1296
Anal. Chem. 1987, 5 9 , 1296-1301
Determination of Atmospheric Carbonyl Sulfide by Isotope Dilution Gas Chrornatography/Mass Spectrometry Ella E. Lewin, Rebecca L. Taggart, Marija Lalevic, a n d Alan R. Bandy* Department of Chemistry, Drexel University, Philadelphia, Pennsylvania 19104
A gas chromatography/mass spectrometry (GC/MS) method for determining atmospherlc carbonyl sulfide (OCS) with a preclslon better than 2% is reported. Hlgh precislon and insendthrltyto sample loss and changes In detector response were achieved by using isotoplcally labeled OCS as an internal standard. Tenax, Molecular Sieve 5A, Carbodeve B, and Carbosieve S were evaluated for collectlng atmospheric OCS. Molecular Sieve SA provided the best trapping and recovery efflciencles.
The objective of this study was to develop a highly selective and precise method for determining atmospheric carbonyl sulfide (OCS). Carbonyl sulfide is the most abundant gaseous atmospheric sulfur species ( I ) . It has an atmospheric residence time of at least 1year (1-3);therefore, its global distribution is very uniform (&IO%). The long tropospheric residence time of OCS, due in part to its low water solubility, allows most of it to escape to the stratosphere where it is converted to SO2 and subsequently to H2S04. The H2S04vapor quickly combines with available water to produce small particles that scatter the sun's radiation and absorb the Earth's radiation and thereby can have an important influence on the Earth's radiation budget and climate (2, 4 ) . Much of our knowledge of the atmospheric chemistry of OCS is obtained from measured temporal and spatial fluctuations of OCS. Since these fluctuations are &lo% or less, a measurement precision of 1%or better is desirable. The gas chromatographic method used by Maroulis et al. ( 1 ) and Torres et al. (5) for atmospheric OCS determinations has sufficient sensitivity but not the required precision. In this work high precision has been achieved by using isotopically labeled OCS, 1s013C32Sand 16012C34S, as internal standards. A similar method for the determination of CS2 has been reported by Bandy et al. (6). The work described below shows that cryogenic preconcentration is a very effective means of increasing OCS concentrations to measurable levels. Unfortunately cryogenic preconcentrated samples usually require immediate analysis which in turn requires an on-site GC/MS. However, there are many applications, such as sample collections using small aircraft platforms, for which an on-site GC/MS is not feasible. Needed is another sampling method that does not require immediate sample analysis and thus direct access to the GC/MS. One approach would be to collect samples on suitable solid adsorbents and return them to the laboratory for analysis. Several reports of successful preconcentration of sulfur gases on solid adsorbents have been published. Black et al. (7)used Molecular Sieve 5A for the collection and determination of H2S and SOz. Steudler and Kijowski (8)tested an adsorber based on molecular sieve 5A and Tenax for collecting OCS, HzS, CS,, dimethyl sulfide, dimethyl disulfide, and CH3SH. Shalaby (9) has reported some preliminary work on the determination of atmospheric OCS by GC/MS using both cryogenic enrichment and preconcentration on Carbosieve B. 0003-2700/87/0359-1296$01.50/0
Based on this previous work, Molecular Sieve 5A, and Carbosieve B adsorbents were chosen for study. Consultations with various manufacturers and our previous experience (6) suggested that we should also evaluate Carbosieve S. Carbosieve S is spherical in shape and more dense than the granulated form of Carbosieve B. According to the manufacturer, type S should be the best for the more volatile sulfur gases, such as OCS. EXPERIMENTAL SECTION A Finnigan Model 4023 GC/MS (Finnigan Instruments, Sunnyvale, CA) was used for these studies. The sample was ionized with 30-eV electronsat a source pressure of about 0.04 torr. Higher energies degraded the signal-to-noise ratio while lower energies resulted in very short filament lifetimes. Carbonyl sulfide was separated from interfering compounds by use of a Teflon column, 4 m long, 0.125 