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

The mechanism of a dehalogenation reaction that occurs during fast ato m bombardment (FAB) mass spectrometry was examined using halogenated n ucleosides as model compounds. For aglycone-halogenated nucleosides, a n inverse linear relationship exists between the extent of FAB dehalog enation and the calculated electron affinity of an individual nucleosi de. The degree of dehalogenation for a given nucleoside also varies in versely with the calculated electron affinity of most FAB matrices. Th e observed dehalogenation reaction can be completely inhibited when ma trices with positive electron affinities, such as 3-nitrobenzyl alcoho l and 2-hydroxyethyl disulfide, are used. High-performance liquid chro matographic analysis of the bulk glycerol matrix following exposure to the FAB beam indicates measurable amounts of dehalogenated product, s uggesting that this reaction occurs in the condensed phase prior to ga s-phase ion formation. A dehalogenation mechanism involving thermal el ectron capture and subsequent negative charge stabilization is consist ent with these observations.