Gas-phase reactions for selective detection of the explosives TNT and RDX

Abstract

Highly selective gas-phase reactions with ethyl vinyl ether (EVE) of major electron (EI) and chemical ionization (CI) fragment ions of the explosives TNT and RDX have been uncovered. The fragment ion of m/z 210 from TNT undergoes [4⁺ + 2] cycloaddition with EVE to form an oxo-iminium ion of m/z 282, which dissociates by acetaldehyde loss after a [1,5-H] shift to form a quinolynium ion of m/z 238. The fragment ion of m/z 149 from RDX reacts with EVE by a formal vinylation reaction, that is, the elusive cyclic adduct loses ethanol to yield a nitro-iminium ion of m/z 175, which reacts further with EVE to form a second cyclic product ion of m/z 247. Calculations and MS/MS experiments support the proposed structures. These highly characteristic reactions of diagnostic EI and CI fragment ions improve selectivity for TNT and RDX detection.

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Explosives detection is a topic of increasing interest.¹ For such an important task, several analytical methods have been developed, including gas chromatography either with electron capture or nitrogen phosphorus detectors, mass spectrometry² (MS) and high performance liquid chromatography with ultraviolet or MS detection.³ Improved performance, including greater selectivity (to minimize false positives) and sensitivity, remains highly desirable and tandem mass spectrometry⁴ is an obvious candidate. While collision-induced dissociation is very selective, highly complex matrices call for additional selectivity. This might be provided by ion/molecule reactions of mass-selected ions with appropriate neutral reagents. Previous experiments on trinitrotoluene (TNT) suggest that such gas-phase reactions can provide an excellent MS method for rapid, sensitive, and highly selective chemical analysis.⁵

Gas phase cycloadditions are highly selective for compound class and geometry, and are therefore prime candidates for diagnostic reactions. We have systematically studied a variety of such reactions by tandem MS.⁶ For instance, N-alkylpiperidienes form N-alkyl-2-azabutadienyl cations upon 70 eV electron ionization (EI) and these ions undergo structurally diagnostic [4⁺ + 2] cycloadditions with vinyl ethers followed by prompt alcohol loss.^6d Herein we report that highly selective cycloaddition or cyclization reactions (Scheme 1) occur for the TNT fragment ion of m/z 210 (1) and the hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) fragment ion of m/z 149 (2) with ethyl vinyl ether (EVE). Proposed product ion structures are corroborated via MS/MS experiments and AM1 calculations. The EVE cycloaddition of 1, an iminium ion, is similar to that of N-alkyl-2-azabutadienyl cations,^6a,b whereas the EVE cyclization reaction of 2 (a formal vinylation reaction) is unprecedented.

Scheme 1 Reaction mechanisms of vinylation for the ion 2 of m/z 149 and [4⁺ + 2] cycloaddition for the ion 1 of m/z 210.

Interestingly, we found 1 and 2 also to be formed as the major product ions via chemical ionization (CI) of TNT and RDX with 1,2-dichloroethane through H₂O and CH₂NNO₂ loss, respectively.⁷ This increases the sensitivity of the analysis making 1 and 2 excellent candidates for both EI and CI diagnostic ions in TNT and RDX detection. Ions 1 and 2 were individually mass-selected and reacted with EVE and Fig. 1 shows the resulting mass spectra. The ion of m/z 282 (Fig. 1a) is likely the expected regioselective [4⁺ + 2] EVE cycloadduct^6c of 1, which loses acetaldehyde after a [1,5-H] shift to form the second product ion of m/z 238. For 2, the intact [4⁺ + 2] EVE adduct of m/z 221 is not observed likely owing to prompt ethanol loss that forms the ion of m/z 175 (Fig. 1b). Then, cyclization of m/z 175 with a second EVE molecule forms m/z 247. Competitive reactions in both cases form protonated EVE of m/z 73 and ethylated EVE of m/z 101,^6b whereas CID of 1 forms m/z 193, m/z 164, and that of 2 forms m/z 85 and m/z 75 (not shown).

Fig. 1 Product ion mass spectra showing the ion/molecule reactions of EVE with a) ion 1 of m/z 210 and b) ion 2 of m/z 149.

Potential energy surface diagrams for the proposed reactions of 1 and 2 with EVE as elaborated from semi-empirical AM1 calculations⁸ show that both reactions are highly exothermic (Fig. 2). Because they are abundant in 70 eV EI and CI spectra, 1 and 2 are important target ions for TNT and RDX, respectively; hence, their characteristic reactions with EVE herein described function as a highly diagnostic method for TNT and RDX detection.

Fig. 2 Potential energy surface diagram for [4⁺ + 2] cycloaddition and subsequent aldehyde loss from 1 and EVE (right pathway) and cyclization with fast ethanol loss and a second EVE addition from 2 and EVE (left pathway). The energies of the full geometry optimized species in kcal mol⁻¹ are: 1 ion (258.38), EVE (−31.33), adduct1 (1 − EVE) (192.59), adduct2 (1 − EVE − acetaldehyde) (185.81), product (quinolynium) (226.07), acetaldehyde (−41.56), 2 ion (265.39), adduct3 (2 − EVE) (197.38), adduct4 (2 − EVE − ethanol) (264.12), adduct5 [(2 − EVE − ethanol) − EVE] (192.33) and ethanol (−62.66).

This work was supported by NSF (grant CHE327190) and ONR (N00014-02-0834) and the IDHM Program, Center for Sensing Science and Technology (Purdue University) and the NSWC (Crane, Indiana). E.M. and M.N.E. acknowledge funding from FAPESP – Brazil and L.R. from Sandia National Lab.

Notes and references

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