Analysis of VX on Soil Particles Using Ion Trap Secondary Ion Mass

May 14, 1999 - The direct detection of the nerve agent VX (methylphosphonothioic acid, S-[2-[bis(1-methylethyl)amino]ethyl] O-ethyl ester) on milligra...
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Anal. Chem. 1999, 71, 2318-2323

Analysis of VX on Soil Particles Using Ion Trap Secondary Ion Mass Spectrometry Gary S. Groenewold,* Anthony D. Appelhans, Garold L. Gresham, and John E. Olson

Idaho National Engineering and Environmental Laboratory, Idaho Falls, Idaho 83415-2208 Mark Jeffery

U.S. Army West Desert Test Center, Dugway Proving Ground, Dugway, Utah 84022 J. B. Wright

U.S. Army Edgewood Research, Development and Engineering Center, Aberdeen Proving Ground, Maryland 21010-5423

The direct detection of the nerve agent VX (methylphosphonothioic acid, S-[2-[bis(1-methylethyl)amino]ethyl] Oethyl ester) on milligram quantities of soil particles has been achieved using ion trap secondary ion mass spectrometry (IT-SIMS). VX is highly adsorptive toward a wide variety of surfaces; this attribute makes detection using gas-phase approaches difficult but renders the compound very amenable to surface detection. An ion trap mass spectrometer, modified to perform SIMS, was employed in the present study. A primary ion beam (ReO4-) was fired on axis through the ion trap, where it impacted the soil particle samples. [VX + H]+, [VX + H]+ fragment ions, and ions from the chemical background were sputtered into the gas-phase environment of the ion trap, where they were either scanned out or isolated and fragmented (MS2). At a surface concentration of 0.4 monolayer, intact [VX + H]+, and its fragment ions, were readily observable above background. However, at lower concentrations, the secondary ion signal from VX became obscured by ions derived from the chemical background on the surface of the soil particles. MS2 analysis using the ion trap was employed to improve detection of lower concentrations of VX: detection of the 34S isotopic ion of [VX + H]+, present at a surface concentration of ∼0.002 monolayer, was accomplished. The study afforded the opportunity to investigate the fragmentation chemistry of VX. Semiempirical calculations suggest strongly that the molecule is protonated at the N atom. Deuterium labeling showed that formation of the base peak ion (C2H4)N(i-C3H7)2+ involves transfer of the amino proton to the phosphonothioate moiety prior to, or concurrent with, C-S bond cleavage. To manage the risk associated with working with the compound, the vacuum unit of the IT-SIMS was located in a hood, connected by cables to the externally located electronics and computer. The compound VX (methylphosphonothioic acid, S-[2-[bis(1methylethyl)amino]ethyl] O-ethyl ester) has recently received a 2318 Analytical Chemistry, Vol. 71, No. 13, July 1, 1999

high level of topical interest because of its potential for use in terrorist or military operations.1 First discovered in 1952, the compound was adopted for use as a chemical casualty agent because of its high degree of toxicity (LD50 estimated at 8 µg/ kg)2 and the fact that it is very persistent.3 The attribute of persistence implies that the compound can be applied to surfaces and allowed to stand for prolonged periods of time and still deliver a lethal dose through dermal contact. The molecular weight (267), and the presence of amino, O-, and S-phosphoester moieties are no doubt responsible for the strongly adsorbent character of the compound4 and contribute to the compound’s reputation for being difficult to detect (Figure 1). In most environments, only a small fraction of the compound will be partitioned into the gas phase; the vast majority will exist adsorbed to solid surfaces, including particulate surfaces. This behavior can defeat characterization approaches based on detection of gas-phase molecules. When the compound can be appropriately separated from its environmental matrix, it can be readily analyzed using GC/MS;5 in particular, ammonia chemical ionization has been shown to be effective for VX and many of its neutral degradation products.6 The success of this type of analysis depends on the efficacy of the extraction step, but this can be difficult; for example, a recent application of supercritical fluid extraction to the separation of VX was unsuccessful.7 Other more conventional extraction approaches have been developed and combined with GC and or MS detection.2,8,9 An additional mass spectrometry approach that has been suc(1) (a) Ember, L. Chem. Eng. News 1998, (Aug 31), (b) Zurer, P. Chem. Eng. News 1998, (Aug 31), 7. (c) Rouhi, M. Chem. Eng. News 1999, (Feb 22), 37. (2) Kientz, Ch. E. J. Chromatogr., A 1998, 814, 1-23. (3) Compton, J. A. F. Military Chemical and Biological Agents; The Telford Press: Caldwell, NJ, 1987; p 9. (4) Verweij, A.; van Liempt-van Houten, M. A.; Boter, H. L. Int. J. Environ. Anal. Chem. 1985, 21, 63. (5) Witkiewicz, Z.; Mazurek, M.; Szulc, J. J. Chromatogr. 1990, 503, 293-357. (6) D’Agostino, P. A.; Provost, L. R.; Visentini, J. J. Chromatogr. 1987, 402, 221-32. (7) Kuitunen, M.-L.; Hartonen, K.; Riekkola, M.-L. J. Microcolumn Sep. 1991, 3, 505-12. (8) Verweij, A.; Boter, H. L. Pestic. Sci. 1976, 7, 355. (9) Wise, M. B.; Thompson, C. V.; Buchanan, M. V.; Merriwether, R.; Guerin, M. R. Spectroscopy 1993, 8, 14. 10.1021/ac981391r CCC: $18.00

