Formation of M+. ions under matrix fast atom bombardment conditions: Does charge exchange reaction occur?

The formation of molecular ions, M^+., under fast atom bombardment (FAB) conditions using a liquid matrix was examined by using a new type of synthesized compounds in which preferential M^+. peaks appear in their FAB spectra. The FAB spectra were compared with the corresponding mass spectra obtained by the electron impact (EI) ionization, chemical ionization (CI) and charge-exchange ionization (CEI) methods. All of the spectra showed preferential peaks of M^+. ion and a characteristic intense fragment ion peak originating from a β-fission. The FAB spectra were similar in the fragment ions appearing in the EI spectra and were very similar in the fragmentation pattern to the CEI spectra using Ar^+. and Xe^+. as the reagent ions. Further, the FAB spectra did not show any doubly charged ion peaks, while the 70 eV EI spectra showed the peaks of doubly charged molecular and/or fragment ions. The isobutane CI spectra of the synthesized compounds suggested that the formation of M^+. ions occurred through the CE reaction with isobutane ion, C₄H₁₀^+., and the CI spectra showed a marked intense fragment ion peak originating from the β-fission which seemed to occur characteristically in CEI processes. The results obtained suggested that the formation of M^+. ions under matrix FAB conditions occurred mainly by CE reactions between the analytes M and matrix molecular ions B^+. and/or fragment ions b^+..     

Competitive formation of M+˙ and [M + H]+ ions under fast atom bombardment conditions

The competitive formation of molecular ions M^+˙ and protonated molecules [M + H]⁺ under fast atom bombardment (FAB) conditions was examined using various kinds of organic compounds. The use of protic/hydrophilic matrices such as thioglycerol and glycerol resulted in relatively large values of the peak intensity ratio I([M + H]⁺)/I(M^+˙) compared with the use of relatively aprotic/hydrophobic matrices such as m-nitrobenzyl alcohol and o-nitrophenyl octyl ether. The change of matrix from thiol-containing such as thioglycerol and dithiothreitol to alcoholic such as glycerol and pentamethylene glycol increased the I([M + H]⁺)/I(M^+˙) ratio. Furthermore, the change of matrix increased the peak intensity ratio of the doubly charged ion [M + 2H]²⁺ to [M + H]⁺ in the FAB mass spectra of angiotensin I and gramicidin S. The addition of acids to the matrix solution increased the I([M + H]⁺)/I(M^+˙) ratio, although such an effect did not always occur. The acetylation of simple aniline compounds markedly increased the I([M + H]⁺)/I(M^+˙) ratio. It was concluded from these results that the hydrogen bonding interaction between hydroxyl groups(s) of the matrix and basic site(s) of analyte molecules in solution acts advantageously as a quasi-preformed state for [M + H]⁺ formation, and that the presence of significant proton acceptor(s) such as carbonyl group in analytes hinder the M^+˙ formation which may generally occur under FAB conditions. The formation of M^+˙ and [M + H]⁺ ions seemed to occur competitively, reflecting or according to the interaction or solvation states between the analyte and matrix molecules in solution and the structural characteristics of the analytes.     

Origins and structures of background ions produced by fast-atom bombardment of glycerol

Accurate mass measurements were used to assign elemental compositions and tandem mass spectrometry was used to characterize the peak-at-every-mass background ions produced by kiloelectron-volt-particle bombardment of neat fast-atom bombardment matrices. The majority of the background ions observed in the mass spectrum of neat glycerol was identified. On the basis of the experiments with glycerol, a theory for the formation of background ions is presented. Results are discussed according to the chemical and physical changes that ygoe;on-volt-particle bombardment produces in the matrix.     

The behaviour of caesium iodide cluster ions produced by fast-atom bombardment

Metastable ion and collisionally-activated dissociation spectra of several cluster ions of the type [Cs_n+1I_n]⁺, formed by fast atom bombardment mass spectrometry (FABMS) of CsI, have been studied: It is found that particular cluster ions are more stable than others. Less stable ions undergo more unimolecular dissociation and have larger cross-sections for collisional dissociation than do the more stable ions, and this leads to unexpected anomalies in the FAB mass spectrum of CsI. The collision spectrum of [Cs₃₅I₃₄]⁺ (m/z 8966) has been acquired at an accelerating potential of 10 kV; the extent of both unimolecular and collisionally-activated dissociation of this ion is remarkably high and leads to collision/ transmission efficiencies of 85% for our tandem mass spectrometer. Clusters formed by FAB of an equimolar mixture of CsI and KI have also been studied by their collisionally-activated dissociation (CAD) and metastable ion (MI) spectra.     