in. (3.18 mm) o.d., and approximately 0.082 in. (2.08 mm) i.d., packed with 3% Carbowax 20M and 1%H3P04 on Carbopack B (Supelco, Inc., Bellefonte, PA). The attributes of this column for atmospheric OCS determinations have been discussed by Maroulis et al. (I). The column was conditioned overnight at 100 "C while being purged with helium. The column was operated at 60 "C and a helium flow rate of 20 mL/min. The retention time of OCS was about 1 min. The sample was cryogenically preconcentrated and loaded onto the column by using the apparatus shown in Figures 1 and 2. The OCS was preconcentrated in an unpacked Teflon trap constructed from FEP Teflon tubing, 1 m long, 0.125 in. (3.18 mm) o.d., and 0.082 in. (2.08 mm) i.d. (5, 6). The GC/MS operating conditions and calibration procedures were similar to those used by Bandy et al. (6). The instrument was carefully tuned before each series of measurements. The ion multiplier, quadrupole offset, and source lens voltages were adjusted to obtain the best compromise between sensitivityand peak shape (resolution). Changes in sensitivity were evaluated periodically by direct injection of a standard OCS sample (Scott Specialty Gases, Plumbsteadville, PA). The data acquisition was carried out in the multiple ion detection mode of the INCOS data system (Finnigan Instruments, Inc., Sunnyvale, CA). The parent peak of OCS was monitored by using mass windows of 59.518-60.518 au for 16012C32S, 60.518-61.518 au for 16013C32S, and 61.51842.518 au for 16012C"S. Only two isotopomers were monitored in any given analysis. Each window was monitored for 0.2 s. The minimum fragment width and minimum peak area parameters of the INCOS data system were set to five. Chromatograms of 16012C32C and 16012C34S are shown in Figure 3. Isotopically labeled OCS was purchased from U.S. Services (Summitt, NJ). Standard mixtures of both labeled and unlabeled OCS were prepared by Scott Specialty Gases (Plumbsteadville, PA). The standard having the terrestial isotopic abundance, OCS-60, had a concentration of 689 ppbv. The standard enriched in I3C, OCS-61, had an overall OCS concentration of 169 ppbv, whereas the standard enriched in %S, OCS-62, had an overall OCS concentration of 106 ppbv. Mole fractions of OCS isotopomers in ambient air were calculated from known terrestrial abundances of 0, C, and S (10). The percentage contents of the isotopomers in the standards were calculated from isotopic abundances provided by the manufacturer. The total OCS concentration in the standard mixtures was determined by comparison to test atmospheres containing OCS prepared by use of permeation tubes (Metronics, Santa Clara, 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 9, MAY 1, 1987
1297
n
ROOF
M A S S FLOW
c
VACUUM PUMP P1
M A S S FLOW METER MFM2
VACUUM
M A S S FLOW METER
'
-
COLUMN HELIUM
VALVE
-
SYSTEM
MASS
C E E Z Z T A R Y INTERFACE
SPECTROMETER
r CONTROLLER
T R A P PURGE HELIUM
Figure 1. Block diagram of the calibration and collection system for in situ measurements of OCS in air. OVEN
a
b
t
CALIBRATION MAWIFOLD
MA61 FLOW CONTlOLLEl
MASS
MASS METER FLOW
SPECTROMETER INTERFACE
LIOUID I R O O N
Figure 2. Schematic diagram of the valving system used for collection (a) and desorption (b) of ambient air samples.
CA). The permeation tube calibration devices were calibrated by monitoring their weight loss for at least 3 months (after stabilization). The block diagram of the apparatus used for the preparation of test atmospheres is shown in Figure 1. All flowmeters were calibrated against either soap bubble flowmeters or wet gas meters. The standard mixtures were introduced into
the mixing chamber at rates determined by mass flow controllers MFCl and MF'C2 where they were diluted with a zero air stream whose flow rate was monitored by flowmeter MFM1. The resulting gas mixture was then added t o the main manifold mar the ambient air inlet just upstream of the particle filter. The flow rate in the main manifold was measured with the mass flow meter
1298
* ANALYTICAL CKMISTRY, VOL. 59, NO. 9, MAY 1. 1987
K
-1
!I i"
Figure 4.
a
ocs.
U"1,"1,*"1
Flgura 3. mlz 60 and m l z 62 chrwnatogams ot ambient ah wim added OCS62.
