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

✍ Scribed by Mitsuo Takayama; Yoshiaki Tanaka; Taro Nomura

• Publisher: John Wiley and Sons
• Year: 1993
• Tongue: English
• Weight: 703 KB
• Volume: 28
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/oms.1210281227

✦ Synopsis

The competitive formation of molecular ions M+' and protonated molecules I M + HI + 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 + HI +)/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 + HI +)/I(M+*) ratio. Furthermore, the change of matrix increased the peak intensity ratio of the doubly charged ion [ M + 2HI '+ to [ M + HI + in the FAB mass spectra of angiotensin I and gramicidin S. The addition of acids to the matrix solution increased the

Z([M + H]+)/Z(M+'

) ratio, although such an effect did not always occur. The acetylation of simple aniline compounds markedly increased the Z( I M + HI +)/Z(M+') ratio. It was concluded from these results that the hydrogen bonding interaction between hydroxyl group@) of the matrix and basic site@) of analyte molecules in solution acts advantageously as a quasi-preformed state for [ M + HI + 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 IM + HI + 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.