M-Nitrobenzyl alcohol electrochemistry in fast atom bombardment mass spectrometry

Abstract

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 ( E 1 2 ). From the known electrochemistry of the complexes studied, it can be estimated that 1020 Mv > E 1 2 > 500 mV for the reaction of NBA serving as a reducing agent, while 500 mV > E 1 2 > 424 mV for the reduction potential of NBA. However, in the presence of bombardment the former E 1 2 must be at least as low as 356 mV, and the latter E 1 2 must be at least as high as 1188 mV. 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.