Table 1. Background Loading in Picomoles per Centimeters of Packing OCS-60
Tenax Carbosieve B Carbosieve S Molecular Sieve
0.15 0.2 0.5 0.5
OCS-61
0.2 0.15 0.0 0.22
MFM2. The test atmosphere produced was sampled a t a rate of about 400-1000 mL/min. For test atmospheres prepared in zero air, the flow rate of zero air/standard gas mixture (F5 = F 1 + F2 + F3) was larger than the flow rate in the sample manifold 0%).Under these conditions zero air flowed out of the end of the manifold thus excluding ambient air from the manifold and the sample. For calibrations in zero air, the OCS-61 or OCS62 concentration was maintained at about 500 pptrv, while the OCS-60 concentration was varied over the range 300-830 pptrv by varying F2. During ambient air measurements of OCS, F5 was maintained a t a much lower flow rate than that of the main manifold, F6, to minimize the reduction of the 'W'CmS signal hy dilution. For these measurements, only isotopically labeled OCS was added to the sample air. A diagram of the valving system used for sample collection is shown in Figure
Adsorbent trap used for collection of ambient samples of
silanized glasa wool plugs that had been treated with H,POI. The tubes were sealed with a nylon nut, nylon fermle, and brags caps. The pressure drop across the packed trap was about 50% at a sampling flow rate of 1 L/min. A diagram of the valving system used for collection and desorption of ambient air samples is shown in Figure 2. A photograph of the adsomtion trap used in this work is shown in Figure 4. Prior to use, the traps were conditioned for a t least 1 h a t 300 OC while being continuously purged with helium. The traps were allowed to cool without intempting the helium flow. When cool, they were sealed and stored at room temperature. The traps were checked for background by desorbing them in exactly the same manner (temperature, time, helium flow, etc.) as was used for atmosphericsampling. The first desorption always yielded higher values than those for subsequent desorptions. The degree of contamination did not depend on storage time: thus contamination during storage apparently was negligible. Therefore, we concluded that the desorbed OCS probably arose from contamination introduced during cooling and sealing of the traps. Table I contains measured OCS background loadings for the adsorhents tested. Carbonyl sulfide was desorbed by heating the trap in a small oven while purging the trap with helium at a flow rate of 200 mL/min. The OCS in the purge gas was cryogenically trapped and later volatilized and analyzed. Above 200 "C OCS recovery was complete after 3 min and depended little on the trap purge rate if the purge rate was greater than about 200 mL/min. As a precaution each trap was desorbed for an additional 3 min to verify that no residual OCS remained in the trap after the first desorption. RESULTS AND DISCUSSION
2.
The air inlet to the eample manifold was located on the roof of a four-story building on the Drexel University campus, near downtown Philadelphia. Atmospheric partides were removed at the manifold inlet system with an 0.5 wm pore Teflon filter. The standard mixture, enriched in I3C or =S, was added to the main manifold between the manifold inlet and the particle filter, so that both the analyte and the internal standard were affected identically by the system. The sample manifold was constructed from 0.375 in (9.53 mm) 0.d. FEP Teflon tubing and was approximately 6 m long. The sample was cryogenically preconcentrated in a FEP Teflon trap cooled with liquid argon ( I ) . Samples were volatilized with warm water prior to injection onto the gas chromatographic column and were eluted into the MS with a 20 mL/min flow of high-purity helium (6). Adsorbent Traps. Adsorbent traps were constructed from F'yrex tubes, 20 cm long and 4 mm i.d. The center section of each tube was filled with the adsorbent material. The packing length was 4 cm for Tenax and Carbosieve B and 2 em for Carhosieve Sand Molecular Sieve 5A. The packing was held in place with
The algorithm for converting GC/MS data to concentration data using an isotopically labeled standard has been derived by Bandy et al. (6) K2 - K,R c. = K& - K,'* Here C. and C. are the total OCS concentrations and Haand H. are the peak heights for ambient air and isotopically labeled standard, respectively, and R is the ratio of the peak heights, R = H./H.. When the separation of the ions of the isotopomers of the analyte is complete, the constants in eq 1are determined solely hy the isotopic ahundances of oxygen, carbon, and sulfur in the standard and ambient air (6). Under these conditions K , and K3 are equal to the fractions of OCS, F,, having masses 60and 61 (or 62) in ambient air, respectively, and K2and Kdare equal to the respective fractions, C,,of these isotopomers in the standard mixture. By use of the isotopic
Table 11. Sulfur Isotope Abundances for Ambient Air and Sulfur Gas Standards Enriched in '"c or
OCS-62 standard. OCS-61 standard"
ambient ai@
12 au
13 au
32 au
98.9 8.9 98.9
1.1 91.13 1.1
3.52 95.01 95.01
'Manufacturersanalysia. 'Reference 8.
