Internal energy distribution in charge inversion mass spectrometry using alkali metal targets

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A charge inversion mass spectrometer (PMS/NMS) by using alkali metal targets has been developed which produces clearer differentiation of hydrocarbon isomers than collision induced dissociation (CID). In PMS/NMS, mass-selected positive ions are made to collide with an alkali metal target, and the resulting negative ions formed upon two-electron transfer are mass analyzed. On the basis of the observed target-density dependence of product ion intensity and thermochemical considerations, we propose that the process of dissociative negative ion formation in PMS/NMS is by way of near-resonant neutralization, followed by spontaneous dissociation of the neutrals and then endothermic negative ion formation. CID spectra and PMS/NMS spectra have been measured for two types of so-called thermometer molecules, namely partially deuterated methanol and W(CO)6. The differences between the CID spectra and the PMS/NMS spectra of partially deuterated methanol demonstrate that the major process in PMS/NMS involves dissociation of the excited neutral species. The internal energy deposition of the charge inversion measured for W(CO)n+ (n = 4–6) ions indicates that dissociation occurs in the energy-selected neutrals formed by way of near-resonant neutralization. The relative peak intensities in the PMS/NMS spectra for some hydrocarbons depend strongly on the alkali metal target used, indicating the importance of internal energy in the dissociation of the excited neutral intermediates. These results demonstrate the utility of PMS/NMS as a technique for the investigation of the dissociation of energy-selected neutral intermediates and for isomeric differentiation.