Environ. Sci. Techno/. 1995, 29, 2107-21 11
Detection of 2=Chloroethyl Ethyl Sulfide and Sulfonium Ion Degradation Products on Environmental Surfaces Usinm GARY S. GROENEWOLD,* JAN1 C . I N G R A M , ANTHONY D. APPELHANS, JAMES E. DELMORE, A N D DAVID A. DAHL Idaho National Engineering Laboratory, P.O.Box 1625, Idaho Falls, Idaho 83415-2208
2-Chloroethyl ethyl sulfide (CEES) is a simultant for the chemical warfare agent bis(2-chloroethyl)sulfide (also known as HD or mustard), and both molecules undergo hydrolysis and subsequent condensation in aqueous solution to form stable sulfonium ions. The sulfonium ions derived from CEES are directly detected on quartzic surfaces using static SIMS instrumentation, which employs a molecular Re04- (250 D) primary ion and pulsed secondary ion extraction. Pulsed extraction mitigates surface charging, and the Re04- primary particle is efficient at sputtering molecular surface species into the gas phase. CEES eliminates CI- to form an ethyl thiiranium intermediate, which is susceptible to nucleophilic attack by water and methanol to form 2-hydroxyethyl ethyl sulfide and 2-methoxyethyl ethyl sulfide. These two products and unhydrolyzed CEES also function as nucleophiles that condense with the ethyl thiiranium intermediate, resulting in the formation of sulfonium ion aggregates that are observable using SIMS. The previously unreported methoxy-substituted sulfonium ion suggests that a variety of derivatives are possible if different nucleophiles are present in the vicinity of the ethyl thiiranium intermediate. This work demonstrates that the sulfonium ion aggregates are stable on mineral surfaces and also demonstrates the potential value of SIMS for the detection of unanticipated ionic species in monitoring applications where mustard and its degradation products are suspected.
Introduction The detection of the chemical warfare agent bis(2-chloroethyllsulfide (HD or mustard) is complicated by the fact that the compound can undergo a variety of decomposition reactions in the environment (1). Many of the decomposi* Fax: (208) 526-8541.
0013-936x/95/0929-2107$09.00/0
0 1995 American Chemical Society
tion products have a strong tendency to adsorb to surfaces or are ionic, and these factors tend to complicate detection strategies that rely on converting the decomposition products into gas-phasespecies. Because of the importance of HD, the development of transportable instruments capable of the rapid detection of HD has been the focus of considerable effort. In contrast, the detection of low-volatile decomposition products originating from the environmental hydrolysis of HD has not been probed in depth because the decomposition products are hydrophilic and can also be highly surface adsorptive. These attributes frequently confound conventional analytical strategies. The chemistryand detection of sulfoniumions that result from the hydrolysis and subsequent condensation of HD are subjects of increasing interest (I,2). Rohrbaugh,Yang, and co-workers have shown that intermediate sulfonium ions are remarkably stable and can revert to HD as well as other toxic compounds. It has been speculated that sulfonium ions may be responsible for persistent toxicity, exhibited by samples that were exposed to HD as long as 80 years ago (World War I) and hence were not expected to be toxic based on a consideration of typical decomposition rates for HD (2). Among the literature describing the decomposition of CEES (31, the formation of sulfonium ions in 2-chloroethylethyl sulfide (CEES)solutions has also been reported, which is significant because CEES is the most commonly used simulant for HD (4). The focus of the present study is the application of static secondary ion mass spectrometry (SIMS) to the detection of sulfonium ions resulting from the hydrolysis and condensation of CEES. CEES is used because it is less toxic and, hence, easier to handle than HD. Static SIMS applications are being investigated because the technique has advantages compared to more conventional analysis techniques (e.g.,extraction/GC/MS): SIMS requires little or no sample preparation and hence is rapid; it generates no laboratory waste: and it can be performed on a very small sample (1mg is typical). In many respects, SIMS is complementaryto extraction-basedmethods: SIMS detects surface contaminants (as opposed to bulk), and it works best with ionic and/or polar (and hence involatile) species that have surfactant character. Busch and co-workers had successfullyemployed SIMS for the detection of a sulfonium ion species on a thin-layer chromatography plate (5). The attributes of SIMS are derived from the conceptual simplicity of the experiment in which solid samples are analyzed by bombarding them with high-energy particles (primary ions), which causes the emission of secondary ions by a process called sputtering. The secondary ions are subsequently mass analyzed. Recent advances in primary ion gun technology have resulted in improved sensitivityfor molecules adsorbed on surfaces: in the work reported herein, perrhenate (Reo4-)is used as the primary ion (6'). Polyatomic primary particles have been demonstrated to be more efficient at generating molecular secondary ions by an order of magnitude or more when compared to atomic particles such as Cs' (7). A limitation that has hindered the application of SIMS has been the fact that many electricallyinsulating samples tend to build up surface charge when under particle bombardment in a vacuum (8). Consequences of surface
