110
Anal. Chem. 1988, 60,110-114
Cluster Ion Formation under Laser Bombardment. Studies of Recombination Using Isotope Labeling Inga H. Musselman,1'2Richard W. Linton,'* and David S. Simons2
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514, and Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
The analysis of natural abundance NIS particles mounted on an isotopicaiiy enrkhed 34slilm u~lng a transmisskngeometry laser microprobe mass spectrometer provides a means for observing recombhation reactions in the laser-lnduced piasma. Sulfur-34 from the isotopically enrlched thin film Is extensively combining In the plasma with nickel from the NIS particle. The data suggest that this recombination phenomenon occurs to a M a r extent for the three cluster ions NIS', NIS,', and NI,S+. One exception for the NIS,' case is the 5eNi34S34S+ ion, whkh has a much larger contribution to recombination by the '% film. This probably reflects Ni attachment to %'% dimers which are formed almost exclusively by contributions from the a4Sfilm.
The investigation of cluster ions is a current area of interest in laser microprobe mass spectrometry (LAMMS) research. A better understanding of ion formation mechanisms will improve the ability to interpret the laser mass spectra of solids with respect to chemical speciation and structure. A review of the applications to solids of the laser ion source in mass spectrometry by Conzemius and Capellen (I) and a bibliography of laser mass spectrometry of solids by Conzemius et al. (2) provide background to this research problem. Processes including volatilization, ionization, and cluster ion formation by pulsed lasers occurring at various places and times in the plasma surrounding the point of sample-laser interaction have been discussed (3-7). Throughout this paper, the term plasma will be used to describe the high energy vapor formed by the high power density laser ablation process even though the charge densities in these experiments are unknown. Several research groups have used LAMMS specifically to study cluster ion formation in inorganic and organic compounds. The LAMMS studies have included experiments concerned with polyatomic ions generated from silica particles (8),thin carbon foils (9), thin foils of various metals and semiconductors (IO), binary oxide particles ( I I ) , and substituted pyridines (12). Cluster ions, formed by the laser ionization process, also have been used empirically to differentiate among closely related compounds including inorganic perrhenates (13),titanium oxides (14),inorganic sodium and calcium oxide and calcium salts (17). sulfoxy salts (15,16), Cluster ion formation in laser ionization mass spectrometry may occur either by direct ionization of the sample by the pulsed laser beam or by a recombination of ions and neutral atoms or molecules in the laser-induced plasma formed above the sample surface. For example, cluster ions of Ag,Cu,+, formed by recombination in the plasma, were observed by Wurster et al. in laser mass spectra obtained from physically separated Cu/Ag sandwich foils (18). Recombination ions of Ag,,Au,+ were present along with cationized species in positive ion spectra collected by Benninghoven and Anders when inUniversity of North Carolina. National Bureau of Standards.
ducing Ag cationization of leucine on Au-Ag sandwich substrates (19). Recently, Bruynseels and Van Grieken studied recombination reactions occurring in elemental 12C/13Cbilayers (20). The experiments reported here evolved from a cooperative project with the Environmental Protection Agency (EPA)/ Environmental Monitoring Systems Laboratory, Research Triangle Park, NC, to study new methods for nickel speciation in airborne particles produced by stationary sources. The development of new techniques to provide speciation information about elements present in trace quantities within complex and highly heterogeneous samples is a challenging area of analytical chemistry. Microscopic analysis techniques, such as LAMMS, which combine the advantages of high spatial resolution and high sensitivity, are being developed to provide information about chemical species and their distribution among, or within, individual particles. Model nickel species (Ni, NiO, NiS04, NiS, and Ni&) and pollution source samples were examined in prior analytical technique development projects. These nickel species were analyzed, and in many cases differentiated, by LAMMS using characteristic cluster ion fingerprint mass spectra (21-23). The production of these unique cluster ion spectra sparked a more fundamental interest in ion formation mechanisms of nickel/sulfur compounds. In this study, a unique approach was developed to study ion formation permitting the recombination process to be observed directly in the production of cluster ions from nickel sulfide particles deposited on an isotopically enriched sulfur thin film.
