Anal. Chem. 1904, 56.871-873
power. Thus, structural information about the compound can be studied in detail. At threshold energy, the LMS spectrum generally contains fewer fragment ions than the SIMS spectrum. In those diquaternary salts where charge repulsion between the two positive charges is large, dissociation to form two separate monocations is favorable in both SIMS (7) and LMS. The preference of charge separation in these compounds is responsible for the absence of M2+in SIMS as well as for (M -H )' and (M - CHJ+ in LMS. Finally, a process analogous to halide substitution in the cation in LMS has been reported in SIMS of diquaternary salts (6).
ACKNOWLEDGMENT The authors thank T. M. Ryan of Purdue University for providing some diquaternary ammonium salts. Registry No. 1, 85-00-7; 2, 89043-66-3; 3, 22383-16-0; 4, 2315-97-1;5,3868-84-6;6,89043-67-4;7,23970-73-2;8,6952-20-1; 9, 14870-72-5; 10, 5369-38-0; 11, 23484-62-0; 12, 23045-52-5; 13, 56-97-3; 14, 52-62-0. LITERATURE CITED Balasanmugam, K.; Dang, T. A.; Day, R. J.; Hercules, D. M. Anal. Chem. 1981, 5 3 , 2296-2298. Hercules, D. M.; Day, R. J.; Balasanmugam, K.; Dang, T. A,; Ll, C. P. Anal. Chem. W82, 54, 280A-305A.
071
(3) Heinen, H. J.; Meier, S.; Vogt, H.; Wechsung, R. Adv. Mass Spectrom. 1980, 84, 942-953. (4) Katakuse, I.; Matsuo, T.; Wollnik, H.; Matsuda, H. Org. Mass Spectrom. 1979, 14, 457-458. (5) Eicke, A.; Slchtermann, W.; Benninghoven, A. Org. Mass Spectrom. 1980, 15, 289-294. (6) Day, R. J.; Unger, S. E.;Cooks, R. G. Anal. Chem. 1980, 52, 557A572A. (7) Ryan, T. M.; Day, R. J.; Cooks, R. G. Anal. Chem. 1980, 5 2 , 2054-2057. (8) Barber, M.; Bordoll, R. S.; Sedgwick, R. D.; Tyler, A. N. J . Chem. SOC.,Chem. Commun. 1981, 325-327. (9) Barber, M.; Bordoli, R. S.; Elliott, G. J.; Sedgwick, R. D.; and Tyler, A. N. Anal. Chem. 1982, 54, 645A-657A. (IO) McNeal, C. J.; Macfarlane, R. D. J . Am. Chem. SOC. 1981, 103, 1609-1 610. (11) Krueger, F. R.; Wlen, K. Adv. Mass Spectrom. 1978, 7 , 1429-1432. (12) McEwen, C. N.; Layton, S. F. Presented at the 24th Annual Meetlng of the American Society for Mass Spectrometry, San Diego, CA, 1976. (13) McEwen, C. N.; Layton, S. F.; Taylor, S. K. Anal. Chem. 1977, 49, 922-926. (14) Heller, D. N.; Yergey, J.; Cotter, R. J. Anal. Chem. 1983, 55, 13 10-131 3. (15) Barofsky, D. F.; Giessmann, U. Int. J . Mass Spectrom. Ion Phys. 1983, 46, 359-362. (16) Coffey, S . "Rodd's Chemistry of Carbon Compounds"; Elsevler: Amsterdam, 1965; Chapter 6. (17) Unger, S. E.; Ryan, T. M.; Cooks, R. G. Anal. Chlm. Acta 1980, 118, 169-174.
RECEIVED for review August 18, 1983. Resubmitted January 23, 1984. Accepted January 23, 1984. This work was supported by the National Science Foundation under Grant CHE81-08495.
Laser Microprobe Mass Spectrometric Identification of Sulfur Species in Single Micrometer-Size Particles Frank J. Bruynseels and Ren6 E. Van Grieken*
Department of Chemistry, University of Antwerp (U.I.A.), B-2610 Wilrijk, Belgium
Laser mlcroprobe mass analysis spectra of mlcrometrlc partlcles of Na,SO,, Na,SO,, and Na,S,O, have been studled. The negative Ion mode spectra are domlnated by SO,- Ions; thelr pattern with respect to the fragment valence can be modeled but It Is ldentlcal for the three compounds and hence does not allow Identllcation. Thls Is also true for the Na20n+ clusters In the positive Ion mode spectra. The ratio of the strong Na3S03+to Na,SO,+ mass peaks, which Is not crltlcaliy affected by the laser energy, does offer the posslblllty for unamblguous ldentlflcatlon that can be confirmed via the Na,S+ and Na2S02+Intensltles.
