The role of oxygen in the formation of TNT product ions in ion mobility spectrometry

Abstract

The atmospheric pressure ionization of 2,4,6-trinitrotoluene (TNT) in air yields the (TNT-H) − product ion. It is generally accepted that this product ion is formed by the direct proton abstraction of neutral TNT by O 2 − reactant ions. Data presented here demonstrate the reaction involves the formation of an intermediate (TNT·O 2) −, from the association of either TNT+O 2 − or TNT −+O 2. This intermediate has two subsequent reaction branches. One of these branches involves simple dissociation of the intermediate to TNT −; the other branch is a terminal reaction that forms the typically observed (TNT-H) − ion via proton abstraction. The dissociation reaction involving electron transfer to TNT − appeared to be kinetically favored and prevailed at low concentrations of oxygen (less than 2%). The presence of significant amounts of oxygen, however, resulted in the predominant formation of the (TNT-H) − ion by the terminal reaction branch. With TNT − in the system, either from direct electron attachment or by simple dissociation of the intermediate, increasing levels of oxygen in the system will continue to reform the intermediate, allowing the cycle to continue until proton abstraction occurs. Key to understanding this complex reaction pathway is that O 2 − was observed to transfer an electron directly to neutral TNT to form the TNT −. At oxygen levels of less than 2%, the TNT − ion intensity increased with increasing levels of oxygen (and O 2 −) and was larger than the (TNT-H) − ion intensity. As the oxygen level increased from 2 to 10%, the (TNT-H) − product ion became predominant. The potential reaction mechanisms were investigated with an ion mobility spectrometer, which was configured to independently evaluate the ionization pathways.