VOL. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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charge are that secondaryions (dependingon their polarity) have either insufficientenergyto escape the charged surface or have excess kinetic energy as a result of the potential on the surface. In either case, observation of the sputtered ions with the secondaryion mass spectrometer is inhibited. This problem has been overcome in the instrument employed in the present study: a technique referred to as “pulsed extraction” alternately extracts anions and then cations in a ratio that can be adjusted so as to mitigate surface charging (9). The pulsed extractionsystem conveys the added benefit of permitting the near-simultaneous collection of both the anion and the cation spectra. The instrument that combines pulsed extraction with the Reo4- primary ion has been used successfully for the detection of the organophosphothioatepesticide malathion on leaves (IO),the detection of alkyl methylphosphonic acids on leaves (111, and tributyl phosphate on minerals (12). The purpose of the present study was to determine if static SIMS could be used to detect sulfonium ions resulting from the hydrolysisIcondensation of CEES, provided they exist on mineral surfaces.
Experimental Section SIMS Instrumentation. The instrument used for the majority of the studies has been described in detail previously (9); a brief description will be provided here. The instrument uses Reo4- at 10 keV as the primary bombarding particle, which is produced by heating a Ba(Reo& ceramic in vacuum (6‘). The ceramic was synthesized in our laboratories and processed in a form that could be used as an ion source. Reo4- ions that are emitted upon heating are acceleratedto 10keV. The ion gun was typically operated at 80 PA (the ion current from the gun could be continuously adjusted by adjusting the current passing through the heating element that supported the ceramic). The focusing of the primary ion gun was adjusted so that the sample was just silhouetted on an image intensifer located behind the sample. Thus, most of the primary ion beam is directed onto the target. A typical acquisition required 168 s, and a typical sample had an area of about 0.03 cm2;thus, a normal primary ion dose was 2.8 x 10l2 ions/cm2,which is less than the commonly accepted static SIMS limit (13). The secondary anions and cations were extracted from the sample target region using pulsed secondary ion extraction (9). This technique alternately extracts anions and cations from the sample surface by alternating the polarityof the secondaryionextractionlens. This technique mitigates charge buildup on the surface of the sample and, thus, permits facileanalysis of electricallyinsulatingsamples like minerals, leaves, and polymers. The ratio of [time extracting cations]/ [time extracting anions] is adjustable in this instrument, and in the case of the mineral samples studied in this report, a value of 3 worked well. The total period for the pulsed extraction sequence was 120 ms, divided as follows: cation extraction,84 ms; electronic settle time, 4 ms; anion extraction, 28 ms; electronic settle time, 4 ms. This sequence was repeated for each 0.2 u step of the scan of the quadrupole secondary mass spectrometer, which was scanned from 10 to 310 u. The quadrupole was a 2-2000 uinstrument, manufacturedby Extrel (Pittsburgh, PA) and modified in our laboratory. The quadrupole was tuned for unit mass resolution and optimum sensitivity for mlz 81- and 198+ in the SIMS spectrum of tetrahexyl ammonium bromide (14). 2108
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 8, 1995
severa measurements were made to v e r y rragmentation pathways using an ion trap SIMS instrument recently constructed in our laboratory (15). A rigorous description of this instrument is beyond the scope of this paper, in part because the instrument is still undergoing substantial refinement. Briefly, an Reo4- ion gun is located coaxially behind one end cap of a modified Finnigan ion trap mass spectrometer (Finnigan-MATCorporation, San Jose, CAI, and an offset electron multiplier is located next to the ion gun. The sample, attached to a probe, is located 3 mm from the opposite end cap, and the Reo4- beam at 3.5 keV is focused on the sample through the ion trap. A diagram of the instrument is found in ref 15. Ions in the mass range of interest were collected using filtered noise fields applied with a Teledyne system (Teledyne ElectronicsTechnology, MoutainView, CA),and collision-induceddissociationwas performed using a supplementary rf field on the end caps. Sample Origin, Exposure, and Preparation. The samples that were analyzed were sandy soil that was collected at the U.S. Army Chemical Materiel Destruction Agency (CMDA)site in Raritan, NJ. This soil is of interest because Raritan is under study by CMDA. Scanning electron microscopy (SEMI and energy dispersive X-ray spectroscopy (EDS) of these samples revealed that they were mostly SiOa (quartzic) but also contained some iron. The sandy grains which comprised the soil appeared tan in color. The soil samples were attached to sample holders (small wires which attach to a direct insertion probe) using doublesided tape, approximately2 mm x 2 mm. Once attached, the soil was exposed to solutions of 2-chloroethyl ethyl sulfide (CEES) and CEES hydrolysate by spiking with approximately 1 pL of solution and allowing this to dry. Sample evacuation was rapid, and sample off-gas did not appreciably degrade the instrument vacuum. CEES was procured from Aldrich (Milwaukee,WI) and was dissolved in methylene chloride at a concentration of 11pglpL. Protic solventswere not used because these will hydrolyze CEES. Intentional hydrolysis of CEES was accomplishedby dissolvingthe compound in 1:1methanol/ water or acetonelwater at a concentration of approximately 50 pg/pL. This was stirred vigorously, and 1-pL samples were periodically removed for analysis.
