Applicability of continuous-flow fast atom bombardment liquid chromatography-mass spectrometry in bioanalysis. Dextromethorphan in plasma

Continuous-flow fast atom bombardment (CF-FAB) is an interface for combined liquid chromatography-mass spectrometry using FAB as the ionization method. The applicability of CF-FAB for quantitative bioanalysis was studied for a model compound, dextromethorphan, in plasma samples using conventional high-performance liquid chromatography. The flow-rate reduction was achieved either by splitting or by the phase-system switching approach. The features of both systems are discussed.     

Biomedical applications of high-performance liquid chromatography-mass spectrometry with continuous-flow fast atom bombardment

This report describes the application of high-performance liquid chromatography combined with continuous-flow fast atom bombardment mass spectrometry to analytical problems in the biomedical laboratory. Applications include the compound-specific detection of diagnostic acylcarnitines in human urine, the separation and analysis of acyl-coenzyme A thioesters, and qualitative studies on complex mixtures of modified peptides (dansyl and dinitrophenyl derivatives). For each of these applications standard analytical columns (3.9 mm I.D.) and 1 ml/min flow-rates were employed with post-column stream splitting (1:100) before mass spectrometry. Various mobile phase compositions and solvent gradients were employed. The addition of 1-5% glycerol to the mobile phase was shown to have little effect on the chromatography. For all compounds studied (acylcarnitines, acyl-coenzyme A thioesters, and derivatized peptides) molecular weight information was obtained and sufficient sensitivity was achieved to allow unambiguous identification of trace components in complex mixtures.     

Capillary zone electrophoresis-mass spectrometry using a coaxial continuous-flow fast atom bombardment interface

Mixtures of peptides have been analyzed by capillary zone electrophoresis in conjunction with mass spectrometry (MS) using an on-line coaxial continuous-flow fast atom bombardment interface. MS and MS-MS spectra have been acquired in electrophoretic real time from femtomole levels of the peptides, while maintaining separation efficiencies in excess of 100,000 theoretical plates.     

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.     

Interfacing microbore and capillary liquid chromatography to continuous-flow fast atom bombardment mass spectrometry for the analysis of glycopeptides

This work describes a system to interface either microbore or packed capillary gradient liquid chromatography (LC) to fast atom bombardment mass spectrometry (FAB-MS). The interface incorporates on-line ultraviolet detection and post-column matrix addition to enable independent optimization of both LC and FAB-MS. The glycopeptide antibiotic teicoplanin was chosen as a model system because this group of compounds places severe demands on the chromatographic separation and is difficult to analyze by FAB-MS. For both microbore and capillary LC, high-quality mass spectra of the major components in teicoplanin were obtained; however, the increased sensitivity of the capillary system allowed spectra to be obtained at low picomole concentrations. The sensitivity and ease of use make capillary LC the preferred system for use in LC-FAB-MS.     

An integral probe for capillary zone electrophoresis/continuous-flow fast atom bombardment mass spectrometry

An integral probe for capillary zone electrophoresis/continuous-flow fast atom bombardment mass spectrometry was constructed and operated in either the coaxial or liquid-junction interface mode. Results using these interfaces for the analyses of synthetic peptides are presented. The coaxial arrangement attains the best electrophoretic performance, generally providing a greater number of theoretical plates. However, in that the electrophoresis times are generally greater for the liquid-junction interface because there is no mechanical flow resulting from the source vacuum, the overall separation efficiency of the liquid-junction interface is equal to or greater than that of the coaxial interface. In addition, the liquid-junction interface is easier to set up and operate, and allows larger inner diameter capillaries to be used to achieve higher sample loads.     

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.     

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.     

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.     

Approach to studying proteinase specificity by continuous-flow fast atom bombardment mass spectrometry and high-performance liquid chromatography combined with photodiode-array ultraviolet detection.

Fast atom bombardment mass spectrometry (FAB-MS) and high-performance liquid chromatography using a photodiode-array ultraviolet detector were applied to study a dynorphin-converting endopeptidase from the human pituitary gland. The specificity of the enzyme was tested towards various opioid peptides derived from the prodynorphin precursor, i.e. dynorphin A, dynorphin B and alpha-neoendorphin. Peptide fragments were analysed directly by continuous-flow FAB-MS and those containing aromatic amino acids were detected independently by the photodiode-array ultraviolet detector. The results obtained suggest a similar processing of these structure-related substrates and it appears that the enzyme recognizes the dibasic stretch in their sequence. It is also clear from this study that the combination of the above techniques provides a powerful tool for studies of enzymatic conversion among the prodynorphin-derived peptides and it should be applicable to studies of similar mechanisms in other peptide systems.     

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^?.     

Mass spectrometry of arenediazonium salts by fast atom bombardment

Contrary to expectation, thermally labile arenediazonium salts (M) afford sizeable peaks due to ArN₂ ⁺ and (ArN₂ ⁺)M ions under fast atom bombardment; the main fragment ion is usually the highly reactive aryl cation Ar⁺, the production of which is independent of whether its ground state is singlet or triplet.     