The chemical characterization of individual particles is important in a variety of fields, e.g., aerosol chemistry and geochemical research on sediments and suspensions. Most of the conventional analytical techniques provide information about the bulk elemental composition of a particulate sample but not about the distribution of the elements over and within the particles nor about the stoichimetry of the compounds present. This information is fundamental to assess the origin of a particle and the physicochemical transformations it has gone through. Laser microprobe mass analysis (LAMMA) is one of the novel techniques that allow analysis of single particles or microscopic samples with a favorable sensitivity (1, 2). In
LAMMA, a pulsed focused laser beam is used for evaporating single particles in the micrometric size range and the laser generated ions are separated according to their mass in a time-of-flight mass spectrometer. Quantitative or semiquantitative analysis of individual particles is still limited, because of the present scarce knowledge about the laser induced ion formation mechanism and the influence of the particle size, the laser power density, the matrix effects, and the instrumental parameter settings. However research about these topics is currently in progress (3-5). Besides, LAMMA can be used for structural analysis, since in addition to the atomic mass peaks, many cluster ions appear in the mass spectrum. Until now most attention has been paid to the characterization of organic substances (6),and only a few results are known for inorganic compounds (7, 8). Qualitatively LAMMA spectra are very similar to static SIMS spectra, but the superior lateral resolution of 0.5-1.0 pm and the high speed of LAMMA makes it potentially more interesting for single particle characterization. The major drawback of individual particle analysis, in general, is that it is difficult to achieve statistically meaningful results and, of LAMMA in particular, that the technique is destructive. In the present study, we examine the positive and negative LAMMA spectra of particles of micrometer size of sodium sulfoxy salts with a different stoichiometric sulfur-to-oxygen ratio, namely, sulfate, sulfite, and thiosulfate, with the aim of checking whether information about the sulfur speciation
0003-2700/84/0356-0871$01.50/00 1984 American Chemical Soclety
872
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
Table I. Negative Ion Intensitiesa m/e 48
64
80
96
cluster ion IR(NalS0,) SOSO; SO; SO,-
IR(Na,SO,) IR(Na,S,O,)
0.32 i: 0 . 0 3 b 0.23 i 0.02 1.00 1.00 0.79 i: 0.06 0.67 c 0.05 0.21 e 0.05 0.06 * 0.02
0.49 * 0.04
Table 11. Parameters of the Gaussian Distributions Fitting the Relative Intensities of the SO,- ( n = 1 to 4 ) Cluster Ions as a Function of the Fragment Valence (Laser Energy = 2 pJ) compound Na,SO, Na,SO, Na,S,O, CaSO,
1.00
0.80 * 0.07 0.20 i. 0.05
a Relative intensities (IR)of the SO, ( n = 1 to 4 ) cluster ions for Na,SO,, Na,SO, , and Na,S,O, , normalized to the SO,' cluster intensity (laser energy = 2 Standard deviation on the average. qJ).
can be derived from the LAMMA cluster ion intensity distribution. Sodium is by far the predominant cation in marine aerosols, which are globally most abundant in the atmosphere. Sulfate compounds and sulfur-containing particles play an essential role in atmospheric chemistry and air pollution, and the mechanism of sulfate compound transformation in the atmosphere is currently an area of intense scientific debate.
EXPERIMENTAL SECTION Laser microprobe mass spectrometry was performed with the LAMMA 500 instrument (Leybold-Heraeus, Koln, F.R.G.). A detailed description and various applications have been given in the literature ( I , 2). The pulsed laser light (A = 265; T = 15 ns) of a frequency quadrupled Q-switched neodymium-YAG laser (power density of 101o-lO1l W/cm2) is focused onto the sample through an optical microscope (magnifications of lOOX to 1250X) with the aid of the red spot of a continuous He-Ne laser beam, that follows the same optical pathway as the high power laser. Depending on the UV intensity which can be attenuated over a range of 100% to 1% of full power by means of a 25-step optical filter sequence, the sample is more or less damaged or even perforated (lateral resolution up to 1 pm). The laser-generated ions are accelerated into a time-of-flightmass spectrometer with ion refledor. Either a positive or negative ion mode can be chosen. The ions are detected in an open Cu-Be secondary electron multiplier, and the signals are stored in a 100-MHz transient recorder and fed into a Digital Minc-11 computer for calibration, integration, and further examination. The samples of analytical grade salts were ground with an agate mortar and pestle to particles of micrometer size, that were transferred onto a very thin Formvar foil (-0.1 pm), supported by a copper electronmicroscopic grid. The measurements were performed on particles of about 2 Fm diameter. RESULTS AND DISCUSSION Negative Ion Mode. The major negative ions detected in the LAMMA