Results and Discussion 2-Chloroethyl Ethyl Sulfide (CEES). Soil samples were initially exposed to CEES as a CHzClZ solution in order to identify the mass spectral signature of unhydrolyzed CEES. The spectrum presented in Figure 1is representative of soil exposed to CEES at about 4.5% (wlw), which would correspond to about five monolayers (assuming a soil surface area of 10 m2/g, a molecular area of 25 &, and neglecting any partitioning onto vial walls or evaporative losses). The abundant ions observed at mlz 89+ and 61+ were not present in the spectrum of unexposed soil (Figure 2). Two mechanisms suggest themselves for the origin of mlz 89+: (a) HC1 is eliminated from [CEES + HI+ (which is not significantly above background at mlz 125+);(b) heterolytic cleavage of the C-C1 bond occurring upon bombardment of CEES adsorbed to the surface (Figure3). The latter explanation is supported by the abundance of C1- in the anion spectrum (not shown), which is substantially greater than in the spectra of the unexposed soil. mlz 89+ subsequently undergoes the loss of CzHl to form mlz 61-; this transition was verified in an MS/MS experiment
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FIGURE 3. Mechanismsfor the formation of mh89+ obsenred in the SIMS spactrum of CEES.
using the ion trap SIMS instrument described in the Experimental Section. In addition to C1-, the anion spectrum also contains enhanced ion abundance at mlz 32-, which likely corresponds to S-. The cation SIMS spectra displayed in Figures 1 and 2 contain many other ions that are ascribable to surface contamination (16). Odd mass ions are usually attributed to CxHy+,and examples of these ions include mlz 27+, 29+, 39+, 41+, 43+, 53+, 55+, 57+,67+, 79+, 81+,and 83+. Other 'background' ions in the cation spectrum are the result of cyclohexyl amine surface contamination. This compound is an atmospheric contaminant present in our laboratory and produces ions at m/z loo+,83+, 56+,30+,28+,and 18+. Siloxanecompounds also contribute to the ion background in the SIMS spectrum at m/z 73+, 147+, 207+, and 221+. mlz 89+ and 61' could be observed on soil samples exposed to smaller quantities of CEES. The lowest level at which m/z 89+could be identilied as above the background
FIGURE 4. (a) Partial cation SlMS spectra (@89+ region) of CEES hydrolysate (methanollwater) on soil at 7.5,29,50, and 164 min after initiation of hydrolysis. Spectrum at time = 0 is unaxpored soil. (b) Partial cation SlMS spectra (mh195-215 region) of CEES hydrolyrclto (mathanollwater) on soil at 7.5,29,50, and 164 min after initiation of hydrolysis. Spectrum at time = 0 is unexposed soil.
