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Detection of Chemical Weapon Agents and Simulants Using Chemical Ionization Reaction Time-of-Flight Mass Spectrometry Rebecca L. Cordell, Kerry A. Willis, Kevin P. Wyche, Robert S. Blake, Andrew M. Ellis, and Paul S. Monks*
Department of Chemistry, University of Leicester, Leicester, UK
Chemical ionization reaction time-of-flight mass spectrometry (CIR-TOF-MS) has been used for the analysis of prepared mixtures of chemical weapon agents (CWAs) sarin and sulfur mustard. Detection of the CWA simulants 2-chloroethyl ethyl sulfide, triethyl phosphate, and dimethyl methyl phosphonate has also been investigated. Chemical ionization of all the agents and simulants was shown to be possible using the CIR-TOF-MS technique with a variety of reagent ions, and the sensitivity was optimized by variation of instrument parameters. The ionization process was found to be largely unaffected by sample humidity levels, demonstrating the potential suitability of the method to a range of environmental conditions, including the analysis of CWAs in air and in the breath of exposed individuals. Chemical weapons were first used on a large scale in warfare in World War 1, where chlorine and sulfur mustard were used extensively. Subsequently, chemical weapons have been used in several conflicts including the Iran-Iraq war, when Iraq used mustard gas and possibly the nerve agent tabun against troops and Kurdish civilians.1 In 1997, the Chemical Weapons Convention (CWC)2 came into effect, prohibiting the development, production, stockpiling, and use of chemical weapons coupled with monitoring of the use of precursor and degradation compounds. The CWC is aimed at reducing the possibility of the use of chemical weapons on the battlefield, but there is still serious concern that terrorist groups or rogue states could use chemical warfare agents against military or civilian targets. Certain chemical weapons can be easily and cheaply produced without the need for high-level expertise, so they offer an accessible method of producing casualties and mass panic. An example of this was the attacks using the nerve agent sarin on the Tokyo subway system in 1995 by the Aum Shinrikyo cult that left 12 dead and over 5000 injured.3,4 * To whom correspondence should be addressed. Fax: +44 116 252 3789. Tel: +44 116 252 2141. E-mail:
[email protected]. (1) Marik, P. E.; Bowles, S. J. Intensive Care Med. 2002, 17, 147-161. (2) Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on their Destruction, Technical Secretariat of the Organisation for Prohibition of Chemical Weapons, The Hague, 1997. (3) Tokuda, Y.; Kikuchi, M.; Takahashi, O.; Stein, G. H. Resuscitation 2006, 68, 193-202. (4) Vale, A. Przegl Lek 2005, 62, 528-532. 10.1021/ac071193c CCC: $37.00 Published on Web 09/26/2007
© 2007 American Chemical Society
Many analytical methods have been developed for the detection of the active chemical constituents of chemical weapons, chemical weapon agents (CWAs), and their breakdown products in soil,5-9 groundwater,10,11 and air.12 GC/MS has been favored in the past owing to its suitability for the analysis of volatile, thermally stable agents and its general reliability,11,13 whereas LC-MS is often the favored method for analysis of breakdown products8,14-18 and has the advantage over GC/MS of not requiring sample pretreatment.6,19 Another spectrometric method commonly employed for agent detection is ion mobility spectrometry (IMS), and hand-held monitors currently used by the military for direct agent vapor detection such as the improved chemical agent monitor20 are of this type. Instruments incorporating IMS technology are the mainstay in several countries for military and civilian defense purposes.21 Ionization of the chemical agents with IMS can be through the formation of cations or anions. Sulfur mustards, which are well-known vesicants (see later), are usually detected through formation of oxygenated anions.22 In contrast, G-agents are most