Articles Anal. Chem. 1995, 67, 1026- 1033
Parameter Evaluation for the Analysis of Oxide-Based Samples with Radio Frequency Glow Discharge Mass Spectrometry S. De Gendt and Rene E. Van Grieken Department of Chemistry, University of Antwetp (UIA), Universiteitsplein 1, 8-26 10 Antwetp, Belgium
S. K. Ohorodnik and W. W. Harrison* Department of Chemistry, University of Florida, Gainesville, Florida 326 1 1
The performance of radio frequency-poweredglow discharge mass spectrometry (rf-GDMS)has been evaluated for the direct analysis of nonconducting samples. The parameters under study are the discharge power, the discharge pressure, the sample versus exit orisce distance, and the effect of cryogenic cooling. Optimum conditionsare used to evaluate some analytical figures of merit for the analysisof compacted oxide-basedmaterials. A precision of better than 5%is obtained on all measured species in the discharge, and a semiquantitative analysis delivers results within a factor of 2 from the certitled concentrations. Relative sensitivity factors are reported and compared for various matrices. The analytical data demonstrate the potential capabilities of rf-GDMS for direct analysis of various kinds of nonconductingmaterials. Glow discharge mass spectrometry (GDMS) has developed rapidly in recent years as a technique for the elemental analysis of solid samples. The great majority of that work has used a direct current (dc) glow discharge, although attention is now tuming to a radio frequency (rf) discharge. More than two decades ago, Cobum and Kay1 showed that a combination of rf glow discharge sputtering and mass spectrometry allowed elemental analysis of thin surface layers for either conducting or insulating matrices. They reported comparable sensitivities for almost all elements. Studies by Donohue and Harrison,2 who modified a spark source mass spectrometer to power an rf hollow cathode discharge ion source, indicated that similar spectra and sensitivities were obtained for dc and rf discharges, with the latter having the added advantage of sampling insulators directly. Despite this demonstrated advantage of rf sources, the next decade of research in the field of analytical GDMS was dedicated toward the development of the dc counterpart. Significantly, the commercial instrumentation for GDMS employed dc sources, which were thought to be simpler to use. Although dc-GDMS (1) Cobum, J. W.; Kay, E.Appl. Phys. Lett. 1971,19, 350. (2) Donohue, D. L.; Hamson, W. W. Anal. Chem. 1975,47,1528
1026 Analytical Chemistry, Vol. 67, No. 6, March 75, 7995
seems to have found its place among the techniques available for direct analysis of conducting and (to a lesser extent) semiconducting material^,^-^ the analysis of nonconducting materials still presents a major challenge. Due to the nature of dc-GDMS, a limiting factor is sample conductivity. To circumvent this limitation, powdered nonconductors have been mixed with various high-purity conducting binders (e.g., Ag, Cu, Ta, etc.), typically in a mixing ratio between 1:20 and 1:2, and compacted into a pin or disk ~ h a p e .This ~ - ~approach does allow analysis of most nonconductors, but it imposes stringent demands, including the intrinsic purity of the mixing binder, the grain size of the materials, and the extent of surfaceabsorbed impurities introduced, such as water vapor. Another approach to analyze flat solid nonconductors is presented by the use of the “secondary cathode technique”,I0 where the sample under examination is exposed to the discharge through an aperture in a conducting diaphragm. The discharge produces controlled redeposition of the secondary cathode material onto the overall cathode surface, yielding an analytical signal comprised of both the secondary cathode and the nonconducting sample. Obvious drawbacks of both approaches to nonconducting samples are the dilution of the analytical signal, which affects obtainable limits of detection, and the introduction of matrix impurities into a methodology that routinely exhibits sub-parts-per-billion detection limits. Extensive efforts by Marcus and co-workers during the last few years have revitalized the use of an rf discharge for direct solid analysis. This group has studied the ion source design and presented mass spectra of both conducting and nonconducting (3) Jakubowski, N.; Stuewer, D.; Vieth, W. Int. J. Mass Spectrom. Ion Processes 1986,71,183. (4) Vassamillet, L. F. J, Anal. Atom. Spectrosc. 1989,4,451. (5) Mykytiuk, A. P.; Semeniuk, P.; Berman, S. Spectrochim. Acta Rev. 1990, 13, 1. (6) Jakubowski, N.; Stuewer, D.; Vieth, W. Anal. Chem. 1987,59, 1825. (7)Dogan, M.; Laqua, IC; Massmann, H. 2. Anal. Chem. 1973,263,1. (8)Woo, J. C.; Jakubowski, N.; Stuewer, D. J. Anal. Atom. Spectrosc. 1993,8, 881. (9) Tong, S. L.; Harrison. W. W. Spectrochim. Acta 1993,48B,1237. (10) Milton, D.; Hutton, J. C. Spectrochim. Acta 1993,48E, 39. 0003-270019510367-102659.0010 0 1995 American Chemical Society