was 1 pg of CEESlmg of soil. flhis would correspond to about 0.1 monolayer of CEES, if one assumes a molecular area of 25 A2/gfor CEES and a surface area of 10 m2/gfor the soil. These values are consistent with those derived for similarly sized contaminants and similar soil samples.) Significantly, the differences between the SIMS spectra generated from samples having lower and higher loadings were minor. At concentrations below 1 pg/mg, it was not possible to unequivocallyidentify CEES because of isobaric 'background' ions observed in the SIMS spectra of soil not exposed to CEES. The high contamination levels required for SIMS observation of CEES are attributable to the following: (a) the volatility of the compound (some of the CEES will evaporate in the spectrometer vacuum); (b) the absence of a facile ionization mechanism; and (c) the isobaric ion problem. CEES Hydrolysate. CEES was dissolved in 1:l water/ methanol (50 pg/pL) and stirred vigorously. Periodically, a 1-pL sample was withdrawn from the reaction mixture, applied to afresh soil sample (approximately1mg), allowed to dry, and admitted to the instrument for SIMS analysis. The m/z 89+ ion could be observed in the initial analysis (7.5 min after initiation of hydrolysis) together with low abundance ions at mlz 195+, 209+, and 213+/215+. The abundancesof these ions increased until the 50-minanalysis and then slowly decreased at longer times (Figure 4a,b). Significantly, mlz 89+ in the 50-min analysis (Figure 5) was about 20 times more abundant than in the case of the unhydrolyzed CEES at the same loading (Figure 1). The VOL. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE 81 TECHNOLOGY
2109
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+.
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markedly enhanced abundance of the hydrolysate is consistent with the idea that the contaminant is primarily present in an ionic form and therefore is less volatile and more amenable to the production of abundant secondary ions (17). It is unlikely that any significant portion of the rnlz 89+ abundance is originating from unhydrolyzed CEES. This conclusion stems from a sample that was extracted with CHzClz and analyzed using GC/MS subsequent to spiking with CEES hydrolysate: no CEES was detected using an instrumental protocol capable of easily detecting 100ng in the sample (the sample was spiked with approximately 50 P@*
The higher mass ions observed at rnlz 195+,209+,213+, and 215+ are attributed to sulfoniumion aggregates, which are produced as the result of CEES chemistry occurring in solution. The first step in the hydrolysis of CEES involves the elimination of C1- to form an ethyl thiiranium ion (ion a, Figure 6 ) . This ion subsequentlyundergoesnucleophilic attack at the a-carbon. If the nucleophile is water, hydroxyethyl ethyl sulfide (HEES)results, if a second CEES molecule attacks ion a, the sulfonium ion designated as ion b is formed. Ion a can also be attacked by HEES to form the sulfonium ion c (18). rnlz 213+ observed in the SIMS spectrum of the CEES hydrolysate is in agreement with the mass calculated for the 35C1-containingion b. This assignment is supported by the presence of rnlz 215+,whose abundance relative to 213+ is within 2% of that calculated for C8H&Cl. In a similar fashion, mlz 195+ agrees with the mass calculated for ion c. The observation of ion b and ion c in the SIMS experiments indicates that the sulfonium ions are stable on the surface of quartzic soil, in the open atmosphere, long enough to be observed in the SIMS analysis. The time between samplinglsoil spiking and SIMS analysis was 2110 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 8,1995
FIGURE 8. Partial cation SIMS spectrum of soil exposed to CEES hydrolysete (acetonehvater) 325 min after initiation of hydrolysis.
typically 30-90 min; this time was needed to allow the samples to dry at ambient temperature. The most abundant ion in the 190-220 region is rnlz 209+,which does not correlate with any sulfonium ions previously observed using NMR spectroscopy(1,2,4). The most reasonable hypothesis for the formation of rnlz 209+ is that ion a is attacked by methanol to form methoxyethyl ethyl sulfide, which subsequentlyattacks a second ion a to form sulfonium ion d (Figure 7). This explanation is supported by the fact that methanol was used as a solvent in the CEES hydrolysis and by the fact that rnlz 209+ was not observed when the reaction was carried out in 1:l acetonelwater. In this latter experiment, only ions at rnlz 195+ and 213+/215+were observed (Figure 8). The identification of mlz 209+ was supported by a MSl MS experiment in which one of the soil samples exposed to the methanol/water hydrolysate was allowed to stand for 2 days and then analyzed using the ion trap SIMS instrument. Ions in the 209+ mass region were selectively trapped and then collisionally activated (see experimental Section). The sole fragment ion observed rnlz 89+,which correspondsto ion a and would be the dominant fragment ion predicted from fragmentation of the sulfonium ions b-d. A n MSIMSIMS experiment revealed that m/z 61+is a fragment ion of rnlz 89+,which is consistent with the lower mass region of the SIMS spectrum of the hydrolysate and of unhydrolyzed CEES.