EXPERIMENTAL SECTION The JAMMS measurements were made with a laser microprobe mass analyzer, LAMMA-500 (Leybold Heraeus) (24-26). A frequency-quadrupled Nd-YAG laser (A = 266 nm, T = 15 ns, average energy per pulse = 0.28 pJ, average laser power density = 2.8 X lo9 W/cm2) was focused on the sample with the aid of a He-Ne pilot laser. The energy of each laser pulse was monitored by an energy meter. The ions, generated by the laser pulse, were extracted at 180' relative to the incident laser (transmission geometry) by an immersion lens, focused by an ion lens (-1050 V), analyzed by a time-of-flight (TOF) mass spectrometer, and detected by a secondary electron multiplier. The amplified analog signal was digitized by two Biomation Model 8100 transient recorders and transferred in digital form to a computer for storage and further processing. Integral areas were obtained from the peaks of interest for use in isotope ratio calculations. Twenty positive ion mass spectra were obtained from nickel(I1) sulfide particles (Alfa Division, 99.99% pure) deposited on a Formvar-coated (Ernest F. Fullam, Inc., 0.25% Formvar in ethylene dichloride) 200-mesh Cu transmission electron microscope grid, and from NiS particles mounted on an isotopically enriched 34Ssputtered thin film deposited on a Formvar-coated TEM grid (Figure 1). The sulfur film, approximately 150 nm thick as estimated by optical interference microscopy, was made by sputter-depositing 93 atom % 34Sfrom a powder (Monsanto Research Corp., Mound Facility) target onto a Formvar-coated TEM grid via argon bombardment using a Gatan Dual Ion Mill, Model 600. The typical particle sizes analyzed were 1 or 2 Fm. The natural isotopic abundances of sulfur and nickel are as follows: %, (95.0%);33S, 0.8%; 34S,4.2%;36S,0.02%; 5sNi,68.3%; 60Ni,
0003-2700/88/0360-0110$01.50/0 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO.2. JANUARY 15, 1988 v1
E
n N
v) 0
NI0k.l
-
111
Sulflda
-z,.
u I
TEM Grid
Figure 1. NiS particles mounted on an isotopically enriched 3'S spunered thin film deposited on a Formvar-coated TEM grid.
-z D
Figure 2. Transmissbn geomeby of laser microprobe with schematic illustration of recombination reaction which may occur in the laserinduced plasma. NiS particle contains 4 2 % %. Enriched % film contains 93% %. 26.1%; "Ni, 1.1%; @Ni, 3.6%: MNi,0.9% (27) The laser conditions were not typical of laser desorption hut rather allowed for complete ablation of the sample. These conditions were chosen to maximize the production and detection of cluster ions (28-30). The samples were first mounted in the sample holder so that the Formvar film faced the extraction optics. The experiment was repeated with the sample in the other orientation, i.e. with the NiS particles facing the ion extraction optics, to determine whether sample orientation had a major influence on the spectra. The transmission geometry of the laser microprobe requires that the laser perforates the sample so that the ions may he extracted from the hack side of the sample into the TOF mass spectrometer. This geometry, shown in Figure 2, ensures that which both the NiS particles and the isotopically enriched % film, are physically separate for NiS on "S, are vaporized in a single laser shot. By comparison of the isotope ratios from the spectra collected from NiS and from NiS on 34S,it is possible to determined whether recombination reactions in the laser-induced plasma between the nickel from the NiS particle and the sulfur from the isotopically enriched 34Sfilm produce clusters, an example of which is schematically illustrated in Figure 2.