samples.ll A recent review addresses the operation principles and design considerations for rf-powered glow discharge devices.'Z Other work from this same laboratory developed the complementary use of rf-powered glow discharges as sources for atomic absorption and atomic emission applications. Source design considerations and parameter optimization are presented for both conducting and nonconducting material.13-18 Despite the expected benefits of an rf discharge over dc for the analysis of nonconductors, most of the recent reported work using mass spectrometry deals with instrumental aspects (direct insertion probe,Ig rfpowered discharge for double focusing,2oand rf-powered discharge for commercial quadrupole mass spectrometers).21 The analysis of oxide-based materials (e.g., ceramics, refractories, soils, etc.) is gaining importance and is one of the biggest challenges in the field of atomic spectroscopy. The application of graphite furnace atomic absorption and inductively coupled plasma emission spectroscopy and mass spectrometryz2,23 requires the sample to be dissolved, and although many types of dissolution procedures have been developed, the whole procedure remains cumbersome and time consuming. In addition, the sample is diluted and exposed to possible contamination. Therefore, rfGDMS offers great promise and application,since it allows direct analysis of nonconducting materials without dissolution or dilution steps. In the present study, our aim is to evaluate rf-GDMS for the analysis of compacted oxide-based materials. Various parameters, all of which might affect or influence the analysis, e.g., rf power, pressure, sample to exit orifice distance, and cooling, were optimized. Optimum conditions were determined for aluminum, Macor (solid nonconductor with aluminosilicate matrix), and firebrick (powdered nonconductor with composition analogous to Macor). Results are also shown for a Cody shale sample. In addition, plasma stability and relative sensitivity factors (RSFs) for analytical applications are evaluated. EXPERIMENTAL SECTION
The quadrupole mass spectrometerz4and cryogenic cooling devicez5used in these experiments have been described previously, except that multichannel analyzers were used for data acquisitions. The cryogenic coil is positioned 5-7 mm from the ion exit orifice so that the coil surrounds the sample without touching it (or the probe). Radiofrequency power (13.56 MHz) is produced by an rf generator (Model RF5S), impedance matched by an automatic (11) Duckworth, D. C.; Marcus, R K. Anal. Chem. 1989,61, 1879. (12) Marcus, R K. J. Anal. Atom. Spectrosc. 1993,8, 935. (13) Winchester, M. R; Marcus, R. K. J. Anal. Atom. Spectrosc. 1990,5, 575. (14)Winchester, M. R; Lazik, C.; Marcus, R K. Spectrochim. Acta 1991,46B, 483. (15) Lazik, C.; Marcus, R K. Spectrochim. Acta 1992,47B, 1309. (16) Lazik, C.; Marcus, R. K. Spectrochim. Acta 1993,48B, 1673. (17) Absalan, G.; Chakrabarti, C. L.;Hutton, J. C.; Back, M. H.; Lazik, C.; Marcus R K. J. Anal. Atom. Spectrosc. 1994,9, 47. (18) Parker, M.; Marcus, R K. Appl. Spectrosc. 1994,48, 623. (19) Duckworth, D. C.; Marcus, R K.]. Anal. Atom. Spectrosc. 1992,7, 711. (20) Duckworth, D. C.; Donohue, D. L.; Smith, D. H.; Lewis, T. A; Marcus, R. K. Anal. Chem. 1993,65, 2478. (21) Shick, C. R.; Raith, A; Marcus, R K. J. Anal. Atom. Spectrosc. 1993,8, 1043. (22) Grade, T.; von Bohlen, A; Broekaert, J. A C.; Grallath, E.; Klockenkamper, R; Tschopel, P.; Tolg, G. Fresenius 2.Anal. Chem. 1989,335, 637. (23) Bailey, E. H.; Kemp A J.; Ragnarsdottir, K. V. J.Anal. Atom. Spectrosc. 1993, 8, 551. (24) Bruhn, C. G.; Bentz, B. C.; Harrison, W. W. Anal. Chem. 1978,50, 353. (25) Ohorodnik, S. K.; Harrison, W. W. Anal. Chem. 1993,65, 2542.
