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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.

The relative effects of adjustable fast atom bombardment (FAB) parameters (choice of matrix, primary atom flux, and primary atom energy) on the appearance of FAB spectra (including signal-to-noise, signal-to-background, and signal-to-matrix ratios) of several organic dyes have been investigated. Beam-induced chemical damage is 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 > (nominal) primary atom energy, but optimization of the parameters involves a trade-off between sensitivity and damage. The effect of these parameters on thermal damage (fragmentation) is much less. It can be concluded from comparison of the dyes that the extent of beam damage does not depend simply on the standard reduction potential of the analyte.

The abundance of ion pairs (CA+) relative to that of doubly charged ions (C 2 +) in electrohydrodynamic (EH) mass spectra of a series of anions with a common dication in glycerol was found to increase in the order acetate < nitrite < chloride < bromide~ni-trate < iodide < perchlorate. Correlation with enthalpies of hydration for the anions suggests that this trend reflects the solution chemistry of ion association. These spectra also reveal that solvation rather than interactions with the extracting field is more important in determining the overall EH mass spectrometric sensitivity to doubly charged ions. Therefore, the use of anions that promote more extensive ion pairing enhances the overall sensitivity to multiply charged ions that otherwise interact strongly with the solvent, but reduces sensitivity to singly charged ions. These observations hold in fast atom bombardment mass spectrometry, surviving the invasive effects of the primary beam. (J Am Soc Mass Spedrom 1990, 1, 233-237) T h e extreme impact of the choice of solvent [1][2][3][4][5][6][7][8][9] on the appearance of matrix-assisted desorption ionization (DI) mass spectra [10] arises from interacting effects of solvation [11,12], ion pairing [13], surface activity [14,15], mass transport [13,16], and other aspects of solution chemistry. These in turn are convoluted with the effects of primary beam-induced damage intrinsic to most DI experiments [2,3,[17][18][19][20][21][22] and aspects of gas-phase chemistry [23,24]. Although separation of these variables is not generally feasible, an independent understanding of the effects of each would facilitate experimental design for optimum sensitivity. The object of the present study is to isolate the effects of ion pairing on sensitivity in electrohydrodynamic (EH) and fast atom bombardment (FAB) mass spectrometry (MS).Analyte ion-pairing effects have been observed previously in mass spectrometry [14,15,[25][26][27][28][29][30][31]. Although Hand et al. [26] found that counterion effects can be suppressed by use of a liquid matrix, others [14,15,25,[27][28][29][30][31] have explored the use of surface-active counterions to enhance analyte sensitivity. However, such studies have not specifically assessed the extent of ion pairing and its impact on sensitivity.On the basis of evidence of preferential sampling of preformed ions in FAB/MS and EHMS [32,33], it can be predicted that neutralization due to ion pairing of singly charged analytes should decrease sensitivity. The situation is more complex in the case of multi- ply charged ions. Partial charge reduction will decrease solvent-solute interactions, so that the overall sampling efficiency may increase with increased ion pairing, until gains are offset by neutralization. Of course, the effects of beam-induced damage and the possibility of gas-phase ion-molecule reactions subsequent to sampling cloud this picture somewhat.Electrohydrodynamic mass spectrometry [33] provides a means of studying the effects of solution chemistry without interference from an...

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