Discrimination of C 3H 4+ isomeric ions by charge inversion mass spectrometry using an alkali metal target

Abstract

Charge inversion mass spectrometry was performed using an MS/MS instrument in which mass-selected positive ions were made to collide with alkali metal targets, and the resulting negative ions formed upon two-electron transfer were mass analyzed. Charge inversion spectra using Cs targets were measured for C 3H 4 + ions produced from allene (CH 2CCH 2) and propyne (CH 3CCH) by electron impact. The peak associated with C 3H 4 - in the charge inversion spectra is twice as intense for propyne as for allene, whereas the profile of the peaks associated with C 3H n − ( n = 0–2) is similar for both isomeric precursors. The kinetic energy release values at FWHM of the peak associated with C 3H 3 − formed from allene ions and propyne ions are 0.28 ± 0.04 eV and 0.64 ± 0.06 eV, respectively. A clear differentiation between the isomeric precursors can be made on the basis of these differences in the charge inversion spectra. The differences observed are attributed to the formation of excited C 3H 4 states by near-resonant neutralizations of allene and propyne ions. Collisionally activated dissociation (CAD) spectra using He targets were measured for the same C 3H 4 + isomeric precursor ions. The CAD spectra are similar for both isomeric precursor ions, although slight differences were detected in the weak peaks associated with the species formed upon CC bond cleavage. Charge inversion mass spectrometry using alkali metal targets was found to provide a much clearer differentiation between the isomeric ions of unsaturated hydrocarbons than CAD.