Table 1. Elemental Composition of Macor, Firebrick ECRM 776-1, and CdS-1 Cody Shale
element
Macor
Li
composition (%) firebrick geol rock std,
ECRM 7761
C d S l Cody shale
0.009
B Na
2.17
Mg
10.3 8.47 21.5
Al
Si P K Ti Cr Fe Ba
8.30
0.362 0.287 15.5 29.3 0.027 2.42 0.971 0.015 1.00 0.109
0.720 1.62 8.41 28.9 2.32
matching network (Model AM5, both RF Plasma Products, Marlton, NJ), and coupled to the sample. A direct insertion probe is connected via an HN-type connector to the output of the matching network. We constructed the probe similar to the design outlined by Duckworth and Marcus,lgbut the electrical feedthrough and two glass sleeves, instead of one, were inserted into the steel tubing and around the center insulation. The copper sample holder is wrapped with Teflon tape, and a machinable ceramic (Macor, Corning Works, New York, NY) sleeve is placed around the holder. To prevent sputtering of the shield and to restrict the rf discharge onto the sample portion exposed to the discharge, a grounded metal cap is placed on top of the probe. The distance between the sample and metal cap is kept to '0.1 mm. The rf power was varied between 5 and 80 W. Ultrahigh-puritygrade argon (Liquid Air Corp., San Francisco, CA) was used in all experiments, with an operating pressure ranging from 0.3-2.0 Torr. Analytical samples were prepared from NIST standard reference material 601 spectrographic aluminum, Macor, Euronorm certified reference material 7761 firebrick, and US. geological rock standard SCo-1 Cody shale. The elemental composition of the oxide-based materials is given in Table 1. Metal pin samples were machined into 2 mm diameter pins with 5 mm of the pin exposed to the discharge. Oxide-based disk samples, with a diameter of 4.5 mm and thickness of 2 mm, were either machined from solids or compacted from powders. Compacting of the powdered samples was done using a stainless steel die. To compact the firebrick material, 40 ,uLof liquid binder (Chemplex, Tuckahoe, NY) was added to 0.15 g of the sample prior to pelletizing. Disk samples were mounted with doublesided carbon tape (SIX tape, E. F. Fullam Inc., Latham, NY) onto the copper disk-shaped holder. This holder is clamped onto the rf feedthrough. Approximately 1.5 mm of the disk is exposed to the discharge. The distance between the sample and the ion exit orifice was varied between 3 and 12 mm. RESULTS AND DISCUSSION The analysis of solid samples by GDMS has become generally
routine in recent years, but nearly all those results are based on the use of a dc discharge, which has been the standard source and remains so today. Optimized conditions for dc plasmas are well known and successfully applied to many GDMS applications. However, rf discharges exhibit different characteristics, yield Analytical Chemistry, Vol. 67, No. 6, March 15, 1995
1027
unique ionization processes, and require careful optimization to realize the significant rf advantages. Compacted samples introduce special problems that must be evaluated. To evaluate the use of rf-GDMS for the analysis of compacted nonconducting (oxide-based) materials, an optimization of various discharge parameters was performed. Subsequently, these optimized conditions were used to obtain some preliminary analytical results on the analysis of a firebrick ECRM 77&1 disk. Effect of Cryogenic Cooling. One of the most important parameters influencing the analysis is the use of cryogenic cooling. The use of cryogenics to remove discharge impurities is a standard feature of commercial instruments, but there has been little demonstration of the magnitude of this effect. Ohorodnik and Harrisonz5studied an aluminum sample, a material susceptible to oxidation, to show the importance of cryogenic cooling. The spectral “cleanup” achieved by using a cryogenic coil in the discharge could not be similarly obtained in the absence of cooling, even by extended presputtering of the aluminum pin. An increase in analyte ion signals upon cooling was also reported, probably due to removal of analyte reactivity with atomic oxygen (a byproduct of water dissociation in the glow discharge). The magnitude of increase in the analyte ion signal depends upon the reactivity of that element with residual water vapor.26 Oxide-based materials, especially when compacted, not only contribute sputtered oxygen from their natural composition, thus creating oxidizing conditions, but also add air and water, trapped during the compacting process, to the discharge. In such cases, the use of cryogenic cooling turns from being merely a benefit into a necessity, as is