© 1999 American Chemical Society Published on Web 05/14/1999

Figure 1. Structure of VX.

cessfully utilized for VX detection employed thermospray MS with a HPLC.10 When coupled with a preconcentration step, this resulted in the detection of as little as 0.1 ng/mL VX. Research in the authors’ laboratory has been focused on the applications of static secondary ion mass spectrometry (SIMS) for the detection of surface-adsorbed chemical contaminants. Initial studies showed that the organophosphothioate pesticide malathion could be readily detected in the negative ion mode using a singlestage, quadrupole mass analyzer together with a polyatomic primary projectile (SF60).11 Subsequent research led to the development of an anionic polyatomic projectile (ReO4-).12 This projectile was also found to be highly effective for sputtering intact molecular ions, or structurally significant fragment ions, into the gas phase from the surfaces13 of naturally occurring samples (e.g., mineral particles and vegetation). Subsequent studies performed using this instrumentation showed that compounds having other functional groups (e.g., amines,14 sulfides,15 or organophosphates16) were also amenable to detection using SIMS. While SIMS detection using single-stage mass spectrometry (MS1) could be readily achieved for samples having substantial contaminant surface concentrations, it was found that detection of lower concentrations was confounded by chemical ion background derived from ubiquitous organic adsorbates present on the surfaces of environmental samples. To overcome the chemical background problem, the SIMS technique was combined with MS2 in the form of an ion trap SIMS (IT-SIMS) instrument.17 This device is capable of the selective accumulation of secondary ions having a specified mass range, while the sample surface is bombarded. Once formed and isolated, the ions can be collisionally dissociated to form fragment ions diagnostic for the contaminant molecule of interest and distinct from the chemical background. This instrumental approach using the IT-SIMS has been successfully applied to the detection of sulfides18 and organophosphates,19 (10) Wils, E. R. J.; Hulst, A. G. J. Chromatogr. 1990, 523, 151-61. (11) Delmore, J. E.; Appelhans, A. D. Biol. Mass Spectrom. 1991, 20, 237-46. (12) (a) Delmore, J. E.; Appelhans, A. D.; Peterson, E. S. Int. J. Mass Spectrom. Ion Processes 1995, 146/147, 15. (b) Delmore, J. E.; Appelhans, A. D.; Peterson, E. S. Int. J. Mass Spectrom. Ion Processes 1991, 108, 179. (13) Groenewold, G. S.; Delmore, J. E.; Olson, J. E.; Appelhans, A. D.; Ingram, J. C.; Dahl, D. A. Int. J. Mass Spectrom. Ion Processes. 1997, 163, 185-95. (14) (a) Groenewold, G. S.; Gianotto, A. K.; Olson, J. E.; Appelhans, A. D.; Ingram, J. C.; Delmore, J. E. Int. J. Mass Spectrom. Ion Processes 1998, 174, 12942. (b) Groenewold, G. S.; Ingram, J. C.; Gianotto, A. K.; Appelhans, A. D.; Delmore, J. E. J. Am. Soc. Mass Spectrom. 1996, 7, 168-72. (15) Groenewold, G. S.; Ingram, J. C.; Appelhans, A. D.; Delmore, J. E.; Dahl, D. A. Environ. Sci. Technol. 1995, 29, 2107-11. (16) (a) Ingram, J. C.; Groenewold, G. S.; Appelhans, A. D.; Delmore, J. E.; Olson, J. E.; Miller, D. L. Environ. Sci. Technol. 1997, 31, 402-8. (b) Ingram, J. C.; Groenewold, G. S.; Appelhans, A. D.; Dahl, D. A.; Delmore, J. E. Anal. Chem. 1996, 68, 1309-16. (c) Groenewold, G. S.; Ingram, J. C.; Delmore, J. E.; Appelhans, A. D. J. Am. Soc. Mass Spectrom. 1995, 6, 165-74. (17) Groenewold, G. S.; Appelhans, A. D.; Ingram, J. C. J. Am. Soc. Mass Spectrom. 1998, 9, 35-41. (18) Groenewold, G. S.; Appelhans, A. D.; Ingram, J. C.; Gresham, G. L.; Gianotto, A. K. Talanta 1998, 47, 981-6. (19) Ingram, J. C.; Appelhans, A. D.; Groenewold, G. S. Int. J. Mass Spectrom. Ion Processes 1998, 175, 253-62.

