Fundamental aspects concerning the appearance of electrohydrodynamic and fast atom bombardment mass spectra

This work assesses the impact of several controllable parameters on the appearance of electrohydrodynamic (EH) and fast atom bombardment (FAB) mass spectra, with the aims of gaining a better understanding of the ionization/sampling mechanisms and of providing practical guidance to mass spectrometry users. The insight gained from these fundamental studies is then applied to studies which consider the energetics associated with a series of bridged dirhodium complexes. The percent ion pairing evident in EH mass spectra of a dication with a series of anions in glycerol correlates with the enthalpy of hydration for the ions. Ion-solvent interactions are more important than ion-field interactions in accounting for pairing-induced changes in EH MS sensitivity to the doubly-charged cation. Thus, the use of anions that promote more extensive ion pairing will enhance the overall sensitivity to multiply-charged ions that usually interact strongly with the solvent. However, ion pairing will reduce sensitivity to singly-charged ions due to neutralization. Similar trends are observed in FAB MS, despite the invasive effects of the primary atom beam. By careful selection of supporting electrolyte, viscosity and ion pairing can be separately controlled. It was thereby possible to discern that mass transport and ion pairing effects on EH MS sensitivity of a series of quaternary ammonium ions are of roughly the same magnitude. Beam-induced chemical damage during FAB of organic dyes can be minimized by lowering the primary atom flux, by raising the primary atom energy, and by selecting a matrix with radical scavenging properties (e.g., m-nitrobenzyl alcohol). The relative importance in minimizing this chemical damage is choice of matrix > primary atom flux > primary atom energy, but optimization of the parameters can involve a trade-off between sensitivity and damage. The effect of these parameters on thermal damage (fragmentation) is much less than the effect on chemical damage. Comparison of the extent of damage for each dye shows that the extent of beam damage does not depend simply on analyte E^o.