetrafluoroethy1ene) (PTFE) O-ring by a torque bolt to secure a tight vacuum, and the rf potential is applied directly to the back of the sample. This design is also useful for depthresolved analyses of thin films. In these applications,machining a sample to con-
-
Figure 1. Self biasing process. (a) Application of a 2-kV square wave potential to an insulating surface ( Va). (b) Resulting potential on the cathode surface (VJ.
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form to a specific holder geometry would result in loss of the desired information. Furthermore, machining of glass or ceramic samples is not a viable option. The approach illustrated in Figure 2, therefore, is designed with applications involving metal, glass, and thin-film analyses in mind. Much effort has been directed toward improving signal intensities and detection sensitivities of the rf-GD-AES source by optimizing various operating conditions through parametric studies of anode geometry, rf power, and source pressure (6, 11,12).These basic studies have led to the successful coupling of a laboratoryconstructed rf-GD source with a highresolution commercial atomic emission spectrometer (13). rf-GDMS. The advantages of using mass spectrometers rather than emission spectrometers as elemental detectors are the much more simplified spectra, the generally higher sensitivity over the entire periodic table, the relatively uniform signal response to both metals and nonmetals, and the additional isotopic information that only MS can provide. On the other hand, capital costs and analytical speed can be considered as shortcomings relative to AES. Extensive academic research and commercialization of dc GD mass spectrometers have both prompted and paved the way for the development of rf-GDMS (14-16). In addition to the advantage of direct analysis of nonconductive materials, rf powering offers rapid plasma stabilization times in comparison with its dc counterpart. Despite the fact that it is more difficult to interface a plasma source to a mass spectrometer than to an optical emission spectrometer, researchers have successfully coupled rf-GD ion sources to several different types of mass spectrometers, including quadrupole mass filters (3, 17), quadrupole ion traps (18), magnetic sector instruments (19),and Fourier transform ion cyclotron resonance mass spectrometers (20). As was the case for atomic emission source design, rf-GD ion source design for MS has been approached with an eye toward the direct analysis of a wide variety of sample sizes and shapes. Two types of rf-GD sample introduction assemblies have been designed for two com-
Analytical Chemistry, Vol. 66, No. 18, September 15, 1994
G-
Figure 2. External sample mount geometry used in rf-GD-AES. Key: A, RG-213N coaxial cable to matching network; B, glass insulator; C, brass torque bolt; D, Macor spacer; E, O-rings; F,vacuum port; G, thermocouple gauge; H, stainless steel body; I, argon inlet ports; J, fused-silica windows; K, negative glow; L, orifice disk; M, sample; N, female coaxial connector: 0, copper conductor; P, male coaxial connector. (Adapted with permission from Reference 6.)
mercial GD mass spectrometer systemsthe Fisons Instruments Elemental VG 9000 (magnetic sector) and the VG GloQuad (quadrupole mass filter)-although the same basic design types could be implemented on other manufacturers’ instruments. These assemblies have been designed to accommodate both pin-shaped and flat sample geometries, as illustrated in Figures 3a and 3b, respectively. The primary complicating factor in the GDMS source design is the necessity to house the ion source volume within the main vacuum chamber. This permits cryogenic cooling of the discharge cell to remove trace impurities from the discharge gas, which tend to inhibit sputtering rates, and also result in the formation of molecular ions that contribute to spectral complexity (16, 21). Thus, the sample must be transported to the cell volume through a vacuum interlock system by way of a direct insertion probe (DIP).
