電荷逆転質量分析法の基礎 Ｉ：歴史的展望と４種の電荷逆転の違い

Abstract

The most valuable technique for the structure analysis in mass spectrometry, tandem mass spectrometry (MS/MS), involves the excitation of mass-selected species followed by their dissociation into products, the mass-to-charge ratios of which are also measured. Collisionally Activated Dissociation (CAD) has been used as a most common technique to dissociate the mass-selected precursor ions. Charge inversion mass spectrometry is a special branch of MS/MS method, in which the electric charge of the precursor ions is opposite to that of the secondary product ions. Charge inversion mass spectrometry is classified into four types depending on the electric charge and time scales of collisions. Electric charges, which are either positive ions or negative ions, provide the characteristic difference between reactants and products. Either successive single electron transfer in two collisions or double electron transfer in one collision depend time scales of collisions. Charge inversion mass spectrometry using collisions with gaseous targets in the keV collision energy range have provided insights into the structures and reactions of ions and neutral molecules. The characteristics of these four types of charge inversion experiments are presented in terms of the reaction endothermicities, the cross sections and their dependence on the target species. In the case of rare gas or simple molecular targets, double electron transfer in one collision is effective to form positive ions from negative ions, while in the case of alkali metal targets successive single electron transfers in two collisions is effective to form negative ions from positive ions. While CAD is due to dissociation of activated ions with broad internal energy distributions, the charge inversion processes using alkali metal target is due to dissociation of energy-selected neutral species with narrow internal energy distributions.