Combined supercritical-fluid chromatography/mass spectrometry in the analysis of diuron in plasma using on-line phase-system switching

On-line sample pretreatment by means of the phase-system switching approach is an interesting technique for the analysis of aqueous samples, e.g., plasma, by means of supercritical-fluid chromatography. In order to analyse plasma samples the following analytical procedure is used. The plasma sample is injected on to a short precolumn, which is washed with water and subsequently dried with nitrogen. Next, the solutes are desorbed with the supercritical mobile phase, analysed with packed-column supercritical-fluid chromatography and detected with either a UV detector or a mass spectrometer, equipped with a moving-belt interface. The herbicide diuron is selected as a test compound to study the feasibility of this approach. Using a selective detector the procedure is sufficiently sensitive to detect diuron in plasma, but not appropriate to detect the diuron metabolites in a post-mortem plasma sample. These have been identified with liquid chromatography/mass spectrometry. The detection limit of diuron in plasma using the procedure described is about 30 ng/mL.     

Analysis of nucleosides,nucleotides and oligonucleotides using fast atom bombardment mass spectrometry

Mass spectrometry, especially fast atom bombardment mass spectrometry, is playing an ever increasing role in the identification of naturally occurring and synthetic nucleic acid components. Practical applied aspects of this technique are discussed using examples taken from the recent literature describing the mass spectral analysis of nucleosides, nucleotides and oligonucleotides.     

The application of normal phase liquid chromatography/mass spectrometry by using coaxial continuous flow fast atom bombardment

This article describes the interfacing of a normal phase fused silica capillary high performance liquid chromatography system to a magnetic sector mass spectrometer by using continuous flow fast atom bombardment (CFFAB). While the performance of CFFAB using reversed phase techniques is well understood, there is very little if any documentation on interfacing nonaqueous normal phase systems with CFFAB. This article describes the use of packed fused silica capillary liquid chromatography columns and the corresponding normal phase solvent systems. The experimental parameters required with nonaqueous solvent systems differ significantly from those of aqueous solvent systems. Ditallowdimethylammonium chloride (DTDMAC), a cationic surfactant commonly used as the active ingredient in fabric softener products, was chosen as a model compound to demonstrate the technique. DTDMAC was identified in a commercially available fabric softener product by using on-line normal phase liquid chromatography/mass spectrometry with accurate mass and tandem mass spectrometry.     

Characterization of silica dissolved in sodium chloride solution using fast atom bombardment mass spectrometry

The chemical species of silica dissolved in sodium chloride (NaCl) solution were identified by means of fast atom bombardment mass spectrometry (negative ion mode). The concentration of silica in 0.1 M NaCl solution is < 0.6 mmol dm(-3) (mM) and application to the identification of the silicate species at low concentrations such as in natural waters level is also possible. An apparent peak at m/z 95, which corresponds to SiO(OH)(3)(-) in 0.1 M NaCl solution, was not confirmed owing to the interference of the peaks corresponding to NaCl(2)(-); however, peaks for complexes such as Si(OH)(2)O(2)Na(-), Si(2)(OH)(5)O(2)(-), Si(2)(OH)(4)O(3)Na(-), Si(2)(OH)(3)O(4)Na(2)(-), Si(2)(OH)(2)O(5)Na(3)(-), Si(4)(OH)(7)O(5)(-), Si(4)(OH)(6)O(6)Na(-) and Si(4)(OH)(5)O(7)Na(2)(-) were detected. The existence of the trimer and its Na(+) complexes such as Si(3)(OH)(7)O(3)(-), Si(3)(OH)(6)O(4)Na(-) and Si(3)(OH)(5)O(5)Na(2)(-) was not clearly shown. These complexes can be confirmed not only in the form of the anion itself (e.g. Si(2)(OH)(5)O(2)(-)), but also in the form of some complexes with sodium ions, such as Si(2)(OH)(4)O(2)Na(-). Copyright 2000 John Wiley & Sons, Ltd.     

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.     

Routine liquid charomatography/fast atom bombardment mass spectrometry of peptides and enzymatically digested proteins

The development of a high-performance liquid chromatography (HPLC)/fast atom bombardment (FAB) interface and the subsequent commercial availability of such systems has facilitated the routine application of the technique to problems in pharmaceutical research and development. Although many products are amenable to FAB analysis and hence LC/FAB, the greatest benefit of the interface has been in the field of peptide and protein analysis. It has been found that, even with post-column matrix addition, chromatographic resolution is maintained and, by plotting mass chromatograms, the resolution may be greater than that achieved by the less specific UV detector. As only 1% matrix is required in the final eluent, the system is stable for extended periods and has been used for 3 h LC/FAB experiments or used continuously for multiple analyses over 8 h periods. In addition to the acquisition of relative molecular mass information, the constant background can be completely subtracted to yield structurally significant fragment ions which may allow sequencing of components from the single LC/FAB experiment. Applications of LC/FAB to date include the characterization of Iys(78)-plasminogen by the on-line analysis of complex mixtures of peptides resulting from the various enzymatic digests.     

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.