The DIP designs used in these assemblies are similar to those used in organic MS. The rf-powered DIP must meet several design requirements, including efficient coupling of the rf power to the sample, isolation of the sputtering to the sample surface, and effective shielding of possible radiated power losses. The two designs shown here are intended to permit great flexibility and allow the use of the same DIP for the actual sample introduction. Figure 3a is an expanded view of the pin-type sample holder implemented on the quadrupole instrument (22,23).This configuration allows direct mounting of the sample in the recessed region of the &"diameter copper holder (mounted to the end of the rf feedthrough of the probe); the probe can then be inserted into the discharge cell. Samples ranging from 1to 5 mm in diameter can be analyzed directly without excessive machining. This basic cell design has also been successfully implemented on the magnetic sector spectrometer (19).In total, this design allows for easy sample interchange with-
out breaking the source vacuum and provides for complete grounded coaxial protection of the powered rf lead. In addition, the modificationsare compatible with the cryogenic cooling features of the basic GloQuad and VG 9000 systems. For samples > 5 mm in diameter, the flat sample holder shown in Figure 3b allows direct analysis, provided the sample is nominally flat. The holder is mounted to the end of the DIP and comprises a PTFE clamp assembly, support rods, boron nitride spacers, and either stainless steel or aluminum anode orifice plates. The DIP is placed through the clamp assembly, and the rf feedthrough is placed directly behind the sample (cathode), which locks the sample in place. The spacers separate the anode plate and the cathode in the same manner as the O-ring seal in the rfAES source design. The anode body in this case is the Ta ion volume, which is enclosed by the anode plate and the ion exit orifice mounted to it; the entire cell assembly is affixed to the commercial source cryo-coolingring. Operating parameters
and cell dimensions for flat-cell geometry are still being optimized. Previous studies of the rf-AES source have shown that the limiting orifice diameter (which defines the sputtering area) has an important role in analyte sensitivity (12).Other variables under study are the roles of discharge power, gas pressure, and the sampling distance between the cathode surface and the ion exit oritice. Other approaches. A number of other research groups are working in the field of rf-GD spectrometries. For example, Chakrabarti and co-workers have used an rf-GD source similar to that shown in Figure 2 to atomize solution residues for atomic absorption analysis (24). The rf powering is an advantage over dc powering because these residues are nonconductive in nature and are atomized with greater stability. Hieftje and co-workers have developed a configuration that uses a transverse magnetic field (up to 1400 G) at the cathode surface (25).This approach should produce a denser plasma with subsequent increases in analyte emission. Finally, Caruso and co-workershave focused on the role of discharge gas on the analytical characteristics of rf-GDMS, using a DIP for sample introduction on a commercial ICPMS instrument (26). Analytical figures of merit
Discharge
Figure 3. Ion source designs for (a) pin and (b) flat sample types in rf=GDMS. (Figure 3a adapted with permission from Reference 22.)
As optimization of rf-GD source designs continues, the general operating characteristics and analyticalfigures of merit are being established. The pertinent analytical characteristics for direct solids elemental analysis applications are summarized for both AES and MS applications in Table 1. The table lists the general figures of merit based on sample conductivity and, in the case of MS, sample (holder) form. The values reported reflect the implementation of the rf methodologies on the instrumentation available in our laboratory and do not necessarily reflect the best or worst cases. For example,limits of detection in MS would be expected to be far better for magnetic sector instruments, which have higher ion beam throughput and lower degrees of isobaric interferences than the quadrupole mass filter used in our laboratory. tfGD-MS. To provide a realistic, quantitative evaluation of the analytical characteristics of the rf-GD-AESsource, we de-
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stabilization time and a high degree of staveloped a methodical line-selectionprobility (precision) over the signal acquisicess for elemental analysis that uses the tion time frame are imperative. Stability is high-resolution, 1.0-m J-Y 38 (HR-1000) monochromator. Wavelengths were se- also important for depth-resolved analylected in a process based on the signal-to- ses, which can take tens of minutes to background ratio (S/B) values obtained complete. Depth-resolved analyses are not practical with arc emission and X-ray from the pure metal (elemental) form of the analyte and the highest S/N values o b analyses. As shown in Table 1,the rf-GD-AES tained when the analyte is present at source shows excellent temporal charactrace levels in the matrix of interest. The teristics. Both metallic and nonconductive optimum transition for quantitative analysis was chosen on the basis of freedom samples reach levels of greater than 5% RSD stability in an operating time of < 0.5 from spectral interferences and high calibration quality (13).Only the latter evalua- min. Furthermore, analyte signals maintain a high degree of stability over singletion criteria need to be repeated when the element (1min) and multielement (5 min) sample matrix is changed from one base metal to another or from metals to glasses analysis times. These temporal characteristics suggest that the use of simultaneous or ceramics. detection is not required for bulk analyHaving performed such a wavelength ses, as is the case for arc and spark emisselection process for a range of elements and matrices and optimized the plasma o p sion analyses. Analysis times could be more like those in arc and spark analyerating characteristics for analyte signal ses (< 1min) if multichannel instruments response, one must then be concerned with the actual implementation of rf-GD- were used. A key component to realizing high anaAES analyses. In any spectrochemical lytical accuracy and precision is the samtechnique, the ability to generate and maintain a stable plasma (i.e., analyte sig- ple-to-sample repeatability of the intensity measurements. The ability to obtain high nal) is of key importance. To assess the “external”precision relies on the ability to relative merits of the rf-GD-AES techmethodically control the sample internique, one must keep in mind that the competing technologies would be arc and change process and to reproduce the plasma operation characteristics. Table 1 spark emission spectroscopies for metillustrates the external precision obtained als and X-ray fluorescence for metals, glasses, and ceramics. In the former case, for analyte concentrations at the tens of ppm level for both conductive and noncondirect-reading (simultaneous) spectrometers allow rapid analysis even under non- ductive sample types. These values could be improved by the use of internal stanequilibrium conditions; in the latter case, the signal stability is set by the primary dardization methods. Although GD-AESis generally characX-ray source. terized as having good precision, the abilWhen using the rf-GD source with seity to achieve this level of precision with quential (scanning) spectrometers, rapid ...