demonstrated in Figure 1 for the ceramic sample. Figure l a represents the spectrum without cooling, and Figure l b reveals the effect of cooling after 40 min of liquid nitrogen flow, with continuous sputtering of the sample. For an aluminum pin sputtered in a dc discharge,25it typically required 20 min to clean up the spectrum. However, for the oxide-based materials it took closer to 30 min to obtain a spectrum cleaned to the point where analytical data could be taken with confidence. As may be seen from Figure la, the spectrum before discharge cooling is not useful for analytical purposes, since the contribution of water-, carbon-, and air-related peaks is dominant. The magnesium (m/z 24, 25, 26) and silicon (m/z 28, 29, 30) ratios are incorrect due to the presence of gaseous impurities (C2H+ and CzH2+ for magnesium and CO+, N2-, COHT, or NzH+ for silicon). The spectrum taken after 40 min of cooling is much cleaner, and for this spectrum the magnesium and silicon ratios are in general agreement with their natural abundances. Even for solid nonconductors, the effect of cryogenic cooling is significant, as reflected in Figure 2. A Macor ceramic sample was sputtered under the same conditions as shown in Figure 1. The intensity profiles for analyte (llB+,27Al-),argon-related (ArH+, Arz+),and other gaseous (OH+) species were monitored at 5 min intervals. After 20 min of natural discharge cleanup without cooling, the liquid nitrogen flow was switched on. Distinct trends upon cooling are noticed for the various signals. Both the OH’ and ArH+ peaks decrease strongly and almost immediately, reaching a steady state after 25 min of cooling. The analyte species signals only start to increase after most of the gaseous species are removed, reaching a steady state after 30 min of cooling. The Ar2+peak is hardly affected by the cooling. After (26) Ohorodnik, S. R; De Gendt, S.; Tong, S. L.; Harrison, W. W. J. Anal. Atom. Spectrosc. 1993,8, 864.
1028 Analytical Chemistry, Vol. 67, No. 6, March 15, 1995
a
0
10
20
30
40
30
40
mlz
b
0
10
20 m/z
Figure I. rf-GDMS spectrum of a Macor disk at discharge conditions of 50 W rf power, 0.4 Torr, and 7 mm sample to exit orifice distance (a) without cooling and (b) with cooling.
Cooling on
0
20
40
60 60 100 Time (minufes)
120
140
160
Figure 2. Effect of liquid nitrogen cooling on the intensity profiles of argon-related, gaseous, and analyte species for a Macor disk.
the cooling is switched off, the trapped impurities on the surface of the coil are released quickly, as reflected by the strong initial increase in both the ArH+ and OH+ peaks. The boron peak (unobstructed by gaseous species) drops immediately to its initial value before cooling, while the aluminum peak initially rises, probably due to the increase of gaseous interferences at m/z 27, and then returns slowly to its value before cooling. The signal profiles with and without cooling have been compared for the ceramic and firebrick samples, and results are shown in Table 2. The ratio of the peak area after 40 min of cooling versus the same peak area before cooling has been calculated for the analyte, argon, and impurity-related species. A similar behavior for the impurity and argon species is noticed for both the ceramic and firebrick samples. An effective reduction
Table 2. Ion Signal Ratios for Macor and Firebrick ECRM 776-1 Showing the Effect of Cryogenic Cooling
ion
C+ N+ O+ OH+
B+ Mg+
Al+
Si+ K+
SiO+
a
ratio (ion peak area after cooling)/ (ion peak area before cooling) m/z Macor firebrick Impurities 12 14 16 17
0.29 0.28 0.14 0.02
0.20 0.21 0.10 0.01
Analyte Species 11 24 27 28 39 44
1.85 2.50 2.17 1.03 2.13 0.93
0.36 2.45 1.16 0.42 1.16
3mm
0.35
Argon Species Ar2+
Ar+ ArH+ Ar2+
/l
20 40 41 80
1.32 1.06 0.19 1.19
1.43 0.78 0.12 1.38
of 70-100% is noted for the impurity signals, while the argon species, with the exception of argon hydride, are only moderately afEected by cooling. These results for solid and compacted nonconductors are in agreement with earlier work on metal (Cu, Fe, and Ta) samples.26 The analyte species, however, at first appear to behave Werently. All analyte signals increase for the ceramic sample, while in the case of firebrick, the signals at m/z 24 and 39, thought to be magnesium and potassium, respectively, decrease by as much as a factor of 3. This is explainable by the fact that the signal before cooling, for most of the analyte species in this region, is made up of the analyte signal itself plus the impurity species signal (C2+, C2H+,and C2H2+ for magnesium and an unidentified impurity for potassium) in