Continuous-flow fast atom bombardment mass spectrometry

The continuous-flow fast atom bombardment probe performs equally well with or without a high-performance liquid chromatography column producing clean spectra containing little or no background noise. Its function as a liquid chromatography-mass spectrometry interface for labile and involatile samples has been illustrated with reference to dansylated amino acids. The versatility of the new probe has been exemplified by on-line enzymatic peptide sequencing.     

Continuous-flow fast atom bombardment mass spectrometry of oligonucleotides

Although frit-fast atom bombardment (frit-FAB) and continuous-flow FAB mass spectrometry have become standard methods for the analysis of peptides and peptide mixtures, these techniques have not been applied previously to the analysis of oligonucleotides. Mobilephase composition, flow rate, and sample size were optimized for the analysis of oligonucleotides by negative ion frit-FAB mass spectrometry (a type of continuous-flow FAB mass spectrometry). With a mobile phase consisting of methanol/water/triethanolamine (80:20:0.5, v/v/w), flow injection frit-FAB analysis of oligonucleotides showed lower limits of detection compared to standard probe FAB mass spectrometry. For example, in order to obtain a signal-to-noise ratio of 3:1, 38 prnol of d(GTIAAC) were required for frit-FAB mass spectrometry and 62 pmol were required for standard probe FAB mass spectrometry. The largest difference between frit-FAB and standard probe FAB was observed for d(pC)₅, for which the limit of detection by frit-FAB was approximately 11-fold lower than by standard FAB mass spectrometry. Adjustment of the mobile phase to pH 7 with trifluoroacetic acid increased the limit of detection (reduced sensitivity) a minimum of sixfold. Equimolar mixtures of two or three oligonucleotides produced deprotonated molecules in identical relative abundances whether analyzed by frit-FAB or standard probe FAB mass spectrometry. Finally, frit-FAB liquid chromatography mass spectrometry was demonstrated by separating mixtures of oligonucleotides on a β -cyclodextrin high-performance liquid chromatography column with a mobile phase containing methanol, water, and triethanolamine.     

Chemical reactions in fast atom bombardment mass spectrometry

Examples of various chemical reactions occurring in the matrix or in the selvedge region in fast atom bombardment (FAB) mass spectrometry are discussed. These are categorized as oxidations and reductions; substitutions; clusterings and additions; and sample decomposition or transformation. Some reactions observed showed significant time behaviour and in one case it was possible to determine rate constants. These data suggest that chemical reactions can be accelerated significantly by fast atom bombardment.     

Kinetics of acrylamide/glutathione binding by fast atom bombardment and thermospray mass spectrometry

The kinetics of the binding of the neurotoxin acrylamide to the cysteine residue of glutathione has been studied. At 37 °C and pH 7.3 the second order rate constant has been determined to be 0.72 ± 0.06 mol^?1 dm³ min^?1 by thermospray mass spectrometry. The critical energy at pH 11.5 measured over the temperature range 10–37°C by fast atom bombardment mass spectrometry was measured as 24.6 kJ mol^?1.     

Analysis of xanthates by negative-ion fast atom bombardment mass spectrometry

A new technique using negative-ion fast atom bombardment mass spectrometry for the analysis of xanthates and related compounds is described. Electron impact and positive-ion fast atom bombardment mass spectrometry produced no structurally related fragment ions or observable molecular ions at the expected m/z values. It was demonstrated that negative-ion fast atom bombardment ionization was the most suitable method of ionization for structure elucidation studies for the compounds described.     

Direct microanalysis by negative ion fast atom bombardment mass spectrometry

In negative ion fast atom bombardment (FAB) mass spectrometry, the use of a liquid matrix comprised of 15-crown-5—glycerol (1:10, v/v) enabled the molecular weight determination of small amounts of non-volatile acidic molecules. This ionization process could result in detection limits of less than one-fifth of those obtained using only glycerol as matrix. The function of the matrix as a proton acceptor is emphasized.     

Determination of the stability constants of macrocyclic ligand-alkali cation complexes by fast atom bombardment mass spectrometry

Quantitative studies of the complexation of the macrocylic ligands, 18 crown 6 (18C6,1) and cyclogentiotetraose peracetate (CGD4Ac,2), with alkali cations, have been investigated by fast atom bombardment mass spectrometry (f.a.b.m.s.). Complexation curves of 18C6 with Na⁺, K⁺, Rb⁺ and Cs⁺ in glycerol, and of CGD4Ac with Cs⁺ in PEG 200, were obtained by plotting the complex peak intensity against the alkali cation concentration. From these curves we describe a method to calculate the stability constant for an alkali cation-macroyclic ligand complex. There is a good agreement between stability constants obtained either by f.a.b.m.s. or calorimetric techniques for 18C6-alkali cation complexes. These results suggest that the f.a.b. technique can be used to study complex formation and determine the stability constant.     

Analysis of some eicosanoids by continuous-flow fast atom bombardment mass spectrometry

Continuous-flow fast atom bombardment has been used to analyze eicosanoids by selected-ion monitoring on a sector-field mass spectrometer operating in the negative-ion mode. The method has been optimized with respect to solvent composition and flow-rates. Detection limits were below 50 pg, and under optimal conditions a linear relationship between response and amount of substance was achieved. The method was successfully applied to the analysis of two spiked urine samples.     

Negative-ion fast atom bombardment mass spectrometry of N-phosphoamino acids

The fragmentation patterns of N-phosphoamino acids in negative-ion fast atom bombardment mass spectrometry (FABMS) showed different characteristics to those in positive-ion FABMS. Six typical N-diisopropyloxyphorphorylamino acids all had intense [M ? 1]^? peaks, and they underwent similar fragmentation pathways. In general, the elimination of one alkene molecule followed by the loss of one molecule of alcohol occurred. They also favoured an N → O rearrangement reaction, followed by fragmentation to (RO)₂ PO₂^? and (RO) (HO)PO₂^?.     

