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

A fast atom bombardment and matrix-assisted laser desorption/ionization mass spectrometry study of doubly charged porphyrins

In this mass spectrometry (MS) study of doubly charged porphyrin salts, fast atom bombardment (FAB) and matrix-assisted laser desorption/ionization (MALDI) MS techniques are utilized to examine several unique ionic species. The predominant transformation of preformed doubly charged ions in the desorption/ionization mechanism of FAB and MALDI is the result of deprotonation reactions to form singly charged ions of the type [M(2+) - H(+)](+) and of one-electron reductions to form radical cations [M(2+) + e(-)](+.). The dependence of this phenomenon and the formation of a number of additional ionic species on the different matrices and the FAB-matrix additive benzoquinone is examined. The significant analogous behavior of doubly charged porphyrins in FAB- and MALDI-MS leads to the conclusion that one-electron reductions are of distinct relevance in the desorption/ionization mechanism of MALDI.

Determination of flux from a saddle field fast-atom bombardment gun

The flux or beam density (equivalent current/area) of xenon atoms striking the sample target from a saddle field fast-atom bombardment (FAB) gun has been compared with that from a cesium ion gun mounted on the same instrument. A shielded Faraday cup mounted on the end of a solids probe was used to measure directly the flux of the Cs(+) beam. Samples of methylene blue in glycerol solution were then exposed to the ion beam at different fluxes and the extents of reduction were measured. The extent of reduction varied linearly with flux up to a value of about 1.16 × 10(13) particles s(-1) cm(-2) (1.85 μ cm(-2)); above this level, the reduction effect appeared to saturate. FAB spectra were obtained from the same dye solution by using varying settings of the FAB gun. By comparing the extents of reduction of the dye from the two guns, the flux from the atom gun could be estimated. Observation of luminescence from a CsI-coated target allowed estimation of the area of the atom beam. The atom beam "equivalent current" could then be calculated by multiplying the flux times the area. It was noted that for given settings, the flux from the atom gun depended on the physical condition of the gun electrodes. With new electrodes, a flux ≥ 1.16 × 10(13) particles s(-1) cm(-2) was obtained with nominal gun emission currents of 0.60-1.0 mA. Electrodes used extensively, but freshly cleaned, provided a flux of ∼ 8 × 10(12) particles s(-1) cm(-2) at nominal emission currents of 0.40-1.0 mA. With dirty electrodes this flux could only be achieved at the highest (1.0 mA) emission current. This decline in performance occurs over a matter of months as a result of contamination and erosion of the electrodes during use. Such behavior can adversely affect spectral reproducibility even when nominal FAB gun voltage and emission current are carefully reproduced.

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

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.

Characterization of beam-induced reactions occurring in liquid secondary ion mass spectrometry/fast-atom bombardment by tandem mass spectrometry

Department of General and Physical Chemistry, University of Liege, Liege, Belgium A specific beam-induced secondary reaction involving the condensation of hydroxylic matrices with some organic groups (aldehydes, ketones, etc.) accompanied by the loss of a water molecule was investigated by liquid secondary ion mass spectrometry/fast-atom bombardment (LSIMS/FAB). A mechanistic scheme and a structure of the induced product are proposed. The features of this secondary reaction were studied and the influence of the types of solutes, acidic additives, and matrices analyzed. Rather than a drawback, LSIMS/FAB mass spectrometry can take advantage of this matrix effect to infer analytical information through tandem mass spectrometry experiments. Specific neutral loss scans can be conducted to highlight beam-induced reactive molecules, even when the detection of these species is prevented in normal scan spectra by other surface-active components.

Fast atom bombardment-induced condensation of glycerol with ammonium surfactants. I: Regioselectivity of the adduct formation

Fast atom bombardment promotes condensation between trimethyl tetradecyl ammonium cations and the glycerol matrix. Bond formation at both the head and tail of the surfactant is demonstrated by low energy collision-induced dissociation (ClD) of deuterium-labeled precursors, with a preponderance of the reaction apparently occurring at the tail. Two distinct ClD pathways are identified for each kind of adduct (head- and tail-attack). Evidence is presented for the detection of distonic radical cations of the surfactant, complexed (solvated) with glycerol.

m-nitrobenzyl alcohol electrochemistry in fast atom bombardment mass spectrometry

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.

Mass spectrometric signature of S-prenylated cysteine peptides

The fast atom bombardment mass spectra of peptides containing S-prenylated cysteine display signature fragmentations characteristic of this modified amino acid. The fragmentation is independent of the nature of the cysteine carbonyl substituent, easily differentiates prenyl from nonprenyl alkylation, and readily identifies the oligomer count of the prenyl. This screening method, which requires little time, effort, or material (compared with previous analysis methods based on chemical degradation), greatly facilitates the identification of these prenylated proteins.