abundance, 70 33 au 2.15 0.16 0.16
34 au
35 au
36 au
93.1 4.22 4.22
0.00
1.18
0.00 0.00
0.02 0.02
ANALYTICAL CHEMISTRY, VOL. 59, NO. 9, MAY 1, 1987
1299
Table 111. Fraction of OCS Molecules in Ambient Air Having Mass mi
60 au
mole fraction 62 au 63 au
61 au
64 au
65 au
isotopomer 34 13 18 34 12 18 34 13 17 34 12 17 34 13 16 34 12 16 32 13 18 32 12 18 32 13 17 32 12 17 32 13 16 32 12 16
0.9378
Fi
0.9378
0.0000 0.001 0.0000
0.0000 0.0005 0.0416 0.0000
0.0019 0.0000 0.0066 0.0104 0.0170
0.0435
61 au
mole fraction 62 au 63 au
and
Gi
0.0116 0.0102 0.9194 0.0002 0.0210 0.0003 0.0213
Enriched in 34 13 16 34 12 16 33 13 16 33 12 16 32 13 16 32 12 16 Gi
0.9196
0.0102
0.0116
I3C
0.0384 0.0373 0.0069 0.0007 0.8641 0.0841 0.0841
0.8648
0.0442
0.0384
abundances given in Table 11,Kl through K4 were calculated and included in Tables I11 and IV. Substitution of these calculated values into eq 1yields a theoretical data reduction formula. When the OCS-61 standard is used as an internal standard, the theoretical data reduction formula is
When OCS-62 is used as an internal standard, the theoretical data reduction formula is 0.0347 - 0.9196& = (3) 0.0435R62 - 0.9378“
c,
Using the cryogenic preconcentration technique, we were able to verify the accuracy of these theoretical data reduction algorithms. In doing this it is convenient to rewrite eq 2 and 3 0.9378Cr 0.0841 (4) R61 = 0.0170C1 + 0.8648
+
and
+ 0.0347 R62 = O.O435C, + 0.9196 0.9378C1
Here C, = C,/C,.
(5)
For R , < 1, which is the range of interest
= 1.020cr
+ 0.038
(7) The accuracy of these algorithms was determined by conducting an experiment in which the isotopic standard concentration, C,, was held constant at about 500 pptrv (parts per trillion by volume) while the concentration of the OCS-60 standard, C,, was varied over a concentration range of 300-800 pptrv, which includes the range of OCS concentrations found in the atmosphere (450-550 pptrv (I)). A linear regression analysis of the ratio of the accumulated ion counts for the 60 and 61 au channels as a function of the ratio of the concentrations of the OCS-60 and OCS-61 standards yielded a slope of 0.936 (hO.008)and an intercept of 0.207 (h0.012). The correlation coefficient was 0.997. These results are based on five calibration runs made on different days. Each point on the curve was determined at least three times for each run; the total number of measurements was 68. The high value of the correlation coefficient and the small standard deviations of the slope and intercept demonstrate the high precision of the technique; however, both the slope and the intercept of the experimental calibration curve are substantially different from that of the theoretical calibration curve. The major sources of these differences appear to result from errors in the isotopic abundances for the 13C-labeledOCS supplied by the manufacturer and “cross-channeling”caused by inadequate resolution of the mass spectrometer (6). This experimental calibration curve, however, produced atmospheric OCS data in agreement with that obtained with the OCS-62 standard, for which, as described in detail below, much better agreement between the theoretical and experimental calibration curves was obtained. A linear regression analysis of the ratio of the accumulated ion counts for the 60 and 62 au channels as a function of the ratio of the concentrations of the OCS-60 and OCS-62 standards yielded a slope of 0.990 (k0.006) and an intercept of 0.029 (h0.004). The correlation coefficient was 0.998. The intercept is within measurement precision of the background concentration of OCS in the zero air mixture used to prepare the test atmospheres. The slope of this experimental curve is statistically the same as the slope of the theoretical Calibration curve. Therefore, we conclude that the calibration curve for ambient air is given by eq 7 within experimental error. A propagation of errors analysis indicates that the overall precision of this GC/MS determination of atmospheric OCS is about 2%. R62