commonly detected through the positive ions made following proton transfer. This (5) Black, R. M.; Clarke, R. J.; Read, R. W.; Reid, M. T. J. J. Chromatogr., A 1994, 662, 301-321. (6) D’Agostino, P. A.; Hancock, J. R.; Chenier, C. L. Eur. J. Mass Spectrom. (Chichester, Eng.) 2003, 9, 609-618. (7) D’Agostino, P. A.; Hancock, J. R.; Chenier, C. L. J. Chromatogr., A 2004, 1058, 97-105. (8) D’Agostino, P. A.; Hancock, J. R.; Provost, L. R. J. Chromatogr., A 2001, 912, 291-299. (9) Hooijschuur, E. W.; Kientz, C. E.; Brinkman, U. A. J. Chromatogr., A 2002, 982, 177-200. (10) Sega, G. A.; Tomkins, B. A.; Griest, W. A. J. Chromatogr., A 1997, 790, 143-152. (11) Hooijschuur, E. W. J.; Hulst, A. G.; de Jong, A. L.; de Reuver, L. P.; van Krimpen, S. H.; van Baar, B. L. M.; Wils, E. R. J.; Kientz, C. E.; Brinkman, U. A. T. TrAC-Trends Anal. Chem. 2002, 21, 116-130. (12) Stan’kov, I. N.; Sergeeva, A. A.; Sitnikov, V. B.; Derevyagina, I. D.; Morozova, O. T.; Mylova, S. N.; Forov, V. B. J. Anal. Chem. 2004, 59, 447-451. (13) Kientz, C. E. J. Chromatogr., A 1998, 814, 1-23. (14) Smith, J. R.; Shih, M. L. J. Appl. Toxicol. 2001, 21 (Suppl 1), S27-S34. (15) Kireev, A. F.; Rybal’chenko, I. V.; Savchuk, V. I.; Suvorkin, V. N.; Tipukhov, I. A.; Khamidi, B. A. J. Anal. Chem. 2002, 57, 842-851. (16) Wada, T. Appl. Organomet. Chem. 2006, 20, 573-579. (17) Rohrbaugh, D. K.; Yang, Y. C. J. Mass Spectrom. 1997, 32, 1247-1252. (18) Lui, Q.; Hu, X.; Xie, J. Anal. Chim. Acta 2004, 512, 93-101. (19) Black, R. M.; Read, R. W. J. Chromatogr., A 1998, 794, 233-244. (20) Hill, H. H.; Martin, S. J. Pure Appl. Chem. 2002, 74, 2281-2291. (21) Karpas, Z. In Encyclopedia of Analytical Chemistry; Mayers, R. A., Ed.; Wiley: Chichester, 2006. (22) Stach, J.; Baumbach, J. I. Int. J. Ion Mobility Spectrom. 2002, 5, 1-21.
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is illustrated by the G-agent simulant dimethyl methyl phosphonate (DMMP), which forms the protonated monomer (DMMP + H)+ 23 and the homogeneous proton-bound dimer ((DMMP)2 + H)+.23,24 Biological analysis related to CWAs has tended to concentrate on analysis in blood25,26 and urine.27,28 The ability, however, to directly measure these compounds in breath would offer a rapid noninvasive technique that could be used, for example, as a means of triage at an incident. A breath-based analysis would allow the efficient targeting of limited resources, eliminating the problem of the “worried well”, where concerned but uninjured members of the public tie up medical resources. The strain on the emergency services was demonstrated by the subway attacks in Japan, where the number of possible casualties seeking medical attention stretched the medical resources to their limits, but it is estimated that roughly 85% of those reporting to hospitals in the aftermath of the sarin attack were the worried well.29 The two most well-known groups of CWAs are the vesicants and the nerve agents. Sulfur mustards are classic vesicants. The destructive effects on skin and mucous membranes, combined with the lack of an antidote, make them attractive weapons to terrorists. They are also inexpensive, potentially obtainable from stockpiles and relatively easy to manufacture.30 Nerve agents are also attractive for use by terrorists, as demonstrated by the Japanese attacks. Nerve agents are more acutely toxic than the vesicants, and although antidotes exist, they must be administered soon after exposure to be effective. The mode of action of nerve agents is to inhibit acetylcholinesterase (AChE), resulting in acetylcholine excess.31 Inhibition of AChE leads to symptoms including the muscarinic effects of excessive secretions, sweating, miosis, bronchospasm, and bradycardia and the nicotinic effects of muscle spasms, weakness, and finally paralysis. Final results are loss of consciousness, convulsions, and depression of central respiratory drive leading to death.1 Given the choice of CWAs available for use in weapons, the ability to rapidly detect and identify the specific agent responsible for an incident would potentially be of major benefit to the security and medical services. The aim of the work described here was to investigate the feasibility of using chemical ionization reaction time-of-flight mass spectrometry (CIR-TOF-MS) to detect the presence of sulfur mustards and nerve agents in air. CIR-TOFMS32-34 is a newly developed variant of proton-transfer reaction (23) Eiceman, G. A.; Kelly, K.; Nazarov, E. G. Int. J. Ion Mobility Spectrom. 