mass spectrum of Na2S04,NazS03,and Na2Sz03 are l60-,17(OH)-,32S-(and 320z-), &(SO)-,64(S02)-,@'(SO3)-, %(SO,), lo3(NaS03)-,and 11g(NaS04)-.The relative intensities of the major cluster ions are listed in Table I. Since all of these ions appear in the spectra of each of the sulfoxy compounds, their mere presence is no direct indication of the sulfur speciation. The occurrence of SO,- ions with different S oxidation states does not neccesarily reflect the presence of different sulfoxy salts in the sample, in contrast to such suggestions in the literature ( 9 , l O ) . The intensity ratios do not differ very drastically for the three salts. Yet the low S0,-/S02- ratio seems to be indicative for Na2S03while the SO-/SOz- ratio is distinctly higher for Na,S203. In SIMS, the cluster ion intensity distribution can be representative for the sulfur speciation, as was demonstrated by Ganjei et al. (11). Using Plog's empirical model (I2),they fitted the distribution of the relative intensities I- of the SO,(n = 1to 4) cluster ions, as a function of the fragment valence K = q + 2n (with q being the charge of the ion and n being the number of oxygen atoms in the cluster ion) according to the formula
cuso,
a
G-
3.7 f 3.6 c 3.5 i 2.3 f 4.8 i
0.4'
0.3 0.5 0.2 1.0
Y-
X2
1.8 i: 0.1 1 . 5 * 0.1 2.0 t 0.2 1 . 6 c 0.1 3.0 i 0.6
0.59 0.11 0.31 0.30 0.03
Standard deviation on the average.
with G- being the K value at the maximum intensity of the distribution I-- and ( Y - ) ~being the variance of the intensities. It has been demonstrated (8, 13) that Plog's model is also applicable to LAMMA spectra. The parameters describing the fitting of the present experimental data are listed in Table 11. The relatively small range of the G-and y-values for the SO; distributions of the three compounds makes a reliable identification by these parameters not straightforward because the differences are within or close to the experimental errors. It appeared that the SO,- distributions depend more on the salt cation than on the sulfur speciation, e.g., G- = 2.3 for CaS04 (€0,G- = 3.7 for Na2S04,and G- = 4.8 for CuSO,. In the case of (NH4)2S04,the negative mode LAMMA spectrum is completely changed and the predominant peak becomes "(HS04)-, as has already been reported (9,lO);this ion has a formal valence, corresponding to the oxidation state of sulfur in the original salt, which provides H atoms abundantly. Positive Ion Mode. The positive spectra of the sulfur salts show cluster ions with the following composition: Naz+,the metal oxide clusters NaO+, NazO,+ (n = 1 to 3) and Na30,+ (n = 1 , 2 ) , and the sulfur containing ions Na,SO,+ ( n = 0 to 3) and Na3S0,+ (n = 0 to 4). In some cases the exact cluster ion identification can be dubious. Indeed, within the mass resolution of the LAMMA instrument, the m l e ratio of two oxygen atoms and one sulfur atom is identical. The intensity of the corresponding 34Scontaining cluster ion cannot always be indicative because of its low abundance. Measurement on Na2CO3showed that the dominant clusters in the spectrum are Naz+,Na20,+ (n = 1,2), Na30,+ (n = 1,2), and Na3C03+, hence, it is suggested that in the case of the sulfoxy salts, sodium oxide cluster ions with m l e > l o 1 are less plausible than sulfur-containing clusters. Typical LAMMA spectra for the three sulfoxy compounds are given in Figure 1. The relative cluster ion intensities, averages of six measurements, are represented in Figure 2. The given uncertainty was calculated as the standard deviation on the average. The most intense mass peak in the Na2S203spectrum is attributed to Na3S+;in view of the low intensity of this mass peak in the NaZSO4and Na2S03spectra, it is suggested that the sulfur atoms in Na3S+are predominantly coming from the low valency sulfur atoms in Na2S203. The low mass cluster ions of the type Na20,+ (n = 0 to 3) are fitted reasonably well by a Gaussian distribution, with G+ = 1.28 0.04 and y+ = 0.96 f 0.06 (for Na2S203the mass peak a t m / e 94 had to be disregarded because significant interference of the Na2SO+ cluster ion is obvious). The Na20,+ ions are not representative for a given sodium sulfur oxy salt (e.g., for Na2C03similar values are found: Gf = 1.25 and '7 = 0.93) and cannot be used for identification. Identical results were obtained for K salts. While the distributions of the sodium oxide ions are equivalent, the intensities of cluster ions as Io1(Na3S)+, 110(Na2S02)+,149(Na3S03)+,and 165(Na3S04)+in a given
*
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
Table 111. Influence of the Laser Energy on the Ratio, R , of the Na,SO,+ to Na,SO,+ Cluster Ion Intensity for Na,SO,, Na,SO,, and Na,S,O,
No@'
NO'
873
laser
I
f
0.7
No'
I
1.0
NbO'
2.0 2.3 3.9 a
0.28 i 0.23 + 0.28 i 0.24 i
0.04a 0.03 0.03 0.02
0.10
0.04
i
0.84 i 0.03 0.65 i 0.05
0.76 i 0.06 0.74 i 0.06
1.38 i 0.07 1.10 i 0.05
1.05 i 0.03 1.03 i 0.03
Standard deviation on the average.
can even be provided by the intensity of the cluster ions lo1(Na3S)+and 110(Na2S02)+ that show an increasing intensity with increasing S to 0 ratio in the compound.