Conclusions Sulfonium ions resulting from the hydrolysis and subsequent condensation with CEES and HEES can be detected on quartzic surfaces using static SIMS, which shows that sulfonium ions are stable on mineral surfaces in the open atmosphere. The results suggest that the investigation of HD-contaminatedsoils would benefit from SIMS analysis
because the technique is amenable to ionic species that are not detectable using extractionlGCIMS or other techniques that require analyte vaporization. Further, SIMS is capable of detecting unanticipated decomposition species: this is significant because new sulfonium ion derivatives can readily form if other nucleophiles are present in the vicinity of CEES (or HD) during hydrolysis.
Acknowledgments The support and encouragement of the U.S. Army Chemical Material Destruction Agency (CMDA),Non Stockpile Program, is gratefully acknowledged.
literature Cied (1) Yang, Y.-C.; Baker, J.A.; Ward, J. R. Chem. Rev. 1992,92, 172943. (2) Rohrbaugh, D. K.; Yang, Y.-C.; Ward, R. J, Sulfonium Chlorides Derived from 2-Chloroethyl Sulfides, III. Gas Chromatography1 Mass Spectrometry Identifications and Mechanism of Degradation;U.S. Army Chemical Research, Development & Engineering Center Report CRDEC-TR-88140; Government Printing Office: Washington, DC, July 1988. (3) (a) Rohrbaugh, D. K.; Yang, Y.-C.; Ward, J. R. J. Chromatogr. 1988,447,165-9. (b) Rohrbaugh, D. K.; Yang, Y.-C.; Ward, J. R. Phosphorus, Sulfur Silicon Relat. Elem. 1989, 44, 17-25. (4) Yang, Y.-C.; Rohrbaugh, D. K.; Ward, R. J. Detection ofMustard Gas by Gas ChromatographylMassSpectrometry in 2-Chloroethyl Ethyl Sulfide; US. Army Chemical Research, Development & Engineering Center Report CRDEC-TR-88011; Government Printing Office: Washington, DC, Nov 1987. (5) Busch, K. L.; Mullis, J. L.; Chakel, J. A. J. Planar Chromatogr. 1992, 5, 9-15. (6) Delmore, J. E.; Appelhans, A. D.; Peterson, E. S. Int. J. Mass Spectrom. Ion Processes 1991, 108, 179-87.
(7) (a) Appelhans, A. D.; Delmore, J. E.Anal. Chem. 1989,61,108793. (b) Blain, M. G.; Della-Negra, S.; Joret, H.; Le Beyec, Y.; Schweikert, E. J. Phys. 1989, 50, C2-147-53. (8) Beck, S. T.; Appelhans, A. D.; Delmore, J. E.; Dahl, D. A. Int. J. Mass Spectrom. Ion Processes 1992, 120, 129-55. (9) Appelhans, A. D.; Dahl, D. A,; Delmore, J. E. Anal. Chem. 1990, 62, 1679-86. (10) Delmore, J. E.; Appelhans, A. D. Biol. Mass Spectrom. 1991,20, 237-46. (11) Ingram, J. C.; Groenewold, G. S.; Appelhans, A. D.; Delmore, J. E.; Dahl, D. A.; Detection of Alkyl Methylphosphonic Acids on Leaf Surfaces by Static Secondary Ion Mass Spectometry. Anal. Chem. 1995, 67, 187-95. (12) Groenewold, G. S.; Ingram, J. C.; Delmore, J. E.; Appelhans, A. D. Static SIMS Analysis of Tributyl Phosphate on Mineral Surfaces. J. Am. SOC. Mass Spectrom. 1995, 6, 165-74. (13) Briggs, D.; Hearn, M. I. Vacuum 1986, 36, 1005-10. (14) Winger, B. E.; Hand, 0. W.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1988, 84, 89-100. (15) Applehans, A. D.; Dahl, D.A.; Delmore, J. E.; Groenewold, G. S.; Ingram, J. C. BookofAbstracts;PITTCON '94: Chicago, IL, 1994; Abstract 1229. (16) Benninghoven, A. Surf Sci. 1973, 35, 427-57. (17) Pachuta, S. J.; Cooks, R. G. Chem. Rev. 1987, 87, 647-69. (18) Rohrbaugh, D. K.; Yang, Y. C.; Hovanec, J. W.; Ward, R. J. Degradation of 2-Chloroethyl Ethyl Sulfide. In Proceedings of Army Chemical Research, Development and Engineering the U.S. Center Scientific Conference on Chemical Defense Research; US. Army Aberdeen, MD, Nov 18-21, 1986; pp 869-75.
Received for review February 14,1995.Accepted Maya 1995.@
ES950093Y @
Abstract published in Advance ACS Abstracts, June 15, 1995.
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