Study of adduct ions of meso-phenyl-substituted tetrabenzoporphyrins by fast-atom bombardment mass spectrometry

Mass spectra of meso-phenyl-substituted tetrabenzoporphyrins were investigated by fast-atom bombardment mass spectrometry and tandem mass spectrometry. A cluster of adduct ions with mass-to-charge ratio values higher than the corresponding molecular ions of the porphyrins has been observed. The mass number differences among the series of cluster ions are constant depending on the para-phenyl substituents. Under certain conditions, dimers or trimers of molecular ions with low abundances have been detected. To trace the origin of the adduct ions, a series of experiments based on mass spectrometry have been carried out. The mass spectrum of tetrabenzoporphyrin showed no adduct ions with mass number differences of 90 even with the addition of phenylacetic acid. The mass spectrum of meso-tetraphenylte-trabenzoporphyrin ¹³C-labeled at the meso carbons showed adduct ions with mass number differences of 91. Product spectra of [2M + H]⁺ or [3M + H]⁺ of porphyrins exhibited adduct ions. All these results suggest that fragmentations of [2M + H]⁺ or [3M + H]⁺ may be one of the many possible routes to form the adduct ions, and the mass number differences among the series of these cluster ions should correspond to the benzyl group from the meso positions of meso-phenyl-substituted tetrabenzoporphyrins.     

Identification of trichothecenes by thermospray, plasmaspray and dynamic fast-atom bombardment liquid chromatography-mass spectrometry

Thermospray, plasmaspray and dynamic fast-atom bombardment liquid chromatography-mass spectrometry are compared for the identification of six trichothecenes. Thermospray spectra of the trichothecenes exhibit only a very abundant ammonium adduct ion. Plasmaspray, which provides a more energetic ionization process than thermospray, produces some fragment ions in addition to an abundant ammonium adduct ion. The spectra obtained by dynamic fast-atom bombardment exhibit a protonated molecule, a glycerol adduct ion and numerous fragment ions formed by the losses of functional groups as neutrals in various combinations. Thermospray and plasmaspray are suitable only for monitoring of the trichothecenes, whereas dynamic fast-atom bombardment is suitable for monitoring and for structure characterization.     

Metastable ions arising from pseudomolecular [M-H]- ions produced by fast-atom bombardment negative-ion mass spectrometry of ecdysteroids

Metastable ions arising from pseudomolecular [M-H]- ions produced by fast-atom bombardment negative-ion mass spectrometry of a range of free ecdysteroids, ecdysteroid conjugates and polar metabolites were investigated by means of linked scanning at constant B/E. Free ecdysteroids displayed daughter-ion spectra which allow 20-hydroxyecdysteroids and ecdysteroids lacking C-20 hydroxylation to be readily distinguished. The ejection of acetic acid from acetylated ecdysteroids was also readily detectable. Characteristic metastable-ion decomposition of ecdysteroid acids was not observed, presumably as a result of charge localization. High-mass daughter ions were also lacking in the case of phosphate conjugates.     

Matrix-assisted laser desorption using a fast-atom bombardment ion source and a magnetic mass spectrometer.

A conventional fast-atom bombardment (FAB) ion source was used to achieve matrix-assisted laser desorption (MALD) in a high-mass, double-focusing, magnetic mass spectrometer. The pulsed ion signals generated by irradiation of a mixture of sample and matrix (2,5-dihydroxybenzoic acid) with either a XeF excimer laser (353 nm) or a nitrogen laser (337 nm) were recorded with a focal-plane detector. A resolution (full-width at half maximum) of 4500 was achieved at m/z 1347.7 (the peptide substance P), 2500 for CsI cluster ions at m/z 10,005.7, and 1250 for the isotope cluster of the small protein cytochrome c (horse) [M+H]+ = m/z 12,360 (average). Sensitivity is demonstrated with 11 fmol of substance P. A survey scan is taken to locate the m/z of the sample molecular ion. The segment that contains the sample can then be integrated for a longer time to produce a better signal-to-noise ratio. In addition to higher sensitivity and lower matrix interference, the advantage of MALD over FAB is the former's lower susceptibility to the presence of salts, and competition between hydrophobic and hydrophilic components of a mixture. This feature is demonstrated by the complete MALD spectrum of a crude partial tryptic digest of sperm-whale apomyoglobin, containing 24 peptides, representing the entire sequence of this protein.     

