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.