..
parison of basic analytical Characteristics far nd rf-GDMS
MethoA =
AES A€-
Aetals
idoncon -’ xors
M! M!
’in - type metals ’in-type
M! M!
:lat noncondL
a
metals
Defined as 1 min for AES and 15 min for LIv
* Defined as 5 min for AES and 45 min for MS
906 A Analytical Chemistry, Vol. 66, No. 18, September 15, 1994
the rf source while sputtering nonconductive samples is somewhat surprising. The effects of residual atmospheric and water vapors, which effectively quench the important metastable levels of the discharge gas atoms and inhibit the cathodic sputtering process, are well documented in the GD literature (21,27,28). The sputtering of oxide samples such as glasses and ceramics most certainly introduces large amounts of oxygen into the plasma. Glasses and ceramics, which tend to be somewhat porous, also act as sources of water vapor that is liberated from the sample matrix in the sputtering process. It appears, however, that the likely introduction of gaseous contaminants does not affect the basic operational stability or reproducibility of rf-GD sources when they are operating in the AES mode. Having illustrated the basic analytical operation characteristics that are most relevant in laboratory implementation, we now turn to analytical sensitivity. Limits of detection (LODs) for the rf-GD-AES device have been calculated by the method described by Boumans (29)as LOD
=
O.Olkc(RSDB) S/B
where RSDB is the relative standard deviation in the background and c is the concentration of the analyte used in the determination; k = 3 is used to yield a > 99% confidence level. The LODs of various elements present in both metallic and glass samples are summarized in Table 1and indicate an improvement of 1-2 orders of magnitude over those obtained by other direct solids analysis methods such as dc GD-AES (30,31)and traditional arc and spark emission spectroscopies (32,33). They are achieved in large part because of the high spectral radiance of the rf-GD source in the presence of low spectral background (i.e., high S/B) and excellent temporal stability &e., low RSDB) of the devices. A large disparity in the analyticalperformance of metallic and nonconductive sample types appears only in the case of LODs. This can be attributed to the greatly reduced sputtering rates (50-100 times slower) observed for nonconductive samples relative to metals, which are typically 0.2-0.5 mg/min. Recent research under-
taken in our laboratory (12,34) has shown that power losses in passing the rf power through oxide materials can be appreciable, particularly for samples > 6 mm thick. Even so, stable plasmas are readily generated for samples up to 1 cm thick. Exhaustive studies of the analytical performance of nonconductive samples have not been completed, but it is reasonable to expect that sensitivitiesobtained for nominally thick (< 4 mm) samples will be within a factor of 10 of those obtained for the same elements in conductive samples. Regardless, the LODs obtained to date with rf-GD-AESare superior to those o b tained using any competing emission or X-ray fluorescence approach. rf-GDMS. In comparison to the laborintensive selection of analytical transitions (wavelengths) required in AES, the development of analytical methods for MS is far easier. The identification of spectral interferences is also far easier for MS. Unfortunately, as researchers who developed ICPMS found, the number of instrumental/plasma parameters that must be optimized results in a process that is far more complex than simply “making photons.” To develop rf-GD ion sources, the plasma parameters of power and pressure must be evaluated over a range of ion cell volumes, sampling positions, and sample sizes to achieve optimum analyte signal intensities. The production of molecular ions and multiply-charged species must also be evaluated. Such a study has been completed for the pin-type sample holder on the VG GloQuad, and an evaluation of those parameters is just beginning for the flat sample holder. The following discussion of the analytical characteristics of the two cell types for the respective sample classes serves as a good representation of pin-type geometry and is a general preliminary assessment of the flat sample holder. A caveat in consideration of Table 1is the admitted limitation of quadrupole MS instruments in comparison with sector-based instruments, where spectral resolution and ion throughput are far superior. As with the rf-AES sources, plasma stabilization times and short- and long-term precisions are positive attributes of the rfpowered MS sources. The plasma stabilization times observed for metallic samples
(3 min) represent a real improvement over the dc applications, where sputtering times of up to an hour may be required for some metals. The precise reasons for the improvement are not known, although o p eration of the rf plasma under constant power (rather than either current or voltage) may be a contributing factor.