Direct analysis of drugs by continuous-flow fast-atom bombardment and tandem mass spectrometry

The techniques of continuous-flow fast-atom bombardment (CF-FAB) and tandem mass spectrometry (MS/MS) are combined and applied to the analysis of small molecular mass drugs (mol.wt less than 500 Da). The approach involves the interfacing of a CF-FAB inlet with a triple-stage quadrupole mass spectrometer, enabling the acquisition of collision-activated decomposition mass spectra of the drugs after FAB ionization. The relationship between a stable sample surface on the CF-FAB probe tip and the quality of the mass spectrum is discussed, as are practical methods for obtaining and maintaining surface stability. CF-FAB MS/MS spectra for several drugs are presented, including penicillin G, phentolamine, cocaine and benzoylecgonine. Minimum detection limits range from 50-500 pg injected, depending on the compound. The reproducibility of the integrated areas of peaks from repetitive injections is approximately five per cent. Data are also presented for the direct CF-FAB MS/MS analysis of cocaine and benzoylecgonine in spiked urine samples.     

Rapid analysis of a peptide by fast-atom bombardment mass spectrometry after polyacrylamide gel electrophoresis

Gel electrophoresis has been a powerful technique for the separation of peptides and proteins for many years. After electrophoresis separation on a polyacrylamide gel, the peptide bradykinin was localized using Coomassie Blue as a staining dye. Excess dye was removed by washing the gel with water. For mass spectrometric analysis, bands containing the peptide were crushed, extracted with acetic acid and the eluent applied to the fast-atom bombardment probe. Under these conditions the protonated molecule of bradykinin was clearly observed. Also apparent were sequence ions at about the same intensity observed from authentic bradykinin.     

If the ionization mechanism in fast-atom bombardment involves ion/molecule reactions, what are the reagent ions? The time dependence of fast-atom bombardment mass spectra and parallels to chemical ionization

The evaporation in vacuo of the matrices used and the particle-induced desorption of matrix molecules in fast-atom bombardment (FAB) contribute to a proposed high pressure region above the FAB matrix known as the selvedge region. If the neutral number density is sufficiently high, ions formed upon bombardment may undergo collisions with molecules, yielding matrix-related cluster ions and, in cases when the analyte is desorbed in neutral form, protonated and deprotonated analyte molecules. Similarities with the chemical ionization (CI experiment have been pointed out previously and are further developed here. If FAB is similar to CI, then the response depends on the structures of the reagent ions — those ions that react with gas phase analyte molecules. We consider here the time dependence of positive and negative ion FAB spectra to attempt to identify the reagent ions of FAB. A model is suggested for the FAB ion source which evaluates similarities to a CI source, as well as spatial aspects that are unique to desorption/ionization techniques.     

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.     

Structural characterization of steroidal saponins by electrospray ionization and fast-atom bombardment tandem mass spectrometry

The structural characterization of four steroidal saponin compounds involving two and three sugar groups, namely spirostanol saponins and furostanol saponins, were investigated by positive ion fast-atom bombardment (FAB), electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) techniques. Important structural information was obtained from collision-induced dissociation (CID) and FAB-MS spectra with different liquid matrices. It was found that a characteristic fragmentation involving the loss of 144 Da arising from the cleavage of the E-ring was observed when there was no sugar chain at the C-26 position. When a glucoside group was substituted at the C-26 position, this C-26 sugar moiety was preferentially eliminated. All of these compounds produced a major product ion with a stable skeleton structure at m/z 255. The results of this paper can assist structural analysis of mixtures of steroidal saponins.     

Structural analysis of rapamycin and related compounds using [M + Li]+ ions generated by liquid secondary ion mass spectrometry

Cationization of the macrocyclic immunosuppressant rapamycin with lithium ion upon liquid secondary ion mass spectrometric ionization yields a number of fragment ions, which are observable in the full-scan spectrum. These are clearly assigned using B/E linked scanning (fragment ion scanning), B²/E linked scanning (precursor ion scanning) and peak matching for accurate mass measurement. Many of the fragments are produced by processes that open the macrocyclic ring, and it is possible to observe several different pieces of the molecule as fragment ions. The diversity of fragments produced facilitates the elucidation of new rapamycin-like structures through mass spectrometry. Three structurally modified rapamycin analogues have been examined by this technique, and the modifications to the molecule may be located based on the nominal masses of their fragments.