The mass spectrometry of neutral species produced from mass-selected ions by collision and by charge exchange. Experiments with tandem collision gas cells

Abstract

The neutralisation—reionisation mass spectra (NRMS) of a number of organic cations have been measured. Neutralisation was achieved by electron transfer from Xe atoms in the first of two collision cells in the second field-free region of a double focussing mass spectrometer of reversed geometry. Reionisation took place by collisional excitation with He atoms in the second cell; ions passing beyond the first cell were prevented from entering the second by an ion beam deflector electrode. The NRMS of benzene and acetic acid molecular ions were similar to the normal electron impact (EI) mass spectra and collisional activation (CA) mass spectra of the two molecules. Acetic acid molecules generated from the unimolecular dissociation of the proton bound dimer [CH 3COOH] 2H + had a collision-induced dissociative ionisation (CIDI) mass spectrum closely similar to that derived from EI. Effects arising from neutral species generated by normal collisionally induced fragmentations of the ions in the first cell were shown to be small. The significant differences between the EI, CA and NRMS of methyl formate and its CIDI mass spectrum, derived similarly from [HCOOCH 3] 2H +, were considered indicative of the presence of rearranged molecular ions among those sampled by CA, some, but not all, of which produced stable neutral species. The NRMS of the acetyl, carboxyl and formyl cations were measured under conditons of good energy resolution. The observations were interpreted in terms of the dissociation characteristics of the intermediate free radicals, each of which underwent sufficient collisional energisation during the electron transfer from Xe, to decompose by simple bond cleavages. For ▪, the results were in qualitative agreement with recent estimates for the energy barriers for the competing reactions yielding ▪ and ▪.