A comparison of charge-exchange, energy-resolved and angle-resolved mass spectrometric methods for studying the energy dependence of ionic fragmentation reactions

Abstract

The energy dependence of the fragmentation of the molecular ions of benzyl methyl ether, nitrobenzene, cyclohexene, n-propanol and nitromethane have been studied by charge-ex-change mass spectrometry (CEMS) and energy-resolved mass spectrometry (ERMS) (low, variable energy collision-induced mass spectrometry). The majority of these systems had previously been studied by angle-resolved mass spectrometry (ARMS). A comparison of the results obtained by CEMS and ARMS shows that the variations of fragment ion yields with scattering angle in ARMS are consistent with an increase in the translational energy converted to internal energy with an increase in the angle of observation. The comparison of the CEMS and ERMS results shows that the translational energy transformed into internal energy in ERMS increases initially with increasing ion kinetic energy but, in many cases, reaches a maximum. Hence, the technique is only suitable for studying the low energy fragmentation reactions of gaseous ions. Contrasting results are obtained by CEMS and ERMS for the nitromethane system and it is proposed, by comparison with photoelectron/photoion coincidence data, that the collision processes produce ions with relatively high internal energies which show a different fragmentation behaviour than the low energy ions produced by charge exchange. The same explanation may apply for the ARMS data, where it has been proposed that the apparently anomalous results are interpretable in terms of angular distribution of products due to kinetic energy release effects in the fragmentation processes.