acteristics that are similar to those of the AES source. Once the plasma has completed the stabilization phase, the conductivity of the sample matrix does not have an appreciable effect on the o b served signal stability. It should be noted that the precision data in MS sampling involves the use of the matrix element as an internal standard, because this approach is key to the application of relative sensitivity factors (RSFs) in the quantification step (16,35).The internal precision observed in MS is quite good, considering the time frames over which the measurements are taken and the large range (ppm to matrix) of analyte concentrations involved. In fact, the 45min analysis period, consisting of full mass spectral acquisitions every 5 min, is long for quadrupole mass analysis but is in line with sectorbased analyses. Given the difficulties resulting from residual gas introduction in all GDMS analyses, one would expect that the MS technique would suffer with regard to external precision relative to the atomic emission Alternatively,a large discrepancy exapplications. As shown in Table 1,howists in the stabilization times required for ever, the sampleto-sample precision for nonconductors in comparison with metals rf-GDMS devices is not substantially differand even with AES sampling of the same ent from that of AES. The proximity of sample types. The overall increase in stabi- these values points to the beneficial aslization times when operating in the MS pects of the use of RSFs to compensate for mode reflects the more critical vacuum re- raw intensity variations, which are on the quirements and the complexity of the order of 1096RSD. As is the case for the inplasma processes responsible for ioniza- ternal precision, the difference between tion relative to atomic excitation. As a rethe external precision obtained for conducsult of the presence of residual gases, the tive or nonconductive sample analyses is ionization signals obtained from GD not significant. This is surprising because sources do not stabilize until such species this figure of merit is assessed by repetihave been liberated from the sample surtive cycles of sputtering of the samples and face/matrix as well as the plasma. As a re- exposure to the atmosphere, which insult, the cryo-coolingconcept is imporvolves a great deal of residual gas adsorp tant. The effects of metastable quenching tion, especially for the glass matrix samare most pronounced in oxide samples, ples. As in the case of the internal preciwhich by nature tend to release residual sion values, better coupling of the rfgases that are adsorbed onto the sample GDMS ion volumes to the instrument’s surface or entrapped in the sample matrix, crywooling will likely improve the exterThe longer stabilizationtimes for noncon- nal precision to a measurable extent. ductors are probably an inherent complicaThe assessment of detection limits for tion of the technique. Improved coupling GDMS is different from the method deof the sources described here to the instru- scribed for AES because it involves the ment cooling apparatus will likely result measurement of the variation of the rein substantially improved stabilization spective isotopic signals over a succestimes. sion of mass scans and the translation of Having established a stable discharge the standard deviation into units of conand analyte signal, the rf-GDMS devices centration by use of RSFs. As such, the show short- and long-term precision char- presence of large analyte signals or iso-
Barring isobaric overlaps or large analyte signals, the MS method is more sensitive than theAES method.
