Competitive formation of M+˙ and [M + H]+ ions under fast atom bombardment conditions

The competitive formation of molecular ions M^+˙ and protonated molecules M + H]⁺ under fast atom bombardment (FAB) conditions was examined using various kinds of organic compounds. The use of protic/hydrophilic matrices such as thioglycerol and glycerol resulted in relatively large values of the peak intensity ratio I(M + H]⁺)/I(M^+˙) compared with the use of relatively aprotic/hydrophobic matrices such as m-nitrobenzyl alcohol and o-nitrophenyl octyl ether. The change of matrix from thiol-containing such as thioglycerol and dithiothreitol to alcoholic such as glycerol and pentamethylene glycol increased the I(M + H]⁺)/I(M^+˙) ratio. Furthermore, the change of matrix increased the peak intensity ratio of the doubly charged ion M + 2H]²⁺ to M + H]⁺ in the FAB mass spectra of angiotensin I and gramicidin S. The addition of acids to the matrix solution increased the I(M + H]⁺)/I(M^+˙) ratio, although such an effect did not always occur. The acetylation of simple aniline compounds markedly increased the I(M + H]⁺)/I(M^+˙) ratio. It was concluded from these results that the hydrogen bonding interaction between hydroxyl groups(s) of the matrix and basic site(s) of analyte molecules in solution acts advantageously as a quasi-preformed state for M + H]⁺ formation, and that the presence of significant proton acceptor(s) such as carbonyl group in analytes hinder the M^+˙ formation which may generally occur under FAB conditions. The formation of M^+˙ and M + H]⁺ ions seemed to occur competitively, reflecting or according to the interaction or solvation states between the analyte and matrix molecules in solution and the structural characteristics of the analytes.