2002, 5, 22-30. (24) Eiceman, G. A.; Stone, J. A. Anal. Chem. 2004, 76, 390A-397A. (25) Noort, D.; Hulst, A. G.; Platenburg, D. H.; Polhuijs, M.; Benschop, H. P. Arch. Toxicol. 1998, 72, 671-675. (26) Nagao, M.; Takatori, T.; Maeno, Y.; Isobe, I.; Koyama, H.; Tsuchimochi, T. Legal Med. 2003, 5, S34-S40. (27) Riches, J.; Morton, I.; Read, R. W.; Black, R. M. J. Chromatogr., B: Anal. Technol. Biomed. Life Sci. 2005, 816, 251-258. (28) Riches, J.; Read, R. W.; Black, R. M. J. Chromatogr., B: Anal. Technol. Biomed. Life Sci. 2007, 845, 114-120. (29) Sidell, F. L., Proceedings of the Seminar on Responding to the Consequences of Chemical and Biological Terrorism, July 12, 1995. (30) Saladi, R. N.; Smith, E.; Persaud, A. N. Clin. Exp. Dermatol. 2006, 31, 1-5. (31) Gunderson, C. H.; Lehmann, C. R.; Sidell, F. R.; Jabbari, B. Neurology 1992, 42, 946-950. (32) Blake, R. S.; Wyche, K. P.; Ellis, A. M.; Monks, P. S. Int. J. Mass Spectrom. 2006, 254, 85-93. (33) Wyche, K. P.; Blake, R. S.; Ellis, A. M.; Monks, P. S.; Brauers, T.; Koppmann, R.; Apel, E. C. Atmos. Chem. Phys. 2007, 7, 609-620.
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mass spectrometry (PTR-MS) and offers the ability to detect and to quantify trace levels of organic compounds in air. Neither PTRMS nor CIR-TOF-MS has previously been assessed for the detection of CWAs. Here we show, through studies both with CWA simulants and with the “live” agents sulfur mustard and sarin that CIR-TOF-MS holds considerable promise as a rapid and sensitive means for detecting CWAs. METHODOLOGY Instrumentation. All experiments were carried out using CIRTOF-MS, which has been described in detail elsewhere.35 The instrument consists of an ion source containing a 44.4 MBq 241Am R-particle source, which is used to ionize the desired CI reagent while entrained in a nitrogen buffer gas. This source is based on a design developed by Hanson et al.36 In this work, three CI reagent ions have been considered, H3O+, NO+, and O2+. The reagent ions are drawn into a 10 cm long drift tube, where they mix and react with the analyte gas. Unreacted reagent ions along with product ions are guided along the drift tube by an electric field. The instrument was operated at a range of E/N values, where E is the electric field and N is the total gas number density in the drift tube. In fact the drift tube was essentially divided into two different regions by virtue of an increased electric field between the final two electrodes at the downstream end. This higher field section served to reduce the degree of ion clustering and is referred to as the collision dissociation cell (CDC). In reporting E/N values later in this article, we have quoted two numbers, the first for the main part of the drift tube and the second for the CDC. Beyond the CDC is a 200 µm exit aperture, through which ions can leave and enter the ion focusing optics of a reflectron time-of-flight mass spectrometer. Simulant Experiments. The simulants 2-chloroethyl ethyl sulfide (2-CEES; >98%), triethyl phosphate (TEP; >99.8%) and DMMP (>97%) (Sigma Aldrich) were used to simulate sulfur mustard (2-CEES) and the G-agents (TEP and DMMP), respectively. The structures of these simulants are illustrated in Figure 1. The headspace vapor from these simulants was examined with three different reagent ions: H3O+, NO+, and O2+. H3O+ ions were generated from saturated water vapor transported by high-purity nitrogen (BOC gases). The source gas for the NO+ reagent ions was a 600 ppmV mixture of NO in nitrogen (BOC Gases) while for O2+, a cylinder of pure O2 (BOC Gases) was employed. The reagent gas mixtures (except water/N2) were passed through coils cooled by dry ice to remove H2O. All CI reagents were introduced at a flow rate of 30 sccm. To assess the effect of humidity, experiments were also carried out with H3O+ in combination with a heated (35 °C) and humidified (80% RH) analyte flow. The total gas flow through the analyte inlet was 230 sccm. Instrument quantification and calibration was carried out with three of the simulants: 2-CEES (3-40 ppbV), TEP (5-80 ppbV), and DMMP (4-60 ppbV) supplied using diffusion vials (Ecoscientific). Standards were delivered in a flow of nitrogen at a given (34) Wyche, K. P.; Blake, R. S.; Willis, K. A.; Monks, P. S.; Ellis, A. M. Rapid Commun. Mass Spectrom. 2005, 19, 3356-3362. (35) Blake, R. S.; Whyte, C.; Hughes, C. O.; Ellis, A. M.; Monks, P. S. Anal. Chem. 2004, 76, 3841-3845. (36) Hanson, D. R.; Greenberg, J.; Henry, B. E.; Kosciuch, E. Int. J. Mass Spectrom. 2003, 223, 507-518.
Figure 1. Chemical structures of investigated compounds.
concentration using a Kintec gas diluter system (model 491-M). The quantitative effects of humidity on the instrument sensitivity to 2-CEES and TEP were investigated using a warmed (35 °C) sample flow with relative humidities of 0, 20, 40, 60, and 80%. The instrument was operated at an E/N of 123/194 Td for the head spacing experiments and 90/197 Td for all other experiments except in those (detailed later) where the influence of E/N was being specifically explored. Live Agent Experiments. To demonstrate that detection could be extended to actual CWAs a vesicant, sulfur mustard, along with a representative nerve agent, sarin, were chosen for investigation. These experiments were carried out using the specialized laboratory facilities and expertise available at the Defence Science and Technology Laboratory, UK. Calibration tests for the CWAs were carried out by delivering vapor from the agent at room temperature diluted in dry synthetic air. This mixture was delivered to the instrument at a flow rate of 230 sccm over a concentration range of 3-190 ppbV for sulfur mustard and 5-170 ppbV for sarin. All analysis of the CWAs was carried out using only H3O+ as the CI reagent. RESULTS AND DISCUSSION Simulant Detection. The head spaces of all the simulants were examined using each of the reagent ions H3O+, NO+, and O2+. All spectra represent 1-min data averaged over a 10-min time period. The different reagent ions act to ionize the analyte molecules by different means. H3O+ ionizes by proton transfer to any molecule that possesses a higher proton affinity than water. H3O+ is ideal for many applications, but NO+ and O2+ have also been shown to provide alternative information.34 NO+ is a relatively soft ionizer, and the ionization occurs mainly through either charge transfer or hydride ion transfer, thereby yielding different ion products to H3O+, which can sometimes assist in distinguishing between isobaric species.34 O2+ is a more aggressive CI reagent and can be useful for detecting those species which cannot be detected by proton transfer. H3O+, NO+, and O2+ have also
been used extensively in the closely related technique secondary ion flow tube mass spectrometry, where they have been applied to the detection of a wide array of organic compounds, as reviewed by Smith and Sˇ panel.37 2-CEES was examined with all three CI reagent ions, producing distinct mass spectra, as shown in Figure 2. The spectrum obtained using H3O+ is shown in Figure 2a. It is dominated by the protonated parent species, [M + H]+ (m/z ) 125, 127), along with a second peak produced by the loss of HCl (m/z ) 89), and a minor peak produced upon the loss of C2H5Cl (m/z ) 61). A repeat of this experiment using humidified (80%) inlet gas produced a virtually unchanged spectrum, as can be seen in Figure 2b. The spectra obtained with NO+ and O2+ as the CI reagents are shown in Figure 2c and d, respectively. In contrast to