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spectively, based on the measured S/N baric, molecular interferences will ultivalues and a 30 criterion (36).These mately limit the determined LODs. These values were obtained without rigorous resultant values are referred to as instrument detection limits. Assessment of these source and plasma optimization studies. These data suggest that, with nominal imvariations with regard to external preciprovements in cell coupling, the rf-GD ion sion results in method detection limits. sources should have superior or at least The LODs reported in Table 1are the inequivalent analytical characteristics to r f strument detection limits and reflect the ion sources, with the added advantages of general increased sensitivity of the MS faster plasma stabilization times and a approach relative to AES. In fact, barring vastly increased scope of application. the presence of isobaric overlaps or large analyte signals, the MS method is more Sample diversity sensitive across the periodic table than is the AES method, and the values are more Having demonstrated some of the basic characteristics of rf-powered GD devices, uniform from element to element. The degraded detection limits for non- we will illustrate the wealth of sample types and applications of the devices. Alconductive samples (glasses) are not significantly different from those of metal though a particular sample may have been analyzed by rf-GD-AESor rf-GDMS, the samples, considering the inability to obsame sample could be successfully anatain glass standards of high (99.999%)purity. Collaborative studies with the Oak lyzed by either technique. The ultimate choice of how a particular sample type Ridge National Laboratory that involve might be most effectively analyzed would use of a similar flat-cell rf-GD ion source on the VG 9000 instrument demonstrated be based on the type of information desired, analysis times, figures of merit, detection limits of 28 ppb and 16 ppb for elements in metal and glass samples, re- cost, and available instrumentation.
Figure 4. rf=GDmass spectrum of NIST SRM 612. Conditions:35 W rf power, 1 .O sccm Ar flow rate.
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Nonconductive samples. The primary impetus for the analytical development of rf-powered GD was its inherent capability to directly sputter-atomize nonconductive sample types, specifically glasses, ceramics, and fused geological materials. An important question as these developments progressed was whether this ability would be counteracted by a sacrifice in plasma stability when sputtering nonconducting samples. A number of different glass and ceramic sample types have been analyzed, and all exhibit similar operational and spectral characteristics. Differences are found as sample thickness affects energy coupling (and thus sputtering rates) and as adsorbed or trapped vapors are released into the discharge. Figure 4 shows the rf-GD mass spectrum of NIST SRM 612 (trace elements in glass). The sample is a 3-mm-thick, lcm-diameter disk and is certified with a range of elements in the 50-ppm range. Analysis simply involves mounting the sample in the flat sample holder (1min) , introducing the probe into the chamber (1 min), evacuating the source (1min), presputtering for a period of - 30 min to remove residual gases, and collecting the spectral data. The mass spectra for glasses and ceramics are characterized as predominantly atomic, although some contributions from metal oxides are seen. Even though the sample matrix is > 50%oxygen (as oxides), it is surprising that intense signals for atomic ions are o b tained while single- and higher order oxides are minimal. The same is also true for atomic emission spectra. Most surprising is that the total analyte ion signal is equivalent to what would be expected for the same concentration from the sputtering of an alloy target, even though the sputtering rate is 100 times lower for the glass (12,34).Particularly noteworthy is the rare earth element region of the spectrum where the 13'La+ (36 ppm isotopic concentration; uncertified) signal intensity of is observed. Because the de- 8x tector dynamic range extends to - 2 x 10-l8and no systematic optimizationhas been performed with the flat sample cell, we believe it is not unreasonable to expect that LODs < 100 ppb could be achieved for such samples. Metallicfilms. One of the earliest advantages noted for the application of GD
devices over arc and spark emission methods was their ability to perform depthresolved elemental analyses. In general, the application of GD atomic emission (with Grimm-type devices) for depth profiling has far outpaced the use of GDMS; however, there has been a recent increase in activity in this area (37,38). One of the limiting factors in the use of GDMS rather than AES has been the low sputtering rates obtained in the MS sources. The slower rates result because different source conditions, which do not favor atomization, are necessary for optimum analyte ion production rather than for atomic excitation. Although the rfGDMS and AES sources do not operate in the same power/pressure regimes, the rf MS source yields higher sputtering rates under optimum conditions than its dc counterpart. This improvement is illustrated in Figure 5, which shows the temporal profiles of the major elemental components of NIST SRM 1361a, a magnetic susceptibility standard. This standard consists of a Cr layer of unspecified thickness on a 12pm-thick Cu layer situated on a stainless steel substrate. The successive removal of the material in each layer is clearly visible using a sputtering rate of - 0.8 pm/ min in the Cu layer. This profile was obtained under plasma conditions that
