IONIZATION IN LIQUID SECONDARY-ION MASS-SPECTROMETRY (LSIMS)

The ionization processes in fast atom bombardment (FAB) are reviewed. The subjects covered include the ionization processes in the collision cascade, the nature of preformed ions in the liquid surface, the desorption process, the distinction between prompt and delayed chemistry, reactions of secondary electrons, ion-ion recombination and ionic cluster formation, ion-molecule reactions in the selvedge and preformed ion formation. Finally, the ejection of multiply charged ions and prompt reduction processes are briefly discussed. It is considered that the initial collision cascade ionization processes in FAB are similar to those observed in radiation chemistry. In glycerol, these processes result in protonated and de-protonated glycerol molecules. During a 'bulk desorption' process, the matrix disintegrates and the solvation of the ions gradually weakens. Ion-ion recombination becomes a dominant reaction. Ion-ion cluster formation and proton transfer from solvated analytes are seen to result from reactions between molecules that were close neighbors in the original matrix.