isotopomer
0.0347 0.0347
0.0000
+
64au
Enriched in 34S 36 12 16 34 13 16 34 12 16 33 13 16 33 12 16 32 13 16 32 12 16
0.0001
in atmospheric studies, the term 0.0170Cr in eq 4 and the term O.0435Cr in eq 5 are small relative to other terms; hence eq 4 and 5 can be simplified to Re1 = 1.084Cr 0.097 (6)
Table IV. Fraction of OCS Molecules in Standard Gas Mixture Having Mass mi 60au
0.0005
1300
ANALYTICAL CHEMISTRY, VOL. 59, NO. 9, MAY 1, 1987
Table V. Recovery of OCS Collected on Carbosieve S and Molecular Sieve 5A as a Function of Desorption Temperature" material
temp, "C
amt collected, pmol
amt recovered, pmol
70recovery
mean, ?&
200
68.0 72.1 36.6 38.0
56.4 62.0 30.7 33.4
83 86 84 88
85 f 2
215
12.5 36.6 37.0 33.9 33.6 33.6 33.4
12.9 37.3 33.3 31.9 36.6 46.4 54.1
103 102 90 94 1096 138* 163*
97 f 6
235
36.9 38.4 38.4 61.9
36.9 33.0 36.1 62.5
100 86 94 101
95 f 7
275
39.6 40.7 40.8
20.6 19.5 17.9
52 48 44
48 f 4
200
34.7 36.4 9.1 9.6
41.6 41.5 11.0 12.6
120 114 121 131
121 f 7
215
13.2 10.5 11.9 14.3
10.4 10.7 11.3 12.4
79 102 95 87
91 f 10
230
17.8 17.9 15.1 18.5 29.5 35.1 32.7
15.7 16.6 21.4 17.2 34.2 31.2 70.0
88 93 142 93 116 89 2146
108 f 30
Carbosieve S
Molecular Sieve 5A
270
99 f 15
"The test was carried out for OCS-61 added to ambient air. bConsecutivecollections on the same trap without regenerating; not included in computation of mean.
Evaluation of Adsorbent Traps. Trap capacity and recovery efficiencies were evaluated. The trapping process should be viewed as frontal elution chromatography. Therefore, breakthrough volumes were used as a measure of capacity. The breakthrough volume was defiied as the volume of air sampled when 1% of the analyte loaded on the trap had penetrated the trap. These tests were conducted with test atmospheres prepared by adding a known amount of OCS-61 to ambient air. The trapping flow rate was about 500 mL/min. The trap was operated a t room temperature. The concentration of the standard in the mixture entering the trap was about 500 pptrv; however, breakthrough was independent of the OCS-61 concentration for the range of concentrations used (100-200 pptrv). Analyte, which had passed through the trap, was cryotrapped in increments of 1 to 3 L and analyzed for OCS content. The results of these investigations for the four adsorbents used are given in Figure 5. Here breakthrough is plotted as a function of the volume of air having passed through the trap. The shape of these breakthrough curves and their insensitivity to analyte concentration supports the validity of the frontal elution model for the trapping process. Molecular Sieve 5A and Carbosieve S had breakthrough volumes greater than 2 and 3 L/cm of adsorbent, respectively. The breakthrough volume for Carbosieve B was less than 2 L/cm of packing while for Tenax the breakthrough occurred almost immediately. On the basis of these findings, further investigations were concentrated on Molecular Sieve 5A and Carbosieve S properties. For recovery evaluation, zero air with added OCS-61 was collected on Carbosieve S and Molecular Sieve 5A traps.
