Improving fast atom bombardment mass spectra: The influence of some controllable parameters on spectral quality

Abstract

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.

Evaluation of the true effect of experimental parameters on the reduction / oxidation processes observed in fast-atom bombardment/liquid secondary spectrometry

“…Instrumental parameters associated with the FAB/LSIMS source conditions may also play an im-portant role in determining the extent of reduction observed. Those parameters that have been studied include time of irradiation [8,14,16,17,23]; primary beam energy [15,20,28); and bombarding beam density (flux) [12,15,20,23,28]. Wirth et al [8] observed a time dependence in their FAB analysis (Xe O primary beam) of azo-group-eontaining peptides.…”

“…The primary beam density, however, plays a more important role in determining the extent of reduction. An increase in the beam density caused substantial increases in the enhancement ratios measured for methylene blue [20,30] and for different peptide systems [281. For systems in which the extent of reduction is large, therefore, it is more important to lower the primary beam density than it is to raise the primary beam energy [20].…”

“…Several contributions, including natural isotopic abundances, background signal (due to beam-induced chemical damage of analyte, matrix, and combined analytey'matrix species), and reducedy oxidized species are usually present in the molecular ion region, and they are not easy to isolate. The methods currently used to express the extent of reduction, commonly referred to as "enhancement factors," assign the excess signal in the A + n peaks entirely to reduction products [15,20]. This approach, however, measures the "apparent" reduction rather than the "true" reduction extent by overlooking contributions of beam-induced background signal and isotopic carryover from other processes affecting peak intensities in the molecular ion region.…”

“…This filtering of beam-induced background peaks from the mass spectra is essential in the isolation of sequence ions from the peak-at-every-mass distribution commonly observed in FAB/LSI spectra. It is surprising, therefore, that the problem of background interference, which can affect the apparent redox processes in the molecular ion region, had not been addressed until recently [20][21][22]28]. To determine quantitatively the contribution of redox processes to the intensity distribution of peaks in the parent ion cluster, the natural isotopic abundances and the background signal must be deconvoluted from the overall peak intensities measured in this mass region.…”

Reduction processes in fast atom bombardment mass spectrometry: Methylene blue in glycerol‐thioglycerol and glycerol‐nitrobenzyl alcohol matrices

“…1 + H ' + 2 (5) a + H'-+[M + 11' ' (6) 1 + H + -* [M + 11" The electrochemical reduction of a to the leucomethylene blue, 2, is a two electron process (reactions (I) and (2)). If a similar reduction occurs as a result of the bombardment process, the subsequent protonation that there must be pathways to the two reduced species which are parallel rather than consecutive.…”