an unknown ratio. Most isobaric interferences disappear after cooling is applied, and the peak area after cooling can be considered mostly interferant-free (correct abundancy ratios for magnesium and silicon isotopes). Therefore, the contribution of gaseous species at both the magnesium and potassium peaks for the firebrick sample became dominant over the analyte species in the precooling period. This explains the decreasing ratios in Table 2 for both elements in the case of firebrick. An increase in analyte signal with cooling has been reported earlies53for metal samples, and the same effect is noted for the ceramic sample, where an increase with cooling can be seen for all analyte peaks (e.g., unobstructed boron peak). The contribution of isobaric interferences also explains the initial rise in signal for the aluminum peak in the ceramic sample (Figure 2), after cooling is switched off. These results show that there is a need for adequate cooling to remove interfering gaseous species, and therefore all subsequent experiments include cryogenic cooling. Effect of Dischaqge Parameters. Optimization studies have been reportedlgfor a diflerent glow discharge source, spedrometer, and sample type (brass). We felt it important to optimize our system also. Although our interests were primarily with nonconductors and powdered samples, a brief evaluation was first done for the aluminum NIST 601 sample in order to compare the instrumental behavior with the previously reported data for brass. Ultimately, we were interested in any differences between solid
0.55
0.7
0.85
1
1.35
1.85
pressure (Torr)
b
3mm 5mm
0.35
0.55
0.7
0.85
1
1.35
1.85
pressure (Torr)
Figure 3. Intensity versus pressure and sample to exit orifice distance for an aluminum NIST 601 sample: (a) AI+ and (b) N+.
and powdered nonconducting materials versus metals. Duckworth and Marcuslg showed an %fold increase in Cu+ signal intensity by changing the pressure and sample to exit orifice distance. Also, by careful selection of discharge parameters, discrimination against argon-based species was possible. The optimum conditions for analyte species in the brass material were reported as 0.42 Torr, 5.3 mm, and 21 W of rf power. We found generally similar trends for the aluminum sample sputtered at 20 W rf power in comparison to the previously reported brass data.lg Figure 3a displays the effect of two critical source parameters. Signal intensity for the Al+ matrix peak is plotted versus discharge pressure and sampling distance. Contrasted in Figure 3b are data for a typical gaseous impurity, in this case N+, arising from residual trace air in the discharge. A comparison of these plots shows that the analyst can discriminate against the undesired background gas and favor the analyte signals by judicious selection of conditions. Optimum conditions for the aluminum signal were between 0.35 and 0.55 Torr argon pressure and 3-5 mm sample distance. The gaseous species tend to optimize at higher pressure and are quasiindependent of the distance to the exit oritice. A more detailed evaluation was done for both the solid (Macor) and compacted (firebrick) nonconducting material. The sample Analytical Chemistry, Vol. 67, No. 6, March 15, 1995
1029
was positioned at 3,5,7,8,9, and 12 mm distances from the exit orifice and studied at three different pressures (0.4, 0.7, and 1.0 Torr). Increasing the sample to exit orifice distance to values greater than 12 mm, or raising the pressure above 1.0 Torr, did not result in useful data. The applied rf power was 50 W, and liquid nitrogen cryocooling was started 30 min prior to data recording in order to maximize removal of the gaseous impurities. Results were obtained for analyte species (see Table 1for listing), argon-related species (Ar2+,Ar+,ArH+,Ar2+), and the remaining gaseous species (C+, N+, O+). Because the results of these experiments were quite similar for both the compacted and the solid material, only data obtained for the firebrick sample are included in the optimization profiles. In Figure 4, an evaluation of the intensity profiles for Mg+, N+, and ArH+versus pressure and distance from the exit orifice is shown. These ions represent three distinct spectral constituents groups, based on the conditions where the optimum signal is obtained. The fist group, behaving similarly to the profile shown in Figure 4a, contains all the analyte species, including the SiO+ peaks, as well as the argon-related species with the exception of ArH+.The optimum conditions for these species are at the lowest pressure, 0.4 Torr, and a distance of 7 