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Some of the factors that influence the reduction of disulfide-containing peptides under fast-atom bombardment have been investigated using two neurohormonal peptides that include disulfide bridges in their structures. Deaminoarginine-vasopressin (DAVP) and arginine-vasopressin (AVP) have been analyzed as their acetate and trifluoroacetate salts. Results obtained in a thioglycerol matrix indicate that the peptides analyzed as their acetate salts are completely reduced under bombardment, whereas the trifluoroacetate salts show little evidence of reduction. Addition of trifluoroacetic acid to the acetate sample prior to bombardment inhibits reduction whereas addition after bombardment shows no effect on the reduction, thereby indicating the irreversibility of the process. Time-monitoring experiments conducted with the acetate salts of DAVP and AVP in common matrices such as thioglycerol, dithiothreitol + diethioerythritol, glycerol, hydroxyethyldisulfide and nitrobenzyl-alcohol demonstrate an important effect of the chemical nature of the matrix on reduction. In matrices containing thiol groups, the reduction is extensive, whereas it is almost suppressed in matrices such as hydroxyethyldisulfide and nitrobenzylalcohol. However, the addition of trifluoroacetic acid to all of these matrices essentially eliminates reduction and provides measured isotopic peak ratios that are in agreement with theoretically calculated values for these peptides.

The peak intensities observed in the molecular ion regions of fast-atom bombardment/liquid secondary ion mass spectra contain contributions from the parent ion species, its one- and two-electron reduction/oxidation products, and chemical background signal due to beaminduced damage. There are several solution and instrumental parameters that can affect the distribution of peak intensities in the molecular ion region. In this study, the analyte concentration and primary beam density and energy were varied systematically to investigate their effects on the measured peak intensities. A computer algorithm, Simbroc (Simulated Background and Reduction/Oxidation Calculations), was designed to deconvolute the observed intensities into their individual components so that the true effects of experimental parameters on redox extent and background levels could be evaluated. The algorithm is based on a comprehensive seven-variable mathematical model for experimental data simulation. The results obtained using the algorithm after its validation indicate that the primary beam energy does not significantly affect redox extent or background levels. Changes in analyte concentration and primary beam density tend to play a more important role in the generation of redox products and beam-induced damage. The background level generally increases as the analyte concentration is lowered for the peptide systems used in this study. An increase in the primary beam density often leads to higher background levels, although the effect is less detectable for samples that have a low (less than 3%) background signal. The apparent two-electron reduction is generally lower at the higher concentrations; however, the "true" reduction occurring for pentaphenylalanine does not show a significant concentration effect.

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The use of 4-hydroxybenzenesulfonic acid as a reduction-inhibiting and general purpose matrix in fast-atom bombardment liquid secondary-ion mass spectrometry (FABILSIMS) is discussed. Results obtained in a comparative study using the disulfide-bridge containing peptide deaminoarginine-vasopressin and several commonly used matrices indicate that the reduction-inhibiting efficiency of 4-hydroxybenzenesulfonic acid is comparable to that of 3-nitrobenzyl alcohol while reduction is found to be important in matrices such as thioglycerol and glycerol. Experiments conducted on peptides such as met-enkephalin and bradykinin, nucleosides and other polar compounds reveal that 4-hydroxybenzenesulfonic acid is a good solvent and has physical properties which are similar Lo those of glycerol. This new matrix material is a radical scavenger, surface active, allows persistent secondary-ion beams and produces a low background signal. The matrix, 4-hydroxybenzenesulfonic acid, generally yields high quality mass spectra which compare well with those obtained in either glycerol or 3-nitrobenzyl alcohol.Since its introduction by Barber in 1981,'3* fast-atom bombardment (FAB) has been used extensively and this technique along with other desorption-ionization techniques has been instrumental in broadening the scope of compounds that can be analysed by mass spectrometry. The technique allows the analysis of thermolabile and non-volatile compounds to be performed on a routine basis and encompasses a wider molecular weight range as compared to the more conventional techniques of electron ionization and chemical ionization. One of the key factors involved in the success of FAB and liquid secondary-ion mass spectrometry (LSIMS) has been the use of a liquid matrix in which the analyte is dissolved prior to analysis.',2 The matrix acts as a sample reservoir which not only permits the replenishment of the surface during analysis but also reduces the effect of the accumulation of secondary products generated by radiation damage. It can, furthermore, play an important and active role in the ionization process through strong matrixlanalyte interaction^.^ In FABILSIMS analysis, the success of the experiment usually depends on the choice of the appropriate liquid matrix. In general terms, a matrix should dissolve the compound to be analysed, have a relatively low volatility to allow persistent secondary-ion beams and should not react chemically with the analyte. In specific analyses, however, other factors such as the background generated by the matrix and its radical scavenging properties can become i r n p~r t a n t .~ Although glycerol was introduced originally as a FAB matrix'V2 and is still widely used, several other materials such as thioglycerol ,5 2-hydroxyethyl disulfide,6 3-nitrobenzyl alcohol ,7, 24trophenyloctyl ether ,' thiodiethylene glycol,"' dithiothreitol/dithioerythritol" and 2-methylgly~erol'~ have been reported to be efficient in certain applicatons. Excellent reviews on useful guide-* Author to whom correspondence should be add...