Immediately after collection, the traps were thermally desorbed and the amount of OCS-61 recovered was determined by GC/MS. The concentration of OCS-61 in the mixture entering the tube was about 500 pptrv. The flow rate through the trap was about 500 mL/min. The amount recovered was compared with the amount of OCS-61 collected in a similar experiment, in which the air was collected with an unpacked trap. Desorption of the OCS was carried out a t different temperatures, over the range 200-280 "C, to establish the optimum desorption temperatures. Prior to use, the background signal for each trap was determined. Only traps that showed a constant and acceptably low background level were used. After each use, the traps were desorbed again to obtain a correction for the trap background. Additional desorptions were carried out occasionally. These tests indicated that the trap background remained constant after the first desorption. All results reported were corrected for these blank values. Background originating from OCS in the zero air was checked by collecting a few samples of test atmospheres to which no standard was added. No OCS in the zero air was detected in these samples. Recovery results are given in Table V and in Figure 6. For Carbosieve S maximum recovery occurred a t a desorption temperature of about 230 "C. Below 200 OC the recovery was less than 90%, and a t temperatures above 270 "C recovery was less than 50%. For Molecular Sieve 5A the recovery was close to 100% over the entire range studied although the best temperature appeared to be close to 270 "C. To test the stability of the collected samples during storage, samples of test atmospheres containing OCS-61 were collected
ANALYTICAL CHEMISTRY, VOL. 59, NO. 9, MAY 1, 1987
1301
Table VI. Stability of OCS S a m p l e s Collected on
Molecular Sieve 5A" days stored
a m t collected, pmol
a m t recovered, pmol
1
16
20 10 12 17 15 13 10 16 11
8 3 28 a
0
1.0
2.0
3.0
4.0
VOLUME SAMPLED (L/cm)
Flgure 5. Breakthrough of OCS-61 in ambient air collected on (A) Tenax, (9) Carbosleve 6,(C) Molecular Sieve 5A, and (D) Carbosleve
S.
1
t
Samules stored
15 18 20 16 14 16 15
in clean atmosuhere a t room temperature.
ently the OCS becomes irreversibly bound during storage. We did not investigate procedures for limiting or preventing this loss, although refrigerating the samples would probably greatly slow this process (8). On the other hand, Molecular Sieve 5A appears to be a satisfactory adsorbent especially when isotopically labeled OCS is used as an internal standard to maintain precision in case of small amounts of OCS loss during the sampling and analysis.
CONCLUSIONS Isotope dilution GC/MS is a highly sensitive, specific, and precise method for the determination of atmospheric OCS. It has the required precision to characterize the small fluctuations expected for such compounds having long atmospheric residence times. Collection of OCS on solid adsorbents followed by the determination of OCS by isotopic dilution GC/MS, should allow the global distribution of OCS to be studied using light-weight easily deployed samplers without sacrificing sensitivity or precision. Molecular Sieve 5A is the most suitable adsorption material because of its superior recovery properties and storage characteristics. Although Carbosieve S has even better breakthrough and recovery properties, large l w e s during storage negate these advantages. Registry No. OCS, 463-58-1;T e n a x , 24938-68-9;Carbosieve B, 74749-61-4.
LITERATURE CITED
200
220
240
TEMPERATURE
280 (OC)
Flgwe 6. Recovery of OCS absorbed on (A) Molecular Sieve 5A and (B) Carbosieve S, as a function of temperature.
and stored at room temperature in sealed traps for 1-28 days. The amount of OCS that could be desorbed after this storage period was then determined. The desorption was carried out at the optimum temperature, for 3 min and with the zero grade helium flow of 200 mL/min. The results of these tests are shown in Table VI. Almost no OCS could be recovered from Carbosieve S even for the shortest storage periods of only a few hours. Appar-
(1) Maroulls, P. J.; Torres, A. L.; Bandy, A. R. Geophys. Res. Lett. 1977, 4 , 510-512. (2) Crutzen, P. J. Geophys. Res. Lett. 1978, 3 , 73-76. (3) Graedei, T. E. Rev. Geophys. Space fhys. 1977, 15, 421-428. (4) Sze, Nien Dak; KO, Malcolm K. W. Atmos. Environ. 1980, 1 4 , 1223-1239. (5) Torres, A. L.; Maroulis, P. J.; Goldberg, A. B.; Bandy, A. R . J . Geophys. Res. 1980, 8 5 , 7357-7360. (6) Bandy, A. R.; Tucker, B. J.; Maroulis, P. J. Anal. Chem. 1985, 57, 13 10- 13 14. ( 7 ) Black, M. S.; Herbst, R. P.; Hitchcock, D. R . Anal. Chem. 1978, 5 0 . 848-851. (8) Steudler, P. A.; Kijowski, W. Anal. Chem. 1984, 56, 1432-1436. (9) Shalaby, L. Ph.D. Dlssertatlon, Drexel University, 1982. (10) Sllversteln, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; Wiley: New York, 1974.
RECEIVED for review June 9,1986. Accepted January 12,1987. This research was supported by NSF Grant ATM 8515000.