mm. The second group of species, which follow the trends in Figure 4b, consists of the background gaseous species. These exhibit maxima at the highest workable pressure (1 Torr) and at a distance of 12 mm from the exit orifice. Finally, ArH+differs from the first group in optimum distance (12 instead of 7 mm) and from the second group in optimum pressure (0.4 Torr instead of 1.0 Torr). The presence of the argon hydride peak is related to the presence of gaseous species (water vapor), and therefore, it optimizes at the intersection of both the first and second group parameters. Compared to solids, powdered compacted materials exhibit potentially more serious problems from gaseous impurities. Thus, the ability to discriminate against these species is of significant interest. Contrary to metal samples, oxidebased samples show an optimum analyte signal at low pressure (0.4 Torr) but at 7 mm distance from the exit orifice Figure 4a). Moving the sample closer to the anode (the exit orifice plate) creates stability problems in sustaining the discharge. When the distance is reduced below 4 mm, the discharge will not sustain or ignite for a nonconducting sample. Even at a distance of 5 mm and a pressure of 0.4 Torr, some problems were encountered when igniting or running the discharge. To a certain extent, there is constant residual outgassing of a compacted sample as the discharge erodes its way into the material, perhaps influencing the optimum source parameters compared to the use of conducting solid samples. To evaluate the effect of rf power on the analysis of a solid or compacted nonconductor, the sample was positioned 7 mm from the exit orifice, and the power was varied between 10 and 80 W at three different pressures (0.4, 0.7, and 1.0 Torr). Results obtained for analyte species (see Table 1for listing), argon-related species (W+,Ar+,ArH+,Ar2+), and the remaining gaseous species (C+,N+, 0+)can be divided into two groups depending on the optimum signal for each of the species under examination. Examples of both groups are shown in Figure 5 for the ceramic sample; similar trends were observed for the firebrick species. In Figure 5%K+ is used to represent the trend for the analyte species and argon-related species with the exception of argon hydride, while the typical behavior of the remaining background gaseous 1030 Analytical Chemistty, Vol. 67, No. 6, March 15, 1995
"." I
IA I
1.4 Torr
3
5
7 0 distance (mm)
9
12
3
5
7 0 distance (mm)
9
12
b
C
1
h
c. .-
3 C
.2 C m
-C c
0.4 Torr
3
5
7 8 distance (mm)
9
12
Figure 4. Intensity versus pressure and sample to exit orifice distance for a firebrick ECRM 776-1 sample: (a) Mg+, (b) N+, and (c) ArH+.
species and the argon hydride peak Figure 5b) is represented by the behavior of C+. The fact that ArH+ acts similar to the gaseous species shows that its presence is closely related to the gaseous species (water vapor) in the discharge. It can be seen
a
150
n
.4 Torr
20
30
40
50
60
70
80
power (w)
b
1
10
x5OO
AI
0
10
1
r
20
30
40
50
60
70
80
(W Figure 5. Intensity versus pressure and rf power for a Macor sample: (a) K+ and (b) C+. power
that the signal intensity for all species increases for increasing rf power. Comparing the ratio of the magnesium (analyte signal) to the carbon (background) peak revealed an optimum ratio at the lowest pressure and at an rf power of 70 W. At higher powers, the background signal increased rapidly. Moreover, continuous exposure of the sample to an rf power of 70 W or higher resulted in destabilization of the compacted samples. This is probably due to resistive heat generation in the sample itself, because of the higher dielectric strength of nonconductors compared to conducting materials. From these experiments, it was concluded that analysis of nonconducting samples should be performed at a pressure of 0.4 Torr,a sample to exit orifice distance of 7 mm, and, depending on the sample, an rf power between 50 and 70 W, since these conditions seem to be favorable for both the signal-to-noise ratio and discriminating against unwanted gaseous species. Evaluation of Analytical Figures. For rf-GDMS to be considered as a useful analytical technique for the direct analysis of nonconducting materials, some demonstration of its capabilities must be shown for samples posing real analytical dficulty. The powdered nonconductor, firebrick ECRM 77G1, is a sample that offers potentially daunting problems for a glow discharge. The
10
20
30
40 mlz
50
60
70
80
Figure 6. rf-GDMS spectrum of a firebrick ECRM 776-1 disk obtained at optimum parameter conditions (see text).