the use of H3O+, there is little evidence of parent ion species. Instead, both NO+ and O2+ lead to extensive fragmentation, with very similar spectra being obtained for both of these reagent ions. The main peaks correspond to fragments at m/z ) 89 and 75 produced by loss of Cl and CH2Cl, respectively. There is also another substantial peak at m/z ) 47, which may be due to SCH3+. TEP was successfully ionized with all three CI reagent ions, as shown in Figure 3. The H3O+ spectrum (Figure 3a) shows that almost the only ion produced is the protonated molecular ion [M + H]+ at m/z ) 183, and the spectrum is largely unaltered by using a humidified flow (see Figure 3b). The spectrum generated with NO+ (see Figure 3c) is dramatically different from the H3O+ case, showing a variety of fragment ions. There is also a small amount of [M + H]+ observed, which must result from H3O+ produced by traces of water vapor present in the instrument. The spectrum in Figure 3d derived from the use of O2+ as the CI reagent is dominated by a single, unassigned, fragment ion at m/z ) 99. Once again there is a weak [M + H]+ peak (m/z ) 183) that is almost certainly the result of slight water contamination in the system. Figure 4 shows the findings from DMMP experiments. The spectrum obtained using H3O+ (Figure 4a) shows the production of the protonated molecular ion [M + H]+ at m/z ) 125. This pattern is unaffected by humidity as shown in Figure 4b. Figure 4c shows that when using NO+ as the reagent ion the dominant species produced is the adduct ion [M + NO]+ at m/z ) 154, which is a different outcome from the other G-agent stimulant, TEP (which produced a series of fragment ions). The [M + H]+ ion is also seen in the NO+ experiment due to traces of water vapor yielding H3O+ contamination in the system. Surprisingly, the reaction between O2+ and DMMP seems to be negligible, and thus, the mass spectrum registers only an [M + H]+ peak from trace H3O+ contamination. To conclude this section, CIR-MS mass spectra have been successfully recorded for the CWA simulants 2-CEES, TEP, and DMMP using all three CI reagent ions, H3O+, NO+, and O2+. However, the spectra produced by H3O+ appear to be preferable for simulant detection owing to their relative simplicity. Another advantage of the H3O+ spectra is that the signal is concentrated mainly as [M + H]+, which has a higher mass than the most common organic constituents found in air and expelled in (37) Smith, D.; Spanel, P. Mass Spectrom. Rev. 2005, 24, 661-700.
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Figure 2. Mass spectra from headspace of 2-CEES with (a) H3O+, (b) H3O+ with humidified air (80%), (c) NO+, and (d) O2+.
Figure 3. Mass spectra from headspace of TEP with (a) H3O+, (b) H3O+ with humidified air (80%), (c) NO+, and (d) O2+.
breath.38,39 Consequently, the background mass signals from normal organic constituents in air would be unlikely to confuse CWA identification. Simulant Sensitivities. The concentration of an organic compound in an air sample can in principle be calculated from (38) Moser, B.; Bodrogi, F.; Eibl, G.; Lechner, M.; Rieder, J.; Lirk, P. Respir. Physiol. Neurobiol. 2005, 145, 295-300. (39) Lindinger, W.; Hansel, A.; Jordan, A. Int. J. Mass Spectrom. 1998, 173, 191241.
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the CIR-MS response with knowledge of the instrument operating conditions, the product ion fragmentation behavior, and the rate coefficient for the ion-molecule reaction.33 This methodology is restricted in its accuracy for several reasons, one of which is the uncertainty limits on the rate coefficient. Consequently, the sensitivity response for three test compounds has been determined from stepwise dilution calibration measurements carried out using H3O+ as the CI reagent. For 2-CEES, the sensitivity for the [M + H]+ ion at E/N settings of 90/197 was determined to
Figure 4. Mass spectra from headspace of DMMP with (a) H3O+, (b) H3O+ with humidified air (80%), (c) NO+, and (d) O2+.
