MECHANISM FOR DEHALOGENATION REACTIONS IN FAST-ATOM-BOMBARDMENT MASS-SPECTROMETRY

The mechanism of a dehalogenation reaction that occurs during fast atom bombardment (FAB) mass spectrometry was examined using halogenated nucleosides as model compounds. For aglycone-halogenated nucleosides, an inverse linear relationship exists between the extent of FAB dehalogenation and the calculated electron affinity of an individual nucleoside. The degree of dehalogenation for a given nucleoside also varies inversely with the calculated electron affinity of most FAB matrices. The observed dehalogenation reaction can be completely inhibited when matrices with positive electron affinities, such as 3-nitrobenzyl alcohol and 2-hydroxyethyl disulfide, are used. High-performance liquid chromatographic analysis of the bulk glycerol matrix following exposure to the FAB beam indicates measurable amounts of dehalogenated product, suggesting that this reaction occurs in the condensed phase prior to gas-phase ion formation. A dehalogenation mechanism involving thermal electron capture and subsequent negative charge stabilization is consistent with these observations.