matrix material is primarily a mixture of aluminum and silicon oxides, materials that have very strong metal-oxygen bonds. In addition, the powdered nature produces a compacted sample electrode that contains signilicant quantities of entrapped air and water vapor, both of which can have deleterious effects on a glow di~charge?~?~~ A mass spectrum of the firebrick material is shown in Figure 6, displaying the area between m/z 1 and 85, as well as the region around m/z 139. Other regions showed no useful analytical peaks under these conditions. To allow better assignment of the obtained peaks, some areas are enlarged. The first inset shows the mass range lower than m/z 12, where %+,7Li+, 1°B+, and llB+ are observed. In the mass range between m/z 12 and 41, peaks due to Ar2+,Al+,and Si+ dominate the spectrum. The sensitivity for aluminum is worth noting. Even though its concentration is only half that for silicon, its peak size is almost 50%larger, meaning that its RSF relative to silicon is about 3, not out of line with generally observed RSFs for GDMSF7 Other certified elements that can be seen in the spectrum are Na+, Mg+, K+, and Ba+, the latter in the third inset. It should be noted that the elemental sensitivityfor barium is less than those for the other elements of comparable concentration, probably due to lower transmission at that mass range and tuning artifacts of the quadrupole instrument. Also, the disparity might be explained by the propensity of barium to form doubly charged ions, a competing pathway for barium ions. However, inspection of the obtained mass spectra in the m/z 69 region did not result in observation of a peak due to Ba2+. Experience gained from the analysis of lanthanum oxide with rf-GDMS showed a peak at m/z 69.5, about 3%relative to the La+ peak. Therefore, a decreased sensitivity for Ba+ due to the formation of doubly charged ions is unlikely to make up for the poor sensitivity. The phosphoruspeak at m/z 31 is probably interfered by Si+,given the ease with which silicon is protonated in the gas discharge. Despite the use of cryogenic cooling, some residual C+, CH+, N+, NH+, 0+,OH+, and H20+ are still present in the low-mass region, and an 02+ peak can be observed at m/z 32. The second inset covers the area between m/z 43 and 60, and it displays clearly the iron (m/z 54,56,57 and 58) and titanium (m/z 46,47,48,49 and 50) regions. The peak ratios match approximately their isotope ratios, except for the titanium peak at m/z 46, and to a lesser extent the one at m/z 47, since both peak intensities are affected by molecular interferences (likely SiO+ and SiOH+, respectively). This is (27) Sanderson, N. E.; Hall, E.; Clark, J.; Charalambous, P.; Hall, D. Mikrochim Acta (Wien) 1987,I , 275.
Analytical Chemistry, Vol. 67, No. 6, March 15, 1995
1031
(reo element, an estimation of the concentration (conc) of an analyte (anlt) element can be done through a ratio of signal intensities [it), after an adjustment for isotopic abundance (abun) .
Table 3. Calculated Concentrations for Firebrick ECRM 776-1 by rf-GDMS
concentration (%)
element certified Na Mg
AI Si
calcd
0.362 0.677 i 0.031 0.287 0.374 i 0.013 15.5 32.8 f 0.8 29.3 21.1 f 0.6
concentration (%) element certified calcd
K Ti
Fe
2.42 0.971 1.00
concanlt= int,lt(conc,ef/intref) (abunref/abunanIt) (1)
1.44 f 0.06 0.631 i 0.031 ref element
Table 4. Comparison of RSF Values Obtained by rf-GDMS for Various Matrice
Macor firebrick geol rock std, element (Corning) ECRM 7761 SCo-1 Cody shale B Na
1.76
Mg
1.54 3.06 1 1.35
AI Si K Ti Fe
2.60 1.81 2.94 1 0.82 0.90 1.38
0.48 2.07 1.73 1 1.35
litz8 1.61 j, 0.23 0.78 1.53 i 0.31 1.41 i 0.09 1 0.59 4.67 j, 0.44 1.96
suggested by observation of the peaks at m/z 44-46, whose pattern is almost identical to the silicon range between m/z 28 and 30. Additionally, the peak at m/z 43 is thought to be due to AlO+. When sputtering compacted oxide-based materials, it is virtually impossible to avoid a contribution of monoxide interferences. These interferences arise in part from cosputtering the monoxide molecules together with the analyte atomic species. Another source of monoxide interferences lies in combination reactions of analyte species with gaseous impurities, the latter pressed within the cathode volume during sample preparation and released during the sputter process. To evaluate how completely the monoxide molecules are dissociated to atoms, the fraction of atomic ion signal to the total analyte ion signals (i.e., M+/(M+ MO-)) can be used as an indication.28 As estimated from Figure 6, this ratio is approximately 98%for aluminum, while for silicon it is only 80%. The fact that approximately 20% of the silicon is present as an oxide, while most of the aluminum exists in an atomic state, does not fully account for the difference in sensitivity between the aluminum and silicon signals reported earlier. Nevertheless, as can be seen from Figure 6, an analytically useful spectrum is obtained when a compacted oxide-based material is analyzed with rf-GDMS. To evaluate the plasma stability, and thus the analyte ion signal reproducibility for the firebrick ECRM 77&1 sample, seven repetitive measurements were