Positive-ion fast atom bombardment (FAB) or liquid secondary ion mass spectra (LSIMS) typically provide the molecular mass of the organic sample M via the presence of a dominant protonated molecule (M+H)+ in the mass spectrum. In addition to the proton transfer reaction, cationization processes result in the formation of ions (M+Met)+, in which the metal is derived from metal salts present in the sample as impurities or purposefully added as promoters to the sample solution. Group I metal salts (lithium, sodium, and potassium salts are popular, but rubidium and cesium salts can also be used) result in cationized ions (M+Li)+, (M+Na)+, and (M+K)+. These ions provide multiple confirmations of the molecular mass M of the sample compound. Cationization reactions (as generalized Lewis acid/base reactions) are encountered not only in FAB and LSIMS but also in other desorption ionization methods of mass spectrometry, including field desorption (FD) mass spectrometry. Traditionally, cationization is considered to be the result of the addition of one singly charged metal ion to the neutral sample molecule. If the initial charge state of the metal ion is that of Met2+ or Met3+, the cationized form of the sample molecule observed in the mass spectrum is usually the singly charged species (M+Met)+, and an electron reduction must have occurred. For incorporation of two metal cations, there is always a concomitant loss of hydrogen to form, as an example, (M+2Met-H)+. Recent studies have re-examined the details of the cationization process. For instance, recent work has shown that when the organic sample molecule contains a nitro substituent, a cationization process can occur to form a singly charged ion that contains two alkali ions, viz., (M+2Met)+. The present work highlights yet another form of the cationization reaction.
Krishnan R. Mohan, Michele M. N. Wilson, John Haseltine, and Kenneth L. Busch, "Ion-Pair Cationization Process in Liquid Secondary Ion Mass Spectrometry," Appl. Spectrosc. 50, 537-540 (1996)