RESULTS AND DISCUSSION The following data are from the experiment in which the Formvar film faced the extraction optics. Representative positive ion laser mass spectra obtained from NiS and from NiS on 34Sare presented in Figure 3. The cluster ions NiS+, NiS,+, and Ni2S+,found in both spectra, may he formed by direct ionization of the sample by the laser and/or by recombination reactions in the laser-induced plasma. There are, however, some ohvious differences between the two spectra. In the spectrum of NiS on 34S,the larger peaks at 34Sf and %,(m/z + 68) are evident. In addition, the ratios of the peaks reflecting various combinations of nickel and sulfur isotopes within each of the clusters, NiS+, NiS2+, and Ni2S+, differ significantly between the two spectra. A more detailed ex-
80
40
120
160
2W
w r e 3. Representative positive ion laser mass spectra obtained from NiS and from NiS on 34S. Due to limited dynamic range of transient recorder, atomic nickei isotopes are saturated to bring ciuster ions on scale. Peaks at low mass reflea ubiquitous Na' and KC contaminants. NiS*
92/90
,IS,+ 124/122
Ni2S*
1501148
1.5
Figure 4. Isotope ratios for cluster ions NiS'. NiS,', and Ni,S+ from NiS (dotted bar. 19 spectra) and NiS on 34S(solid bar, 20 spectra) compared with predicted values based on natural nickel and sulfur abundances (hatched bar). Standard deviations of the mean, s l d n . where n is the number of spectra averaged, are as follows: for NiS+. NiS 0.016. NiS on 34S0.072; for NiS,+. NiS 0.030. NiS on 34S0.12: for Ni,S+, NiS 0.022, NiS on "5 0.061. amination of these differences follows. Isotope ratios from the spectra obtained from each sample were calculated for the NiS+ (92,901, NiS,+ (124, 1221, and Ni2S+ (150, 148) cluster ions. The peak areas for the cluster ion NiSz+were corrected for the isobaric interference Niz+. The average isotope ratios are compared in Figure 4 with the predicted values based on the natural isotopic ahundances. The isotope combinations which contribute to the peaks of interest are presented in Table I. For NiS, the average experimental isotope ratios are within 6% of the predicted natural abundance values and therefore provide an estimate
112
ANALYTICAL CHEMISTRY, VOL. 60, NO. 2, JANUARY 15, 1988 0 75
Table I. Nickel and Sulfur Isotope Combinations Which Contribute to the Cluster Ion Peaks of Interest
NiS+
mass 90 mass 92
58Ni32S 58Ni34S,BONi32S
NiS2+
mass 122 mass 124
5sNis2S32S 5sNi32S34S, ssNi33S33S,BONi32S32S
mass 148 mass 150
MNi68Ni32S 58Ni5sNi94S,58NiBONi32S
Ni2S+
Ni,S
0 45 34 s -
32 s
0 3
0 15 70-1
fi4%
0
6C 50 40
30 20
:0
1%
0 90
901
eo
92
94
1
1 2% 96
M/E
93% 3 4 s 64%
704
I
25%
90
92
94
96
M/E
N i S o n 3dS F i l m
80
501
42%
FiQure6. Sulfur ratios calculated from atomic and cluster ion peak areas obtained from NiS on film, average of 19 spectra. Standard deviations of the mean, slv'n, where n is the number of spectra averaged, are as follows: S', 0.042; NiS', 0.072; NiS,' 0.060; Ni,S+, 0.061.
41%
Figure 5. Relative peak Intensities for the NiS' cluster ion: (a) nickel and sulfur present in their natural abundances; (b) sulfur present as 9 3 % 34S; (c) experimental intensities, NIS on %, avera e of 19 spectra. Isotopic contributions are as follows: m/z 90 ( ~ l a sm) /,z 92 (%i%, s'Ni32S),mlz 94 (%I%,@NP2S),m/z 96 (%*Nl%, %Ji%S).