in both the positive and negative ion modes. However, the technique has not been applied to actual chemical warfare agents or to compounds having the combination of functional group moieties possessed by VX. The utilization of the IT-SIMS for the detection of VX on soil (aluminosilicate) particles is reported in the present study. The results show that the detection of the compound on relatively minute quantities of sample (mg) is highly amenable to the ITSIMS approach. The fragment ion chemistry of the protonated VX molecule is also explored, in particular the mechanism for the formation of the base peak (C2H4)N(i-C3H7)2+, which was probed using deuterium labeling. EXPERIMENTAL SECTION Sample Generation. Caution. VX is a highly lethal compound capable of killing or injuring at extremely minute doses. The compound should only be handled in approved chemical warfare surety laboratories by trained agent chemists. All sample preparation and analyses were performed at chemistry laboratory of the U.S. Army West Desert Test Center, Dugway Proving Ground, Dugway, UT; this laboratory is equipped with appropriate engineering and administrative controls for handling chemical warfare agents. A clean soil sample obtained from the Edison, NJ, area, near the former Raritan Army Depot, was used in the present study. This material was chosen because it had been characterized during the course of previous studies conducted in our laboratories. The sample was predominantly silicate in nature, with minor contributions from aluminum and iron, as determined using scanning electron microscopy and energy-dispersive X-ray analysis. The soil was sieved, and the 0.0049 in. < x < 0.0098 in. mesh fraction was used. The surface area of this fraction was measured at 3.1 m2/g using N2 adsorption (BET method).20 The VX used in the study was the property of the U.S. Army and was an analytical reference standard. The compound was received and used as a 1 µg/µL (2-propanol) standard solution. Soil samples (50 mg) were spiked with 36 or 3.6 µL of standard solution, which was equivalent to 0.4 or 0.04 monolayer, respectively. In the case of the 0.04 monolayer sample, it was necessary to add additional 2-propanol to wet the soil sample. The mass of VX corresponding to monolayer coverage was estimated by assuming that the molecular area of the compound was equivalent to 79 Å2, which is the area of a circle having a radius equal to half the length of the molecule on the surface (estimated at 5.0 Å). This radius estimate assumes that the molecule is laid flat on the surface and was generated using molecular mechanics calculations (Cerius2, Molecular Simulations, Inc., San Diego, CA). If the molecule exists on the surface in an upright fashion, or is coiled, then the monolayer coverages will be somewhat less than the values used in this study. This approach for the preparation of coated samples has, in the past, resulted in self-consistent data; viz., the abundance of secondary ions stops increasing with increasing adsorbate mass when the estimated surface concentration exceeds one monolayer.16b The wet soil samples were allowed to dry for ∼2 h under ambient conditions, whereupon ∼1 mg of particles was attached to a sample holder (the head of a no. 18 nail) using double-sided (20) Adamson, A. W. Physical Chemistry of Surfaces; John Wiley & Sons: New York, 1990; p 609.