Figure 5. Calibration plots for (a) 2-CEES at various relative humidities, (b) TEP at various relative humidities, and (c) 2-CEES at various E/N values, under dry conditions.
be 41 normalized counts per second (ncps) ppbV-1, for TEP 65 ncps ppbV-1, while that of DMMP was 86 ncps ppbV-1. Ion count rates when using H3O+ as the primary reagent ion are normalized to a nominal 1 million reagent ion counts of ([H3O+] + [H3O+‚H2O] + [H3O+‚2H2O]) and are expressed as ncps. Small positive intercepts can be observed in the calibration plots due to low background signal levels present in the relevant mass channels. These sensitivities compare quite well to those of other previously measured VOCs with the same technique, where a range of compounds had sensitivities determined to be in the range 5-60 ncps ppbV-1.33 The instrument showed an excellent linear response for 2-CEES, TEP, and DMMP in the concentration
ranges tested. The sensitivities determined show that detection in the low ppbV range on a 1-min time scale is achievable. Limits of detection were calculated as previously described33 and assumes a power relationship between analyte concentration and the signal-to-noise ratio. Sensitivities in this range allow for the detection of nerve agents and sulfur mustards far below their lethal levels. The agents have a lethal concentration-time dose for 50% of the exposed individuals (LCt50) of around 1-60 ppmV min-1 for nerve agents and 230 ppmV min-1 for sulfur mustard.40 (40) Dwyer, A.; Eldridge, J.; Kernan, M. Jane’s Chem-Bio Handbook; Jane’s Information Group: Alexandria, VA, 2003.
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Figure 6. Mass spectra from 160 ppbV sulfur mustard with E/N values of (a) 61/121, (b) 75/150, (c) 90/197, and (d) 123/194, under dry conditions. Table 1. Effect of Relative Humidity on the Detection Sensitivity for TEP and 2-CEES sensitivity/ncps ppbV-1 humidity/%
2-CEES
TEP
DMMP
0 20 40 60 80
41.3 44.0 42.6 44.9 43.2
64.6 79.9 77.5 76.9 76.8
88.1 104.1 99.0 101.4 102.0
(1) Humidity Effects. The effects of humidity on analyte detection are important for applications in environmental monitoring or for casualty triage via breath analysis, since sample humidity may vary over a wide range in this type of work. For 2-CEES, humidity had very little effect on the absolute detection sensitivity, and the linearity with respect to concentration was clearly maintained across the full range of humidities (Figure 5a). Humidity had a slightly larger effect on the ionization of TEP and DMMP, which showed a small increase in sensitivity when exposed to humidity compared to dry conditions but again linearity was maintained (Table 1 and Figure 5b). (2) Effects of E/N. The value of E/N can have a significant effect on the ionization patterns and the sensitivity to specific analytes.33 In particular, at higher E/N there is an increased tendency for ions to fragment because of the increase in average collision energies. Reducing the E/N was shown to improve the detection sensitivity of 2-CEES, TEP, and DMMP (the case of 2-CEES is shown in Figure 5c). Consequently, in order to optimize sensitivity, a value of 90/197 Td looks to be the most suitable; much below this value, the production of water clusters throughout the spectrum starts to be a problem. The combined calibration/humidity experiments with 2-CEES, TEP, and DMMP show that (i) the sensitivity of the CIR-TOF8364 Analytical Chemistry, Vol. 79, No. 21, November 1, 2007
MS technique is suitable for the rapid detection of CWAs in breath samples and (ii) the response of the instrument is linear in the concentration of the CWA. High humidity has only modest effects on the CWA detection process, which bodes well for monitoring in human breath samples where relative humidities are ∼100%. Optimization of E/N can also clearly have a significant effect in improving the sensitivity of the technique by decreasing ion fragmentation. Detection of CW Agents. The experiments on CWAs, sulfur mustard and sarin, employed only H3O+ as the CI reagent, as this was found to be the best choice from the simulant experiments. Sulfur mustard was successfully ionized with H3O+, as shown in Figure 6, over a range of instrument conditions. The spectra show that the dominant ion produced at four different E/N conditions was the protonated molecular ion [M + H]+ at m/z ) 159, 