The acid effect in fast atom bombardment

Fast-atom bombardment of the cyclic acetals: Evidence indicating the predominant involvement of condensed-phase processes in ionization

A series of cyclic acetals, the 2-phenyl-l,3 dioxolanes, and their deuterated analogues were studied by electron ionization (EI), chemical ionization (CI), and fast-atom bombardment (FAB) mass spectrometry to gain insight into the primary ionization processes for these compounds in FAB/liquid secondary ion mass spectrometry. Comparison of EI and CI data with that of FAB led to the conclusion that the predominant [M - H]+ ion observed in FAB for the nondeuterated cyclic acetals cannot to a large extent be rationalized in thermodynamic terms by known gas-phase ion-molecule reactions. Instead, a condensed-phase model in which the multicharged transition state for hydride abstraction is better solvated than the transition state for proton transfer appears to be a plausible explanation for the FAB data obtained for the nonlabeled cyclic acetals; however, this explanation is not entirely sufficient to rationalize the FAB data for the deuterated cyclic acetals. For these compounds, a dramatic time dependence of protonation versus hydride abstraction is observed that suggests that beam-induced reactive species are responsible for hydride abstraction in the condensed phase. This time dependence can be interpreted in terms of a buildup of highly reactive beam-induced species in the bulk of solution. Comparison of the results obtained for deuterated acetals with different surface activities support this hypothesis. (J Am Sot Mass.

m-nitrobenzyl alcohol electrochemistry in fast atom bombardment mass spectrometry

The efficacy of m-nitrobenzyl alcohol (NBA) as a solvent (matrix) for fast atom bombardment (FAB) mass spectrometry of a group of pyrazolate-bridged dirhodium A-frame complexes has been assessed. Although NBA is frequently used to mitigate the formation of artifacts in FAB/MS of organometallics and other materials susceptible to bombardment-induced reactions, substantial evidence indicates that such reactions cause the formation of artifacts in the spectra obtained here. Parallel absorption spectroscopic studies have established that NBA is capable of inducing both oxidation and reduction reactions independent of ion bombardment, depending on analyte reduction half-wave potential (E1/2). From the known electrochemistry of the complexes studied, it can be estimated that 1020 mV > E1/2 > 500 mV for the reaction of NBA serving as a reducing agent, while 500 mV > E1/2 > 424 mV for the reduction potential of NBA. However, in the presence of bombardment the former E1/2 must be at least as low as 356 mY, and the latter E1/2 must be at least as high as 1188 mY. The kinetics of redox reactions involving NBA, and therefore their influence on the appearance of FAB mass spectra, will be highly sample-dependent. However, this study illustrates an important potential role for redox reactions when NBA is used as a solvent, especially in the presence of bombardment in FAB/MS. Although analyte reaction products could be identified, substantial efforts aimed at identifying NBA oxidation and reduction products did not yield any definitive results due to the complexity of product mixtures.