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Figure 2. Ionization/isolation/reaction/detection sequence in the ITSIMS. For MS1 experiments, the filtered noise field ion isolation of period 1, all of period 2, and all of period 3 is omitted. For H-D exchange, period 2 was omitted, and period 3 was used as a delay time to allow reaction to occur. For MS2 experiments, all components of the sequence were used, and period 3 was used for collisional activation. Time is represented (nonquantitatively) on the x axis. For MS1, total experiment time was on the order of 50 ms. For H-D exchange, 150 ms was typical. For MS2, 200-300 ms was typical.

tape. The sample holder was then attached to the direct insertion probe for analysis. SIMS analysis showed only ions derived from the VX-spiked soil (and not from the tape). Therefore, we conclude that the soil particles are efficiently covering the tape surface. When the analyses were complete, samples were immersed in bleach solution, to decontaminate residual VX. The waste from the decontamination procedure was then treated as hazardous waste and disposed of appropriately. Ion Trap Secondary Ion Mass Spectrometry. The instrument used in this study was a Teledyne Discovery 2 ion trap mass spectrometer (Mountain View, CA) modified for SIMS. The physical layout of the IT-SIMS instrument involved placement of the vacuum housing in a chemical warfare agent surety hood, where all agent-contaminated manipulations were performed. The vacuum housing was connected to the power supplies and computer by elongated cables. Two mechanical pumps, which pumped the turbo and the insertion lock, were also located outside the hood but were vented back into the hood using Tygon tubing. This particular instrument has not been described previously in the literature, but is very similar in design and operation to an IT-SIMS instrument based on a Finnigan ion trap that is described in detail in ref 17. Briefly, the instrument is equipped with a ReO4primary ion gun, an offset dynode/multichannel plate detector system, and an insertion lock for introducing the sample using the direct insertion probe. The ReO4- primary particle12 provides enhanced production of molecular secondary ions, which is advantageous when probing surface adsorbates.13 The ReO4beam is directed through the ion trap along the main axis of the device, passing through a hole in one end cap, and striking the sample holder, which is located behind a 1-mm-diameter hole in the opposite end cap. The ReO4- ion gun was operated at 4.5 keV, at a primary ion current for the 50 pA (measured using a Faraday cup probe). The ReO4- beam is gated to impact the sample only during the initial period of the IT-SIMS analysis sequence (period 1, Figure 2). By varying the length of the ionization period, the dynamic range of the instrument may be varied over 3 orders of magnitude. Secondary ions sputtered from the sample surface are focused into the ion trap by a small, cylindrical, electrostatic lens. 2320 Analytical Chemistry, Vol. 71, No. 13, July 1, 1999