of the measurement error for comparison of the data from NiS on 34S. For NiS on 34S,these same ratios deviate positively by more that 60% from the predicted natural abundance values. This clearly demonstrates the presence of excess 34S in the cluster ion peak integrals. Figure 5 summarizes data from NiS on using the NiS+ cluster ion as an example. Figure 5a illustrates the relative peak intensities for NiS+ at m / z 90,92,94, and 96 provided that nickel and sulfur are present in their natural abundances. At the other extreme, Figure 5b shows the relative peak intensities for these same masses under the assumption that the NiS+ cluster ion contains 93% 34S,i.e. essentially all of the NiS+ forms via contributions from the film.The relative intensities obtained from the average of the 19 spectra from NiS on 34Sare presented in Figure 5c. The intensities in Figure 5c lie between the extremes of parts a and b of Figure 5 suggesting that 34Sfrom the isotopically enriched 34Sfilm
is recombining with nickel from the NiS particle, to a significant extent in the laser-induced plasma, to form the NiS+ ion. The fact that the experimental intensity at m / z 96 (Figure 5c) does not lie between the values of natural sulfur and 93% is attributed to the inherent measurement error of the instrument as described earlier. From this experiment, it is difficult to exclude solid-state diffusion as a mixing mechanism; data are not available for elemental sulfur diffusing into NiS. Significant diffusion is unlikely to have occurred at room temperature during the short time that the sample was stored prior to analysis. In addition, the extent of diffusion during the 115-ns laser pulse event is difficult to assess because the temperature that the sample achieved prior to ablation is unknown. The observation of Ni,S,, cluster ion formation by-ecombination, however, has also been observed from laser ablation of a sandwich sample in which elemental sulfur and nickel layers were separated by a film of elemental gold (31). Ratios of 34S/32S for the atomic sulfur ion (34,32) and the cluster ions NiS+ (92, go), NiS2+(124, 122), and Ni2S+ (150, 148), from the spectra of NiS on 34Sare reported in Figure 6 and each may be compared with the natural 34S/32Sratio of 0.044. The 34Sis highly enhanced in both the atomic and cluster ions relative to the natural abundance value but is significantly more enriched in the various cluster ion peaks. There is no reason to assume, however, that the ratio of So/S+/S-is the same for ion formation out of NiS as compared with the enriched film. The ratio of 34S/32Smay therefore not necessarily be reflected in the corresponding isotope ratio of each of the cluster ions. This difference may also suggest that more from the film is present in the periphery of the plasma where gas-phase reactions to form cIusters are more favored than in the more energetic plasma center where atomic ions are preferentially formed (3-5).It is consistent with the fact that the laser perforation usually encompasses a greater area (2-3 pm diameter) than a typical NiS particle occupies (1pm diameter). Thus, the edges of the vaporized sample area would contain almost exclusively 34S. The lower laser power density at the periphery of the performation should contribute to a greater production of neutral, relative to charged, atoms as well as to a greater probability of direct emission of 34Sxclusters. On the basis of differences in ionization potential, it is likely that a major contribution to recombination ions is the cationization of neutral 34S-containing species from the film by Ni+ produced from the particles. The aS/32S ratio from the three cluster ions varies by much less than a factor of 2 (Figure 6) suggesting that the recombination process occurs to a similar extent in the for-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 2, JANUARY 15, 1988
Table 11. Ratios of srS/32Sfor Ions Obtained in Two Sample Orientations
Table IV. Ratios of 34S/32S for Ions Containing from Zero to Three SBNiAtoms
ratio
34Sfilm facing optics
NiS particle facing optics
NiS+/S+ NiS2+/S+ Ni,S+/S+
2.52 1.87 2.43
3.43 1.83 3.28
ratio 1. 2.
Table 111. Ratio of Sulfur Isotopes Compared with Analogous Ratios Containing One ssNi Atom
3.
natural
4.
abun-
experiment (expt)
dance bat)
x = 0.228 0.0444 s / d l 6 = 0.0418a 0.0444 x = 0.580
s / d l 9 = 0.0718 x = 0.700 s / d l 5 = 0.132 z = 0.851 s / d l 9 = 0.120 x = 0.902 s / d l 4 = 0.219 x = 1.05 s / d l 9 = 0.162
113
expt/nat 5.14 13.1
0.0888
7.88
0.0888
9.58
0.00197
458
0.00197
533
a Standard deviation of the mean, s l d n , where n is the number of spectra averaged. m / z 92 corrected for @"i3%. e m / z 64 corrected for B4Ni. d m / z 122, 124 corrected for Niz, m/z 124 corrected for 6oNi32S32S.m / z 64 corrected for MNi. f m / z 122 corrected for Niz, Nil contribution at m / z 126 is negligible; m / z 126 corrected for @"i32S34S and 62Ni32S32S.