Figure 1. Typical LAMMA spectra, in the positive ion mode, of (a) Na2S0,, (b) Na,S03, and (c) Na,S,O,. For each mass peak, the most plausible ion is indicated but its exact identification can sometimes be dubious (one sulfur atom and two oxygen atoms have nearly equal
mass).
CONCLUSION Laser microprobe mass spectrometry allows fast determinations of alkali sulfoxy anions on a microscopic size level. The intensity ratio of the Na3S03+and Na3S04+ions, which is not critically affected by the laser energy, is most indicative for the type of salt. In marine and coastal air, where high sodium and sulfur concentrations are found in the air particulate, LAMMA provides a useful method for determing the sulfur anions in single aerosol particles. Registry No. Na2S04,7757-82-6;Na2S03,7757-83-7;Na2S203, 7772-98-7. LITERATURE CITED (1) Sessions of LAMMA symposium held in Dusseldorf, West Gemany, Oct 6-10, 1980 (Hiilenkamp, F., Kaufmann, R., organizers) Z. Anal. Chem. 1981, 308, 193-320. (2) Denoyer, E.; Van Grieken, R.; Adams F.; Natusch, D. F. S. Anal. Chem. 1982, 54, 26A-41A. (3) Surkyn, P.; Adams, F., J. Trace Mlcroprobe Tech. 1982, 1 , 79-114. (4) Verbueken A. H.; Van Grieken, R. E.; Paulus, G. J.; De Bruijn, W. C., submitted to Anal. Chem . (5) Michiels, E.; Gijbels, R. E. Spectrochim. Acta, Part 8 1983, 388, 1347-1354. (6) Hercules D. M.; Day, R. J.; Balasanmugan, K.; Dang, T. D.; Li, C. P. Anal. Chem. 1982, 54, 280A-305A. (7) Salvati, L., Jr.; Hercules, D. M.; Vogt, H. Spectrosc. Lett. 1980, 13, 243-25 - - - _1
No;
N:,O* No,OH'
No,O;
Nolo'
NozSO' No7S03 NazSOf ha$' No,SO' No,SO;
No3SO;
NolSOt
Figure 2. Cluster ion intensities as a function of m l e for Na2S04, Na,SO, and Na2S2O3. The intensities are the average of 6-fold measurements; they have been normalized to the mle 165 mass peak; the error bars represent the standard deviation of the mean.
spectrum appear to be very representative of the anion stoichiometry and to yield an unambiguous identification criterion. Measurement of the 149(Na3S03)+ to lGS(Na3S04)+ ratio as a function of the laser excitation energy (Table 111) showed that the laser energy is not a critical parameter, whose variability could confuse the identification. Also in this higher mass range the interference of other cluster ions is less probable than for masses below m / e 100 where fragment ions abundantly occur. Further confirmation of the salt identity
(8) Bruynseeis, F. J.; Van Grieken, R. E. Spectrochim. Acta, Part8 1983, 388,853-858. (9) Kaufmann, R.; Wleser, P.; Wurster, R. Scanning Electron Mlcrosc. 1980. II. 607-622. (IO) Seiier, H.f Haas, ;:V Rentschler, I.; Schreiber, H.; Wleser, P.; Wurster, R. Optlk (Stuffgart) 1981, 58. 145-157. (1 1) Ganjei, J. D.; Colton, R. J.; Murday, J. S. Int. J. M s s . Spechom. Ion Phvs. 1981. 37. 49-85. (12) PI&. C.; Wiedman, L.: Bennlnghoven, A., Surf. Sci. 1977, 67, 585-580. (13) Michiels, E.; Celis, A.; Gijbels, R. "Microbeam-Analysis 1982"; Heirnlch, K. F. J., Ed.; San Francisco Press: San Francisco, CA, 1982; pp 3~3-3m.
Received for review February 23, 1983. Resubmitted December 19, 1983. Accepted January 24, 1984. F.J.B. is indebted to the Instituut ter Aanmoediging van het Wetenschappelijk Onderzoek in Nijverheid en Landbouw (IWONL) for financial support. This research was funded by the Interministrial Commission for Science Policy, Belgium, through Research Grant 80-85/10.