were not optimized to obtain either fast or high-resolution depth profiles. Based on the removal rate of the Cu, one can roughly estimate the thickness of the Cr layer to be - 0.5 pm. Raith and co-workers have presented sputtering-rate data for depth profiles obtained under optimized dc GDMS conditions. These data indicate that a much longer sputter time would be required to reach the respective interfaces for dc-GDMS analyses (38). Alternatively, the same sample sputtered under rf-GD-AES conditions would have a sputtering time of e 5 min (8). Nonconductive films on nonconductive substrates. In terms of performing an elemental analysis, the most challenging sample would be a thick (>1pm) ,nonconductive coating deposited onto a nonconductive matrix. In this case X-ray fluorescence is not capable of depth-resolved analyses, and the shear thickness would make the kind of ion sputtering used in secondary ion MS (SIMS) prohibitive with respect to analysis time (39). One such example is the application of a silicate paint - 17 pm thick onto an automotive glass. The primary use of paint in automotive windshields and backshields is to block UV rays that can break down the urethane adhesive used to bond the glass onto the vehicle. Paint is also used for cosmetic purposes (i.e., to hide roof s u p
port pillars and electricalwires behind the body glass windows).A third application (in half-tone moonroofs and certain back windows) is to filter the amount of sunlight entering a vehicle. With the prior knowledge of the presence of Cr, Fe, Cu, and Ni in the paint, we ignited the plasma to the painted surface and monitored the emission signals from these analytes at fixed time intervals. Figure 6a shows the rf-GD-AEStemporal (depth) profile. The plasma emission is stable as the sputtering process proceeds through the paint layer up to the point where the interface region is reached (- 60 min) .The high degree of temporal stability of the source is important, given the slow sputtering rate of glasses (12,34), and makes the sequential monitoring of the four elements (- 2 min total) a viable option in the absence of a multichannel detection system. At this point, the emission signals of each analyte decrease in a uniform fashion until the substrate is cleanly exposed. Equivalent profiles of defective paint regions exhib ited irregular behavior for the Cr signal,
Figure 6. Analysis of silicate paint layer on glass.
Figure 5. Temporal profiles of analytes sputtered from NIST SRM 1361a by tf-GDMS. Conditions: 25 W rf power, 3.0 sccm Ar flow rate.
(a) rf-GD-AES temporal profile of analytes. Conditions: 40 W rf power, 5 torr Ar pressure, emission wavelengths: Ni, 352.4 nm; Fe, 371.9 nm; Cr, 425.4 nm; Cu, 324.7 nm. (b) Scanning electron micrograph of the resulting sputtered paint layer.
Analytical Chemistry, Vol. 66, No. 18, September 15, 1994 909 A
indicating a nonhomogeneous distribution; the other elements responded as shown in Figure 6a. Figure 6b is a scanning electron micrograph of the sample illustrating the bulk removal pattern of the paint film and the exposed glass matrix. The ability to obtain depth profiles of materials of this sort opens up many applications in the glass, ceramic, and semiconductor industries, where such information is either inaccessible or prohibitively expensive. BuZk polymer materials. Although thus far we have dealt with the use of rf-GD for elemental analysis, the capability of the devices to ablate nonconductive material opens up a wealth of alternative possibilities. A number of methods can be used to assess the quality and makeup of polymer materials in the liquid state, but there is now a pronounced need to perform sensitive analyses of bulk polymers, polymer blends, and polymer films of various sorts. MS in general and SIMS and matrix-assisted laser desorption ionization O I ) techniques in particular are increasinglybeing applied (39-41). Static SIMS is applied to such analyses by using one of a number of charge compensation techniques to ablate the nonconductive samples, with the goal of determining structural information of the polymer backbone and termini (39).MALDI involves mixing the polymer in powder form with a photon-absorbing matrix, producing mass spectra that provide predominantly molecular weight/distribution information (41). The d-GD approach has the inherent capability of providing structural information on bulk and layered polymer systems, without the need for any sort of sample preparation. Figure 7 is the rf-GD mass spectrum of a 2-mm-thick PTFE sheet obtained under somewhat milder discharge conditions than those used in bulk metal or glass analysis; however, they are not optimized for beam intensity or fragmentation characteristics. The positive ion spectrum is nearly identical in makeup to those obtained in static SIMS, with the exception that the+?I'' ion observed in the rf-GD mass spectrum is not present in the SIMS spectrum. The ion currents here (obtained on the Faraday detector) are on the same order of magnitude as those of a metal sample, with the dynamic range of
this MS system extending down to lo-''. The 10.5-GHz count rate for the CF+ion (31 amu) implies a far greater bulk sensitivity than the 5@kHz rate produced in the SIMS technique (40).The total analysis time to obtain this spectrum was 10 min from the time of sample mounting. Despite the fact that PTFE is known to form stable fragment ions for MS analysis,
we believe that spectra of this sort provide insights into the advantages of using rfGDMS for polymer analysis applications. In particular, rf-GDMS provides rapid analysis with high analyte ion signal intensities and the capabilityof performing depthresolved analyses of polymer films or blends. Although far removed from the area of direct solids elemental analysis
Figure 7. rf-OD mass spectrum of a bulk PTFE sample. Conditions: 25 W rf power, 1.50 sccm Ar flow rate.