The electrostatic potential of the sample holder is not controlled; instead, the potential of the sample holder floats at the ambient (and variable) potential of this region of the ion trap. Upon entering the trap, the secondary ions undergo collisions with the He bath gas (3 × 10-5 Torr) and lose sufficient kinetic energy such that they are trapped by the oscillating rf field. One particularly advantageous attribute of the ion trap is that by applying a filtered noise field (FNF) to the end caps during the ion formation period, all ions except those within a selected narrow mass range are kept out of the trap.21 This makes it possible to populate the trap with only the ions of interest, thereby greatly improving the signal-to-noise ratio and resultant sensitivity. Once trapped, the ions can be mass selectively ejected out to the detector region (MS1), which is located off axis on the same side of the ion trap as the ReO4- primary ion gun. Alternatively, the ions can be collisionally dissociated through collisions with He, by applying a high-frequency oscillating field to the end caps (MS2). The ionic fragments are then mass selectively ejected to the detector.21 A third experimental possibility is to allow the trapped ions to react with D2O; this is accomplished by inserting a 100-200 ms delay between ionization and scanout/detection. In a typical MS1 experiment, the ion trap was operated at base rf amplitude corresponding to a low-mass cutoff of 40 amu. Ionization time was typically 20 ms, after which the mass spectrum was recorded. A single “scan” consisted of 10 summed spectra. A normal analysis would consist of ∼30 scans. The primary ion dose for a typical analysis can be calculated at ∼2 × 1011 ions/cm2, by using the above information and knowing the area irradiated (8 × 10-3 cm2) and the primary ion current (50 pA). At this dose, the surface of the sample is not considered to have been seriously perturbed.22 For MS2 experiments, the ionization time was typically longer (100-200 ms), during which time a FNF was applied to the end caps, such that m/z 128, 268, or 270 was isolated (Figure 2). Isolation was further improved by scanning the ions having an m/z less than the parent out of the trap, without detection. A typical MS2 acquisition resulted in a primary ion beam dose of (1-2) × 1012 ions/cm2 to the sample. Deuterium Labeling Experiments. Deuterium labeling was performed by gas-phase exchange of D for H in the ion trap after formation of the VX protonated molecules. A simple modification to the sample holder was employed to generate D2O vapor in the ion trap. A small strip of tape was wrapped around the barrel of the sample holder (directly behind the head of the nail). Tenax was adhered to this strip of tape and then doped with a small volume of methanol and with several microliters of D2O (methanol was required to enable the D2O to be absorbed by the Tenax). The soil particles were then attached to the nail head (sample holder) as described above. When this sample holder was admitted to the IT-SIMS, it resulted in a stable pressure of D2O in the trap for several minutes. RESULTS AND DISCUSSION The cation SIMS spectrum of soil particles having 0.4 monolayer of VX was dominated by abundant ions at m/z 268, 128, and 86 (Figure 3). In this respect, the spectrum was remarkably similar (21) Todd, J. F. J. In Practical Aspects of Ion Trap Mass Spectrometry; March, R. E., Todd, J. F. J., Eds.; CRC Press: New York, 1995; Vol. I, p 4. (22) Briggs, D.; Hearn, M. J. Vacuum 1986, 36, 1005.

Figure 3. Cation IT-SIMS spectrum of 0.5 monolayer VX on soil particles. Table 1. Identification of Salient Ions in the Cation IT-SIMS Spectrum of VX on Soil Particles m/z

ion

m/z

ion

268 167 139

[VX + H]+ (EtO)(Me)P(dO)S(C2H4)+ (EtO)(Me)P(O)(S)+

128 97 86

(i-C3H7)2N(C2H4)+ H2PO2S+ (i-C3H7)NH(C2H4)+

to spectra generated by D’Agostino using ammonia chemical ionization.6 These ions correspond to the protonated molecule [VX + H]+ and abundant amine-bearing fragment ions (Table 1), which supports the contention that VX is initially protonated at the tertiary N site. Semiempirical calculations using the PM3 Hamiltonian showed that electron density was highest at the N atom, and the proton affinities of amine-bearing molecules are generally higher than those of phosphates or thiophosphates.23 Once protonated, the base peak (m/z 128) is formed by the elimination of neutral (EtO)(Me)P(S)(O)H, leaving (C2H4)N(i-C3H7)2+; this reaction was the dominant process observed in the MS2 analysis of m/z 268 (Figures 4 and 5). The mechanism of this elimination reaction has not been identified; however, a McLafferty-type rearrangement involving transfer of a methylene hydrogen (R to the N) to the phosphoryl has been ruled out using D labeling. The amino proton was exchanged for a deuteron in the gas phase (see Experimental Section), which shifted the mass of the protonated molecule from m/z 268 to 269. However, the mass of the m/z 128 fragment ion remained unchanged. This indicates that the amino proton must be transferred to the thiophosphonate moiety either before or concurrent with cleavage of the C-S bond. At the present time, the mechanism of the proton transfer has not been identified: it could migrate either to the S through a five-membered transition state (the S atom has the second highest electron density within the molecule) or to the phosphoryl oxygen through a seven-membered transition state. A second issue that is unresolved is the structure of the m/z 128. Two possibilities include diisopropylaziridinium and N,N-diisopropylmethylimminium (Figure 4). MS2 analysis of m/z 128 revealed that the ion undergoes successive losses of one and two C3H6 to form m/z 86 and 44, but this behavior would be expected from both structures. A structure that contains an exchangeable (i.e., amino) proton is unlikely, since m/z 128 has no tendency to shift to m/z 129 when exposed to D2O in the ion trap for long periods of time. These structural problems are currently under study using ab initio methods and will be reported in a subsequent publication. (23) Corbett, K. M.; White, William E. Quantum Calculations on the Conformations of Protonated VX and Analogues. Proceedings of the 1997 ERDEC Scientific Conference on Chemical and Biological Defense Research, 1997; ERDEC-SP-063, pp 881-8.

