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

Abstract

AbstractThe 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.