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and clearly in an embryonic stage of development, we believe that the rf-GDMS a p proach may offer real advantages in the area of polymer analysis. The future
Future developments in rf-GD spectrometries will likely focus on developing quantification schemes for the analysis of oxide (glass and ceramic) materials, with particular emphasis on assessing the role of sample thickness and oxygen content on atomization rates. Additional work will focus on developing more refined depthprofiling protocols (i.e., quantifying the time-versus-depth relationship). Finally, the opportunity to generate meaningful mass spectra from polymer materials holds perhaps the greatest opportunity for diverse applications, but this application will require the greatest amount of fundamental characterization. The availability of commercial instrumentation and a growing list of researchers and practitioners points to continued development in source designs and applications. The basic analytical characteristics of rf-powered GD offer a wealth of possibilities for practical applications across the solids spectrum. The work described here has been supported through financial and instrumentation support from the National Science Foundation under grant No. CHE-911752;Jobin-Yvon,Division of Instruments, SA; and Fisons Instruments Elemental. Research collaborations with the Oak Ridge National Laboratory and the Westinghouse Savannah River Technology Center have also been instrumental in these develop ments. We thank Tom LaFramboise of the Ford Motor Glass Division for the painted glass samples and Patrick DePalma for his assistance in obtaining the scanning electron micrograph. References Hamson, W. W.; Barshick, C. M.; Klingler, J. A.; Ratliff, P. H.; Mei, Y. Anal. Chem. 1990,62,943 A-949 A. Glow Discharge Spectroscopies;Marcus, R K., Ed.; Plenum Press: New York, 1993. Duckworth, D. C.; Marcus, R. K. Anal. Chem. 1989,61,1879-86. Winchester, M. R; Duckworth, D. C.; Marcus, R K. In Glow Discharge Spectroscopies; Marcus, R. K., Ed.; Plenum Press: New York, 1993; Chapter 7. 1 Winchester, M. R; Marcus, R K. J. Anal. At. Spectrom. 1990,5,575-79. Winchester, M. R; Lazik, C. M.; Marcus, R. K. Spectrochim. Acta, Part B 1991, 46B, 483-99. Duckworth, D. C.; Marcus, R K.J. Anal. At. Spectrom. 1992, 7,711-15.
