Applications of switching reagent ions in proton transfer reaction mass spectrometric instruments for the improved selectivity of explosive compounds

Here we demonstrate the use of a switchable reagent ion proton transfer reaction mass spectrometry (SRI-PTR-MS) instrument to improve the instrument's selectivity for the detection of the explosive compounds 2,4,6 trinitrotoluene (TNT), 1,3,5 trinitrobenzene (TNB), pentaerythritol tetranitrate (PETN), and cyclotrimethylenetrinitramine (RDX). Selectivity is improved owing to the production of different product ions resulting from changes in the reagent ion-molecule chemistry. To be of use as an analytical tool for homeland security applications, it is important that the reagent ions (and hence product ions) can be rapidly changed (within seconds) from H₃O⁺ to another dominant ion species if the technology is to be acceptable. This paper presents measurements that show how it is possible to rapidly switch the reagent ion from H₃O⁺ to either O₂ ⁺ or NO⁺ to enhance selectivity for the detection of the four explosives named above. That switching reagent ions can be done quickly results from the fact that the recombination energies of O₂ ⁺ and NO ⁺ are less than the ionisation potential of H₂O, i.e. charge transfer cannot occur which otherwise would result in ions that can react efficiently with water (e.g. H₂O⁺ + H₂O → H₃O⁺ + OH) leading to H₃O⁺ becoming the dominant reagent ion. Reaction processes observed are non-dissociative charge transfer (O₂ ⁺ with TNT and TNB), dissociative charge transfer (O₂ ⁺ with TNT) and adduct formation (NO⁺ with PETN and RDX). O₂ ⁺ is found to be unreactive with PETN and RDX, and under the conditions operating in the reaction region of the PTR-MS only a low signal associated with NO⁺· TNT was observed. No NO⁺·TNB was detected.