Lower abundance ions were observed at m/z 167, 139, and 97. We speculate that m/z 167 is formed by elimination of diisopropylamine from [VX + H]+, which may occur via attack of a sulfur lone pair on the methylene R to the N atom (Figure 6). The resulting thiiranium ion bears some similarity to sulfonium ions recorded in the SIMS analysis of other sulfides.15,18 The ion at m/z 139 represents the loss of 129 u, which corresponds to diisopropylethylamine. We have not speculated at the structure of the resulting cation (EtO)(Me)P(O)(S)+. The ion observed at m/z 97 is also derived from VX and likely has the composition H2PO2S+. Formation of the ion requires substantial fragmentation. Irrespective of the mechanism of formation of m/z 128, the m/z 268 f 128 fragmentation represents a reaction that could be exploited as part of a MS2 analytical scheme. The need for such an approach is demonstrated by the MS1 analysis of soil particles doped with 0.04 monolayer of VX. The salient VX-derived ions can be observed, but their abundance is comparable to that of the background: for example, the abundance of [VX + H]+ at m/z 268 is only 3-5 times higher than that of non-VX-derived ions in the same mass region. The chemical background was markedly reduced by using the MS2 capability of the ion trap. In this experiment, the protonated molecule was isolated using a filtered noise field, notched so that m/z 268 (and some of the surrounding ions in this mass region) was isolated during the ionization period. Since the other ions were ejected as they are formed, this allowed the ion trap to be populated with predominantly [VX + H]+. Following ion formation and isolation, the [VX + H]+ was collisionally activated through application of a frequency to the end caps, which caused kinetic excitation, and collision with He. The collisions resulted in internally excited [VX + H]+, which fragmented to form daughter ions (MS2). The most abundant fragment ion in the MS2 spectrum of [VX + H]+ was m/z 128 (Figure 5), which could be readily detected in the 0.04 monolayer sample. Soil samples having VX surface concentrations lower than 0.04 monolayer were not generated due to operational constraints. Nevertheless, detection of lower concentrations of VX on soil particles was desired. To accomplish this, the [VX + H]+ isotopic ion at m/z 270 was isolated and fragmented. The fractional abundance of [VX + H]+, which is 0.046 at this mass, is predominantly due to the 34S isotope. Thus, for the 0.04 monolayer sample, the concentration of VX that produces [VX + H]+ at m/z 270 is ∼2 × 10-3 monolayer. When m/z 270 was isolated and then collisionally activated, a fragment ion at m/z 128 was clearly observed above background; the ion at this mass is consistent with the loss of (EtO)(Me)P(O)(34S)H from m/z 270. On the basis Analytical Chemistry, Vol. 71, No. 13, July 1, 1999

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Figure 4. Proposed fragmentation reaction of [VX + H]+, forming m/z 128. A five-membered transition state is indicated, with transfer of the amino proton to the S atom.

Figure 5. MS2 of [VX + H]+, m/z 268. Top, isolation of [VX + H]+. Bottom, fragmentation of [VX + H]+.

Figure 6. Fragmentation of [VX + H]+, m/z 268, by elimination of diisopropylamine, forming a sulfonium ion at m/z 167.

of this result, we estimate that the minimum detectable surface concentration is on the order of 1 × 10-3 monolayer. This estimate should be viewed with some caution, because the FNF ejection of neighboring ions at m/z 269 and 268 (which also produce m/z 128 fragments) may not be quantitative. Nevertheless, the estimate is consistent with detection limits established for other compounds using this technique.18,19 Exposed soil samples were analyzed several times over the course of several months, to provide a preliminary assessment of the stability of intact VX on soil particles. When stored at 4 °C over a period of up to six months, the SIMS spectrum was virtually unchanged. We interpret this to be an indication of VX stability on the soil particle surfaces. This conclusion is contrary to the previous studies of Kingery and Allen,24 who determined that VX degraded on soil surfaces at the rate of 50%/day, and with those (24) Kingery, A. F.; Allen, H. E. Toxicol. Environ. Chem. 1995, 47, 155-84.