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 (E_1/2). From the known electrochemistry of the complexes studied, it can be estimated that 1020 mV > E_1/2 > 500 mV for the reaction of NBA serving as a reducing agent, while 500 mV > E_1/2 > 424 mV for the reduction potential of NBA. However, in the presence of bombardment the former E_1/2 must be at least as low as 356 mY, and the latter E_1/2 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.     

Sequencing of cyclosporins by fast atom bombardment and linked-scan mass spectrometry

The mass spectrometric sequence determination of amino acid residues in cyclosporins using fast atom bombardment, collisionally activated dissociations in the first field-free region and linked B/E scan is described. The general fragmentation scheme was derived from the spectra of cyclosporins A, B, C, D, F, G, L and [DH-MeBmt¹]CS. The main fragmentation pathways start by primary splitting between amino acids 2–3, 1–11 and 5–6. The corresponding N-terminal b-type ions are common fragment types in the mass spectra. The 1–11 splitting can be enhanced by the introduction of a lactone group into the peptide ring by conversion of cyclosporins into isocyclosporins. The fragmentation scheme was used for amino acid sequence determination in four new natural cyclosporins, [MeLeu¹]CS, [Leu⁴]CS, [Ile⁴]CS and [Leu⁵]CS.     

Differentiation of diastereomeric chlorin derivatives by fast atom bombardment mass spectrometry

The stereochemistry of six pairs of diastereomeric chlorin derivatives was investigated by electron impact (EI), fast atom bombardment (FAB) and direct chemical ionization DCI mass spectrometry. It was demonstrated that FAB and EI mass spectrometry are convenient and rapid methods for distinguishing between diastereomeric chlorin derivatives due to their different fragmentation patterns.     

Characterization of glucosamine and N-acetylglucosamine oligomers by fast atom bombardment mass spectrometry

The oligomers of N-acetylglucosamine [degree of polymerization (d.p.) 2–6] and of glucosamine (d.p. 3–9) were systematically studied by the fast atom bombardment technique. Positive- and negative-ion detection were used successively in each case. The latter provided more information in respect of the number of fragmentation pathways. In addition to the classical fragmentations usually obtained with other oligosaccharidic chains, new species specific to these two series were observed. They depend mainly on the nature of the substituents located on the C(2) positions of the glucosidic units constituting the chain.     

Characterization of polymers by field desorption and fast atom bombardment mass spectrometry

Summary Field desorption and fast atom bombardment mass spectrometry in the positive and negative modes have been applied to characterize poly(ethyleneadipate). Diagnostic ions up to the mass range of 1,400 have been found which allow elucidation of the degradation pathways of the polymer and its identification.

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Charakterisierung von Polymeren durch Felddesorption- und Fast Atom Bombardment-Massenspektrometrie

Zusammenfassung Felddesorption- und Fast Atom Bombardment-Massenspektrometrie mit positiver und negativer Registrierung wurden für die Charakterisierung von Poly(ethylenadipat) eingesetzt. Diagnostische Ionensignale bis zur Masse 1400 wurden gefunden und ermöglichen die Kennzeichnung der Abbauwege des Polymers sowie seine Identifizierung,

     

Mechanism for dehalogenation reactions in fast atom bombardment mass spectrometry

The mechanism of a dehalogenation reaction that occurs during fast atom bombardment (FAB) mass spectrometry was examined using halogenated nucleosides as model compounds. For aglycone-halogenated nucleosides, an inverse linear relationship exists between the extent of FAB dehalogenation and the calculated electron affinity of an individual nucleoside. The degree of dehalogenation for a given nucleoside also varies inversely with the calculated electron affinity of most FAB matrices. The observed dehalogenation reaction can be completely inhibited when matrices with positive electron affinities, such as 3-nitrobenzyl alcohol and 2-hydroxyethyl disulfide, are used. High-performance liquid chromatographic analysis of the bulk glycerol matrix following exposure to the FAB beam indicates measurable amounts of dehalogenated product, suggesting that this reaction occurs in the condensed phase prior to gas-phase ion formation. A dehalogenation mechanism involving thermal electron capture and subsequent negative charge stabilization is consistent with these observations.     

Analysis of tyrosine- and methionine-containing neuropeptides by fast atom bombardment mass spectrometry

A simple and unambiguous method for the detection of the amino acids tyrosine and methionine in peptide structures has been developed. The procedure, which was applied in studies of opioid peptides, is based on continuous-flow fast atom bombardment mass spectrometry (CF-FAB-MS) following chemical modification of the residue to be analyzed. Thus, for the detection of tyrosine, modification reactions such as acetylation or non-radioactive iodination were performed prior to analysis by CF-FAB-MS. O-Acetylation of the tyrosine residue with N-acetylimidazole was accompanied by a shift of 42 Da in the molecular mass of the peptide under investigation. This modification was reversed by treatment with hydroxylamine hydrochloride. Incorporation of iodine resulted in a molecular weight shift of 126 Da per iodine atom. Methionine residues were detected in methionine-enkephalin-containing peptides following S-oxidation with hydrogen peroxide. The procedures described may have a wide application in peptide chemistry, particularly for the identification of peptide fragments containing the above residues, e.g. in studies of processing or degradation of the enkephalins or other neuropeptides (e.g. endorphins and tachykinins).     

Caesium fluoride as a mass calibrant in fast atom bombardment mass spectrometry

Caesium fluoride was shown to afford advantages over caesium iodide as a mass-calibration compound for fast atom bombardment mass spectrometry.