mation of the three cluster ions NiS+ (92,90),NiS2+(124,122), and Ni2S+ (150, 148). In the case of NiS2+,this refers to the inclusion of one 34Satom in the cluster, i.e. 58Ni32S34S. The variation in the concentration or charge state of various 34S-containingspecies in the laser-induced plasma may also reflect a gradation of laser power density with depth into the sample as a result of positional differences of the NiS particle and the 34Sfilm with respect to laser focus. If such were the case, data obtained with the sample in the alternative orientation should indicate that difference. The same calculations as above were performed on the data set from a second experiment in which 20 spectra were obtained from NiS and from NiS on 34Swith the samples mounted in the alternative orientation, i.e. the NiS particles faced the extraction optics. The 34Scomponent to the cluster ions is a factor of 2 higher in this orientation which could imply that the 34Sfilm experiences a higher laser power density. The relative abundances of the cluster ions, however, are very similar in both orientations (Table 11). The variation of laser power density with depth, therefore, does not appear to be the controlling factor in cluster ion formation. Further insight into possible formation mechanisms for the nickel/sulfur cluster ions can be gained from additional ratio calculations performed on the first data set taken from NiS on 34S. The average values of these ratios are compared in Tables I11 and IV with ratios calculated by using natural sulfur and nickel isotope abundances. Ratios of experimental to natural abundances are presented in the tables to ease the comparison. These ratios have been corrected for the presence of other combinations of isotopes at the same nominal mass as indicated in the tables. Table I11 is divided into three sets of ratio pairs. Within each pair, a ratio of sulfur isotopes is compared with an analogous ratio containing one 58Niatom. The ratio values within each pair differ at most by a factor of 3. This agreement indicates that the relative contributions of sulfur species
experient (expt)
x = 0.228 s / d 1 6 = 0.0418" 58Ni34S/UNi32Sbx = 0.580 s / d l 9 = 0.0718 58Ni58Ni34S/ x = 0.561 58NiS8Ni3?3c s / d l 9 = 0.0613 58Ni58Ni58Ni34S/x = 0.895 58Ni58Ni58Ni32Sd s i 4 1 6 = 0.173 34s/32s
natural abundance (nat) expt/nat 00444
5.14
0.0444
13.1
0.0444
12.6
0.0444
20.2
"Standard deviation of the mean, s l d n , where n is the number of spectra averaged. b m / z 92 corrected for 6oNi32S.e m / z 150 corrected for 58Ni60Ni32S.m/z 208 corrected for 58Ni58Ni60Ni32S. (charged or neutral) that react with a nickel atom (charged or neutral) to form the corresponding clusters NiS,+ (58Ni32S+, 58Ni34S+, 58Ni32S32S+,58Ni32S34Sf 58Ni34S34S+) reflect the respective ion intensities of s,+(32S+,34S+,32S32S+, 32S34S+, 34S34S+). The ratios of experimental to natural abundances for the clusters shown in Table I11 are all larger than one, thus offering another indication of the contribution from ion recombination. The values are similar for pairs 1 and 2, indicating comparable contributions from ion recombination reactions involving sulfur from the "S film.However, the ratio of experimental to natural abundance is greatly enhanced for cluster case 3 in Table 111. This suggests that the 58Ni34S34S+ has a much larger contribution from recombination reactions dimer involving the 34Sfilm, a likely consequence of 34S34S production dominated by "S atoms originating