161, and 163, but as the E/N was increased, a decrease in relative abundance of the [M + H]+ ion occurred accompanied by an increase in the fragment ion [(CH3CH2SCH2CH2Cl) + H]+ at m/z ) 125 and 127 as expected given the comments in the previous section. Figure 7 shows that the ionic products from the reaction of sarin with H3O+ are strongly influenced by E/N. The [M + H]+ ion (m/z ) 141) only occurs as the dominant ion when E/N is extremely low, at 41/100, where the high abundance of water cluster ions also brings about the production of the [(M + H) + H2O]+ adduct ion. As the E/N increases, the abundance of the protonated molecular ion declines and fragmentation begins to dominate. At the highest E/N conditions of 90/197, only the fragment ion [(CH3PFO2H) + H]+ (m/z ) 99) was detected. Agent Calibration. As with the CWA simulants, the response of the instrument to sulfur mustard and sarin was found to be
Figure 7. Mass spectra from 140 ppbV sarin with E/N values of (a) 41/100, (b) 61/121, (c) 75/150, and (d) 90/197, under dry conditions. Table 2. Effects of E/N on Detection Sensitivity for the Simulants 2-CEES, TEP, and DMMP, and for the CWAs Sarin (GB) and Sulfur Mustard (HD)a 2-CEES
TEP
DMMP
HD
GB
E/N
sensitivity/ ncps ppbV-1
LOD / ppbV
sensitivity/ ncps ppbV-1
LOD / ppbV
sensitivity/ ncps ppbV-1
LOD / ppbV
sensitivity/ ncps ppbV-1
LOD / ppbV
sensitivity/ ncps ppbV-1
LOD / ppbV
41/100 61/121 75/150 90/197 123/194
46.5 51.9 41.3 23.9
4.3 3.3 0.8 10.8
55.5 62.5 64.6 52.1
6.4 3.2 4.7 9.2
77.4 81.1 85.7 65.2
1.1 1.7 1.3 9.1
40.8 45.6 42.8 25.9
4.6 3.3 12 15.6
99.2 50.3 1.9 -
3.1 3.4 55.6 -
a
Limits of detection (LODs) represent 1-min detection limits at a signal-to-noise ratio of 3:1.33
Figure 8. Effect of E/N on the detection response for sarin, under dry conditions.
linear with concentration. As an illustration, Figure 8 shows calibration profiles for sarin for five different E/N values. The effect of E/N for sulfur mustard was found to be modest, but for sarin, the detection was found to be highly sensitive to E/N, due to the almost complete loss of the [M + H]+ signal at high E/N values. The sensitivities achieved for the live agents of ∼40 ncps ppbV-1 for sulfur mustard and 50-90 ncps ppbV-1 for sarin at the lower range of E/N values allow rapid and unambiguous detection in the low ppbV range.
From this work, it appears that 2-CEES is acting as an excellent simulant for sulfur mustard, with sensitivities within a similar range of ∼40 ncps ppbV-1 (Table 2). The effects of E/N on the sensitivity of the molecular ion are also similar with a marked reduction in sensitivity at the highest value (123/194) but similar sensitivities for the lower values with the optimal sensitivities achieves at an E/N of 75/150. These results imply that future work carried out with the simulant 2-CEES would be likely to be directly transferable to full sulfur mustard. However, the G-agent simulants TEP and DMMP exhibit behavior somewhat different from sarin despite their chemical similarities. Their behavior in terms of humidity dependence and effects of E/N are comparable to each other, but they demonstrate much lower sensitivity to E/N changes in the range tested when compared to sarin. This implies that TEP and DMMP may not in fact be the most suitable simulants for sarin detection studies using PTR-MS and CIR-MS. CONCLUSIONS The work reported here shows that the technique of CIR-TOFMS can be successfully applied in the detection of chemical Analytical Chemistry, Vol. 79, No. 21, November 1, 2007
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weapons simulants and, more specifically, the single-component
ACKNOWLEDGMENT
synthetic mixtures of the CWAs sulfur mustard and sarin. Using
The authors thank Matthew Bellerby and Gary Hodges of Dstl, Porton Down, for assistance with the “live” agent experiments.
H3O+ as the reagent ion, the response was found to be linear over the concentration ranges tested. Work with the simulants showed only a modest decrease in sensitivity with increased humidity, revealing that humidity would not be a hindrance to analysis. The technique offers the potential for rapid detection of various CWAs
Received for review June 6, 2007. Accepted August 11, 2007.
in the environment and in on-scene triage via patient breath analysis.
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AC071193C
