Discrimination of C3H4+ isomeric ions by charge inversion mass spectrometry using an alkali metal target

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 C3H4+ ions produced from allene (CH2=C=CH2) and propyne (CH3C=CH) by electron impact. The peak associated with C3H3- in the charge inversion spectra is twice as intense for propyne as for allene, whereas the profile of the peaks associated with C3Hn- (n = 0-2) is similar for both isomeric precursors. The kinetic energy release values at FWHM of the peak associated with C3H3- 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 C3H4 states by near-resonant neutralizations of allene and propyne ions. Collisionally activated dissociation (CAD)spectra using He targets were measured for the same C3H4+ 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.