Formation of halide reactant ions and effects of excess reagent chemical on the ionization of TNT in ion mobility spectrometry

The efficiency of chloride reactant ion formation, when chlorinated hydrocarbon reagent chemicals were added to the ionization region of an ion mobility spectrometer, corresponded to the electron attachment rate constant of the chemical. The chemicals investigated here included chloromethane, dichlormethane, trichloromethane, tetrachloromethane and chlorobenzene, with tetrachloromethane producing the greatest amount of chloride reactant ions for the amount of chemical added. Reagent chemicals with smaller electron attachment rate constants required the addition of more chemical to reach functional reactant ion levels. The excess neutral reagent molecules clustered to the chloride reactant ions and reduced the effectiveness of abstracting a proton from 2,4,6-trinitrotoluene (TNT). The effect of clustering was different for each chemical. Tetrachloromethane, which had the least exothermic clustering reaction, had the most effective production of the (TNT–H)− product ion per mole of reagent chemical. Bromide and iodide ions were also investigated as potential reactant ions. Bromide was found to effectively produce the proton abstracted (TNT–H)− ion. Iodide, however, was not a strong enough base to form (TNT–H)− from TNT. There was no apparent transfer of an electron to TNT by chloride, bromide or iodide.