from the film. Such "SUS species could be produced by direct emission from the film or by the recombination of two 34Satoms, especially in the periphery of the plasma where species are apparently more abundant relative to 32Sas described earlier. If the 58Ni34S34S+ ion is formed primarily by recombination in the plasma, however, one would expect the experimental to natural ratio in case 3 to equal the square of the experimental to natural ratios in cases 1 and 2 reflecting the gas-phase concentration of atomic 32Sand %. The ratio in case 3 is far in excess of this prediction suggesting that the formation of the 58Ni34S34S+ ion depends primarily on the production of the 34S34Sdimer from the film by direct emission, followed by reaction with 58Niin the laser-induced plasma. This one event recombination mechanism is also favored based on collisional probabilities. Cationization of the neutral sulfur dimer by Ni+ is an obvious mechanistic possibility. In Table IV, the 34S/32Sratio is compared for ions containing from zero to three 58Niatoms. The experimental to natural ratios in Table IV are again greater than 1 indicating the occurrence of recombination. These ratios exhibit a relatively constant contribution of 34S,the ratios differing at most by a factor of 2 for the clusters, which suggests that formation of these cluster ions is controlled by reaction of Nix with a sulfur atom roughly in proportion to the gas phase concentration of the sulfur isotopes (reflected in the intensity ratio of MS/32S).There is no evidence of a greater probability or direct emission of specific clusters from the particles; i.e. NiS+ has a very similar 34Senrichment to Ni2S+and Ni3S+.
CONCLUSION We have demonstrated in a previous study that particulate nickel species may be differentiated by the LAMMA using their unique cluster ion fingerprint spectra (21). This study, however, shows that ion/molecule recombination reactions do indeed occur in the laser-induced plasma in the formation of these unique cluster ions. Therefore, caution is necessary
114
ANALYTICAL CHEMISTRY, VOL. 60, NO. 2, JANUARY 15, 1988
when assigning molecular structures from particle spectra which exhibit cluster ions that may not be directly representative of the bonds which exist in the solid state. A complication of this study is that all of the cluster ions observed may have contributions from both direct emission and recombination. Additional experiments of ion formation as a function of sample geometry, laser power density, and ion lens potential have been conducted in which only recombination is possible, e.g., Ni/Au/S sandwich films for which Ni,S,+ reflected only recombination mechanisms in the plasma (31).
ACKNOWLEDGMENT We wish to thank W. R. Kelly of the National Bureau of Standards for providing the 34Spowder used to make the sputtered thin film and Frank Butler of the U S . Environmental Protection Agency for his support of this project. LITERATURE CITED (1) Conzemius, R. J.; Capellen, J. M. Int. J. Mass Specbom. Ion Processes 1980,3 4 , 197-271. (2) Conzemius, R . J.; Slmons, D. S.;Shankai, 2.; Byrd, G. D. I n Microbeam Analysis; Gooiey, R., Ed.; San Francisco Press: San Francisco, CA, 1983; pp 301-332. (3) Hercules, D. M.; Day, R. J.; Baiasanmugam, K.; Dang, T. A,; Li, C. P. Anal. Chem. 1982,5 4 , 280A-305A. (4) Novak, F. P.: Balasanmugam, K.; Viswanadham, K.; Parker, C. D.; Wilk, 2. A.; Mattern. D.; Hercules, D. M. In?. J. Mass Spectrom. Ion Phys. 1983,5 3 , 135-149. (5) Hercules, D. M. Pure Appl. Chem. 1983,5 5 , 1869-1885. (6) Hillenkarnp, F. I n Ion Formation from Organic Solids; Benninghoven, A., Ed.; Springer-Verlag: Berlin, 1983; pp 190-205. (7) Furstenau, N. Fresenius' Z . Anal. Chem. 1981,308, 201-205. (8) Michiels, E.; Celis, A.; Gijbels, R. Int. J. Mass Spectrom. Ion Phys. lg83 23-26 .