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of Verweij and Boter,8 who observed 90% degradation after 2 days. The discrepancy may be explicable by sample storage at lower temperatures in the present case, which may decrease the rate of VX hydrolysis on soil surfaces. In the event of hydrolysis, SIMS should be very amenable to the detection of a variety of degradation products, including ethylmethylphosphonic acid (EMPA)25 and S-[2-(diisopropylamino)ethyl]methylphosphothioic acid (EMPTA).26,27 The negative SIMS spectrum of VX on soil particles was also collected, to assess whether an improved analysis could be achieved using the negative ion mode (Figure 7). A distinctive spectrum originating from VX was recorded, although the (25) Ingram, J. C.; Groenewold, G. S.; Appelhans, A. D.; Delmore, J. E.; Dahl, A. D. Anal. Chem. 1995, 67, 187-95. (26) Yang, Y.-C.; Szafraniec, L. L.; Beaudry, W. T.; Rohrbaugh, D. K. J. Am. Chem. Soc. 1990, 112, 6621-7. (27) Yang, Y.-C.; Baker, J. A.; Ward, J. R. Chem. Rev. 1992, 92, 1729-43.

Figure 7. Anion IT-SIMS spectrum of VX, 0.5 monolayer on soil particles. Table 2. Identification of Salient Ions in the Anion IT-SIMS Spectrum of VX on Soil Particles m/z

ion

m/z

ion

111 95

CH3(HO)P(dO)S(Od)2PsS-

79 63

OdPsSOdPsO-, or PS-

spectrum lacked ions indicative of the intact molecule or the alkyl substituents (Table 2). Ions observed at m/z 79 and 63 arise from POS- and PO2-/PS-, respectively, and originate from all phosphonate compounds that have been analyzed in our laboratory.16,19 M/z 79, 95, and 111 are sulfur analogues to ions that are also typical of phosphonates. The fact that larger anions were not observed was somewhat surprising in light of the fact that another organothiophosphorus pesticide, malathion, produced abundant anions corresponding to cleavage of the phosphothioate ester moiety.11 However, in the case of VX, this process may be discouraged by the facile formation of cations; we note that the total cation abundance is on the order of 4 times greater than the anion abundance. Other significant ions in the anion SIMS spectrum are primarily derived from the aluminosilicate matrix of soil, most notably m/z 60, 76, and 77, which correspond to SiO2-, SiO3-, and HOSiO2-. The abundant ions at m/z 155 and 173 are di- and trihydrated AlSiO4-, and the structure and reactivity of these ions will be the subject of another report. CONCLUSIONS The results of this study show that the highly adsorptive VX molecule is very amenable to detection using a surface analysis

approach, viz., SIMS. The molecule produces a very abundant [VX + H]+, which undergoes a facile fragmentation to form an ammonium cation at m/z 128, as well as several other structurally diagnostic fragment ions at m/z 167, 139, 97, and 86. The major fragmentation process (m/z 268 f 128) can be used as part of an MS2 detection scheme, which overcomes chemical background without resorting to wet chemical separation. The mechanism of this fragmentation was explored using the ion trap to perform D-H exchange in the gas phase: the results showed that the amino proton (deuteron) is transferred to the thiophosphonate moiety in the formation of m/z 128, which rules out protonated diisopropylvinylamine as the structure of this ion. Diisopropylaziridine or diisopropylmethylimminium are postulated. Using the MS2 detection approach together with a combination of a less heavily exposed soil sample, and the analysis of the 34S-bearing [VX + H]+, detection down to 0.002 monolayer was demonstrated. This corresponds to a mass/mass concentration of 3 ppm for a soil sample having a surface area of 3 m2/g. The sensitivity, when combined with the ability of the technique to analyze milligramsize samples without sample extraction, indicates the efficacy of the approach in situations where the analyte is nonvolatile and both the sample size and concentration are problematic. ACKNOWLEDGMENT The support of the U.S. Army Project Manager for NonStockpile Chemical Materiel is gratefully acknowledged. Received for review December 16, 1998. Accepted April 2, 1999. AC981391R

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