Lazik, C. M.; Marcus, R K. Spectrochim. tions, and Trends; Benninghoven, A.; Acta, Part B 1992,47B, 1309-24. Rudehaur, F. G.; Werner, H. W., Eds.; Chapman, B. N. Glow Discharge Processes; John Wiley and Sons: New York, 1987. John Wiley and Sons: New York, 1980; (40) Briggs, D.; Brown, A.; Vickerman, J. C. Chapter 5. Handbook of Static Secondary Ion Mass Marcus, R K. J. Anal. At. Spectrom. 1993, Spectrometry;John Wiley and Sons: New 8,935-43. York, 1989; pp. 24-25. Lazik, C.; Marcus, R K. Spectrochim. Acta, (41) Bahr, U. et al. Anal. Chem. 1 9 9 2 , 64, Part B 1993,48B, 863-75. 2866-69. Lazik, C.; Marcus, R K. Spectrochim. Acta, Part B 1993,48B, 1673-89. Harville, T. R.; Marcus, R K. Anal. Chem. 1993,65,3636-43. Eckstein, E. W.; Cobum, J. W.; Kay, E. Int. 1. Mass Spectrom. Ion Phys. 1975, 17, 129-38. Bentz, B. L.; Harrison, W. W. Prog. Anal. Spectrosc. 1988, 11,53-110. Raith, A. et a1.J. Anal. At. Spectrom. 1992, 7,943-49. Duckworth, D. C.; Marcus, R. K. Appl. Spectrosc. 1990,44,649-55. McLuckey, S. A. et al. Anal. Chem. 1992, R. Kenneth Marcus (left) is associate pro64,1606-09. fessor of chemistry at Clemson University Duckworth, D. C. e t al. Anal. Chem. (Clemson, SC 29634-1905). He earned 1993,65,2478-84. Marcus, R. K. et al. Appl. Spectrosc. 1992, B.S. degrees in chemistry and physics fiom 46,1327-30. Longwood College (VA) and a Ph.D. in anOhorodnik, S. K. et a1.J. Anal. At. Specalytical chemistry from the University of trom. 1993,8,859-65. Virginia. His research interests involve the Shick, C. R, Jr.; Raith, A; Marcus, R. K. use of GD devices (particularly $powered) J. Anal. At. Spectrom. 1 9 9 3 , 8 , 1043-48. to address problems in materials and enviShick, C. R, Jr.; Raith, A; Marcus, R K. J. Anal. At. Spectrom., in press. ronmental sciences. Chakrabarti, C. L. et a]. Presented at the 20th Annual Meeting of the Federation of Tina R. Harville (right) earned her B.S. deAnalytical Chemistry and Spectroscopy greefiom Mary Washington College (VA) Societies, Detroit, MI, October 1993; pa,-and is a Ph.D. candidate at Clemson Uniper 4'/ 1. versity. Her research interests lie in the de(25) Heintz, M. J.; Galley, P. J.; Hieftje, G. M. velopment and applications of $-powered Spectrochim. Acta, Part B, in press. (26) Giglio, J. J.; Wang, J.; Caruso, J. A. Preglow discharges in AES. sented at the 20th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Detroit, MI, October 1993; paper 475. (27) Hess, K. R; Harrison, W. W. Anal. Chem. 1988,60,691-96. (28) Larkins, P. L. Spectrochim. Acta, Part B 1991,46B, 291-99. (29) Boumans, P.W.J.M. Anal. Chem. 1994, 66,459 A 4 6 7 A. (30) Broekaert, J.A.C. J. Anal. At. Spectrom. 1987,2,537-42. (31) Banks, P. R; Blades, M. W. Spectrochim. Yuan Mei (left) earned her B.S. degree fiom Acta., Part B 1992,47B, 1435-46. (32) Kudermann, G. Fresenius 2. Anal. Chem. Peking University (China) and a Ph.D. in 1988,331,697-706. analytical chemistry from the University of (33) Genna, J. L. J. Cryst. Growth 1988,89,62- Florida, after which she spent 16 months 67. as a postdoctoral fellow at Clemson Univer(34) Parker, M.; Marcus, R K. Appl. Spectrosc. sity. She recently joined the faculty of the 1994,48,623-29. (35) Vieth, W.; Huneke, J. C. Spectrochim. Acta, Chemistry Department at Western ConnectiPart B 1991,46B, 137-53. cut State University as an assistant profes(36) Shick, C. R., Jr.; Marcus, R. K.; Ducksor, where she will develop an undergraduworth, D. C. Presented at the 1994 Winter ate research program in the area of G D Conference on Plasma Spectrochemisspectroscopy. try, San Diego, CA, January 1994; paper ThP 51. Charles R. Shick, Jr. (right) earned his B.S. (37) van Straaten, M.; Vertes, A.; Gijbels, R. Spectrochim. Acta, Part B 1991, 46B, degree fiom Clemson University, where he 281-90. is currently a Ph.D. candidate. His research (38) Raith, A; Hutton, R C.; Huneke, J. C. focuses on the implementation of r J. Anal. At. Spectrom. 1993,8,867-73. powered GD sources on both quadrupole (39) Secondary Ion Mass Spectromety: Basic and magnetic sector MS systems. Concepts, Instrumental Aspects, ApplicaAnalytical Chemistry, Vol. 66, No. 18, September 15, 1994 91 1 A
