An approach to the quantiative study of reduction processes occurring in fast‐atom bombardment/liquid‐assisted secondary‐ion mass spectrometry

Abstract

An approach which allows the determination of the specific effects of experimental parameters on the extent of reduction observed in fast-atom bombardmenthiquid-assisted secondary-ion mass spectrometry (FAB/LSIMS) has been developed. The methodology is based on the isolation of the different components-background, reduction extent, and isotopic abundances-which contribute to peak intensities in the molecular-ion cluster. The individual contributions of reduction and background are calculated by an algorithm SRBC (simulated reduction and background calculations) which utilizes a Monte-Carlo method to optimize the values obtained for these variables. This approach has been used to examine the effects of analyte concentration, primary-beam energy and beam density on the extent of reduction in FAB/LSIMS experiments. The results obtained indicate that the beam energy has no significant effect on the apparent reduction, as measured by the experimental (A + 2)IA isotopic peak ratio, while analyte concentration and beam density effects are observed. However, background and reduction data generated by SRBC demonstrate that in some cases there is little effect of these parameters on the true reduction of the analyte and that the increase in apparent reduction with the variation in these parameters results mainly from an artefact created by an increase in the background signal at lower concentrations and higher beam densities.The use of desorption-ionization techniques such as fast-atom bombardment (FAB) and liquid-assisted secondary-ion mass spectrometry (LSIMS) has, over the last decade, expanded the scope of mass spectral analysis. The most unique feature of FAB/LSIMS is the use of a liquid matrix in which the analyte is dissolved.',' Initially, glycerol was employed as the liquid matrix'.' but other materials have since been utilized and these have been the subject of excellent reviews.s5 Several physical and chemical phenomena related, to the use of a li uid matrix have been observed in FAB/LSIMS analysis%7 and although numerous studies have been directed at the investigation of fundamental processes occurring in FAB/LSIMS&" our knowledge of the mechanisms involved is still deficient. One of the phenomena which has been observed under FAB conditions and that has attracted considerable interest is the occurrence of reduction processes in the matrix under bombardment .lLZ9 These reduction processes, which cause the intensities of A + n peaks (A = (M+ H), n 3 1) to be higher than the theoretical values calculated using the natural abundances, have been shown to involve electrons and/or hydrogen 14,21 and have been observed in several chemical systems.The extent of reduction observed in FABILSIMS depends on the nature of the analyt~,'"'6~2~''~3~32 the nature of the r n a t r i~, '~~'~~'~~'~~~~~~~ the presence of matrix additive^'^^'^^'^ and other experimental parameters. Matrices that have electrodradical scavenging properties such as 3-nitrobenzylalcohol (NBA), 2-hydroxyethyl disulfide (HEDS) and 4-hydroxybenzenesulfonic ...