obtained over approximately 1 h, using the optimum discharge parameters. Each of these seven measurements is the average of five consecutive scans over the m/z 1-85 range. Results calculated for analyte, argon-related, and gaseous species demonstrate a signal variation between 1.4% (ArZ+)and 4.9% Ti-). An internal precision (within-sample comparison of the plasma stability) better than 5% RSD was observed for all species present in the discharge. Given the type of sample and the length of time for the measurements, these are encouraging data relative to the analytical applicability of rfGDMS. The data obtained for the analyte species in the previous measurements were also used for the semiquantitative analysis of the firebrick sample. Given eq 1and using iron as a reference
+
1032 Analytical Chemistry, Vol. 67, No. 6, March 15, 7995
Semiquantitative results for the firebrick sample are shown in Table 3. Sodium, magnesium, and aluminum are more sensitive than the reference element (iron), while silicon, potassium, and titanium are less sensitive. The semiquantitative analysis calculations were based solely on comparison of ion intensities, resulting in values within a factor of 2 of the certified concentrations. This is in keeping with results reported for dc-GDMS, where semiquantitative analysis yielded results withii a factor of 3.27 A direct comparison of intensities to obtain semiquantitative data is feasible for rf-GDMS as it was for dc-GDMS, since the variation in elemental sensitivities is of the same order of magnitude for both techniques. This opens perspectives for the direct and undiluted analysis of nonconducting samples, where suitable reference materials are often lacking. However, quantitative analysis in GDMS often relies on the use of relative sensitivity factors. These RSF values can be calculated using eq 2. In the case of dc-GDMS, variation in RSFs
RSF = (in~nlt/conc,,lt)/ (infef/conc,J
(abunref/abuna,,J
(2) between different conducting matrices is known to be smalls2*In Table 4, a comparison of RSFs, is made for various nonconducting matrices. RSFs were calculated for the ceramic, firebrick and geological rock standard (SCo-1 Cody shale). The elemental composition of the latter is comparable to those of the two other materials, as can be seen from Table 1. For reference purposes, the average elemental RSF values determined by Vieth and HunekeZ9are also given. As mentioned earlier, these values were obtained from analyzing a range of conducting materials using a magnetic sector dc-GDMS instrument, whereas our results are obtained from the analysis of nonconducting samples with the aid of a quadrupole rf-GD mass spectrometer. All RSF values (including data from ref 29) have been calculated according to eq 2, using silicon as a reference element. Although RSF values reflect contributions from instrumental factors (ion transmission and sensitivity) and glow discharge processes (atomization and ionization), the variation among the reported values for different materials, and even compared with the literature values on other instrument types and discharge types, is relatively small in most cases. CONCLUSION
The above findings demonstrate the potential usefulness of radio frequency-powered glow discharge mass spectrometry for the analysis of oxide-based materials without prior dilution or dissolving steps. This may encourage a broad area of applications, since these kinds of samples are among the most challenging in analytical chemistry. It is shown that the use of liquid nitrogen cooling allows the removal of unwanted spectral contributions that arise due to the nature of the samples and as a consequence of the compacting process. Careful selection of the discharge parameters provides (28) Mei, Y.; Harrison, W. W. Anal. Chem. 1993,65,3337. (29)Vieth, W.; Huneke, J. C. Spectrochim. Acta 1991,46B, 137
additional discriminationagainst undesirable gaseous species and promotes analyte signal intensities. Since all these data were obtained on a versatile but aging mass spectrometer used mainly to study fundamental processes rather than for high-sensitivity analytical experiments, no optimum obtainable limits of detection are reported. Nevertheless, the outcome of the analytical work so far is promising. Semiquantitative results varied only by a factor of 2 from the certitied values, and relative sensitivity factors for various types of matrices differwithin acceptable margins from one matrix to another. The fact that rf-GDMS is capable of rendering useful spectra with good precision and accuracy for all analyte species makes it a potentially
advantageous technique for direct analysis of nonconducting materials, indicating that future investigations are warranted. ACKNOWLEWMENT
This work has been supported by grants from the US. Department of Energy, Division of Chemical Sciences. S.D. is also supported by the Belgian National Fund for Scientific Research "0). Received for review August 22, 1994. Accepted December
28,1994.@ AC940834Y @
Abstract published in Advance ACS Abstracts, February 1, 1995.
Analytical Chemisfry, Vol. 67, No. 6, March 15, 1995
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