- - - * 47 . , - - -. Furstenau, N.; Hillenkamp, F.; Nitsche, R. Int. J . Mass Spectrom. Ion Phys. 1979,3 1 , 85-91. Furstenau, N.; Hillenkamp, F. Int. J. Mass Spectrom. Ion Phys. 1981, 37, 135-151. Michiels, E.; Gljbeis, R. Anal. Chem. 1984, 5 6 , 1115-1121. Dang. T. A,: Day R. J.; Hercules, D. M. Anal. Chim. Acta 1984, 758, 235-246. Saivati, L., Jr.; Hercules, D. M. Spectrosc. Lett. 1980, 13, 243-251. Michlels, E.; Gijbels. R. Spectrochim . Acta, part B 1983, 388 1347-1354. Bruynseels, F. J.; Van Grieken, R . E. Anal. Chem. 1984, 56, 871-673. Marien, J.; De Pauw, E. Anal. Chem. 1985,5 7 , 361-362. Bruynseels, F. J.; Van Grieken, R . E. Specfrochim.Acta, Part 8 1983, 388, 853-656. I
(16) Wurster, R.; Haas, U.; Wieser, P. Fresenius' Z . Anal. Chem. 1981, 308, 206-2 11. (19) Benninghoven. A.; Anders, V. The Ionization of Organic Molecules in LAMMA and SIMS, A Comparison. LAMMA Workshop, Forschungsinstitut Borstel, Sept 1-2, 1983, Leybold-Heraeus GMBH. (20) Bruynseels, F. J.; Van Grieken, R. E. I n t . J . Mass Spectrom. Ion Processes 1988,74, 161-177. (21) Musselman, I. H.; Linton, R. W.; Simons, D. S. I n Microbeam Analysis; Armstrong, J. T., Ed.; San Francisco Press: San Francisco, CA, 1985; pp 337-341. (22) Musselman, I. H.; Rickman, J. T.; Linton, R. W. I n Microbeam Analysis: Geiss, R. H., Ed.; San Francisco Press: San Francisco, CA, 1987; DD r r 361-364. --(23) Musselman, I.H.; Rickman, J. T.; Linton, R. W.; Butler, F. E., to be submitted for publication in Environ. Sci. Technol. (24) Vogt, H.; Heinen, H. J.; Meier, S.;Wechsung, R. Fresenius' Z .Anal. Chem. 1981,308, 195-200. (25) Denoyer, E.; Van Grieken, R.; Adams, F.; Natusch, D. F. S. Anal. Chem. 1982,5 4 , 26A-32A. (26) Kaufmann, R.; Wieser, P. I n Particle Characterization in Technology; Beddow, J., Ed.; CRC Press: Boca Raton, FL; 1984, pp 21-57. (27) Holden, N. E.; Martin, R. L.; Barnes, I.L. Pure Appl. Chem. 1984,56, 675-694. (28) Mauney, T.; Adams, F. Int. J. Mass Spectrom. Ion Processes 1984, 5 9 , 103-119. (29) Michieis, E.; Mauney, T.; Adams, F.; Gijbels, R. Int. J. Mass Spectrom. Ion Processes 1984,6 1 , 231-246. (30) Michiels, E.; De Wolf. M.: Gijbels, R . Scanning Electron Microsc. 1985, I I I 947-958. (31) Linton, R. W.; Musselman, I. H.; Bruynseeis, F.; Simons, D. S. I n Microbeam Analysis; Geiss, R. H., Ed.; San Francisco Press: San Francisco, CA, 1967; pp 365-368. ~
RECEIVED for review April 6, 1987. Accepted September 9, 1987. Support of this research under U.S. Environmental Protection Agency (EPA) Cooperative Agreement CR812908-01-1is gratefully acknowledged. Although the research described in this article has been funded in part by the EPA, it has not been subjected to agency review. Therefore, it does not necessarily reflect the views of the agency and no official endorsement should be inferred. Certain commercial equipment, instruments, or materials are identified in this paper. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it or equipment are necessarily the best imply that the available for the purpose. Portions of this work were presented at the 1987 Pittsburgh Conference and Exposition on Analytical Chemistry and Spectroscopy, City,
NJ.
