Systematic study of the effects of experimental parameters on the beam-induced dehalogenation of chlorpromazine in liquid secondary ion mass spectrometry

The effect of experimental parameters such as time of irradiation, analyte concentration, primary beam density, matrix selection and matrix additives on the beam-induced dehalogenation of chlorpromazine in liquid secondary ion mass spectrometry (LSIMS) was investigated. It was found that dehalogenation of chlorpromazine in glycerol increased with increasing time of irradiation, analyte concentration and primary beam density. These results were compared with those obtained using 4-chlorophenylalanine ethyl ester and the differences observed were rationalized in terms of compound surface activity. Evidence is given that matrix selection is the key experimental parameter affecting the extent of beam-induced dehalogenation of chlorpromazine in LSIMS. Of the eleven matrices used, the greatest extent of dehalogenation was observed in glycerol. Sulfur-containing matrices consistently exhibited a lower extent of dehalogenation than oxygen-containing aliphatic matrices, implying that sulfur is implicated in mitigating the reduction process. Dehalogenation was totally inhibited in 2-hydroxyethyl disulfide, 4-hydroxybenzenesulfonic acid and 3-nitrobenzyl alcohol. Similarly, the use of matrix additives such as 3-nitrobenzyl alcohol and trifluoroacetic acid was found to be useful in inhibiting the extent of dehalogenation occurring in glycerol.     

Investigation of the ionization processes occurring in liquid-assisted secondary ion mass spectrometry of derivatized monosaccharides

Several derivatized monosaccharides, the 2-deoxy-D -ribofuranoses, have been studied by liquid-assisted secondary ion mass spectrometry (LSIMS) in order to gain insight into the factors affecting ionization in FAB/LSIMS. Examination of the mass spectra for these compounds obtained in eight liquid matrices (diethanolamine, ethylene glycol, glycerol, 2-hydroxyethyl disulfide, 2-hydroxyphenethyl alcohol, 3-nitrobenzyl alcohol, sulfolane and thioglycerol) reveals that in all cases the anomalous [M – H]⁺ ion is the predominant species in the molecular ion region and that [M + Na]⁺ species are observed in the presence of Na⁺. The analysis of these compounds by chemical ionization with ammonia shows [M + H]⁺ as the major species while [M – H]⁺ is essentially absent. This indicates that the ionization processes occurring in the two techniques are not analogous. Thermodynamic considerations based on the gas-phase hydride ion affinities of the protonated matrices do not support a predominant gas-phase mechanism for the formation of [M – H]⁺ in LSIMS. However, it is possible using solvation energies to rationalize the formation of [M – H]⁺ in terms of condensed-phase ionization processes which take place either in the liquid matrix or in the dense selvedge region immediately above the surface where extensive solvation is present. Electrospray data obtained for one of the derivatized monosaccharides indicates that the [M – H]⁺ is not performed in the condensed phase in LSIMS and that it is the product of fast ion beam-induced processes. While the nature of the matrix is seen to have little effect on the intensities of [M – H]⁺ and [M + H]⁺ it is observed to be an important factor for the intensity of M⁺˙ for one of the monosaccharides. This effect can be related to the electron-scavenging properties of the matrices and reinforces the hypothesis that condensed phase processes are significant in ionization.     

Detection of a thermally unstable intermediate in the Wittig reaction using low-temperature liquid secondary ion and atmospheric pressure ionization mass spectrometry

The signal of an extremely air sensitive and thermally unstable intermediate, four-membered ring oxaphosphetane in the Wittig reaction was detected for the first time at subambient temperature using liquid secondary ion mass spectrometry (LSIMS) and atmospheric pressure ionization (API) mass spectrometry. A copper probe and interface were constructed to perform LSIMS analysis at the temperature below ?70 °C. A stable signal from a oxaphosphetane ion [in dry tetrahydrofolate (THF)] was obtained by LSIMS without using a viscous matrix. The signal of the oxaphosphetane was also detected using a low-temperature API source (?40 °C) connected to a reaction vessel.     

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.     

The analysis of salen complexes by fast atom bombardment mass spectrometry and atmospheric pressure chemical ionization mass spectrometry

Fast atom bombardment (FAB) mass spectrometry provides useful structural information about salen complexes and salen-based oxo transfer catalysts that are not appreciably soluble in organic solvents. It was discovered that initial dissolution of these complexes in trifluoroacetic acid was crucial for producing good FAB mass spectra. Trifluoroacetic acid helps dissolve the salen-based catalysts, concentrates the analyte molecules at the matrix surface, and most importantly, suppresses the reduction process, which is a well-known phenomenon when protic matrices are used. The best FAB matrices for these catalysts were found to be thioglycerol and “magic bullet.” However, dechlorination occurred under the acid conditions for complexes containing iron chloride and manganese chloride. Demetalation also occurred for nickel-containing oxo transfer salen-based complexes. When the salen-based complexes are soluble in LC solvents, they can be analyzed easily by atmospheric pressure chemical ionization (APCI) mass spectrometry without the employment of relatively nonvolatile matrices. In addition, APCI/MS provides much more sensitive detection for manganese-salen complexes when compared with FAB results. No dechlorination or demetalation were observed when a negative ion mode APCI was employed. To our knowledge, this is the first time that an intact molecule of this type of complex has been observed by mass spectrometry.     

Stereospecific fragmentations in the mass spectra of stereoisomeric isoindoloquinazolines

The mass spectrometric behavior of stereo- and regioisomeric, partially saturated isoindoloquinazolines was studied by positive-ion electron ionization (EI) and fast-atom bombardment (FAB/LSIMS) mass spectrometry combined with collision-induced dissociation (CID). A highly stereospecific retro-Diels-Alder process was observed in the cyclohexene-fused isomers under the EI conditions, and a corresponding (although less specific) fragmentation was observed in their FAB spectra. In the absence of RDA fragmentations, regio- and stereoisomers of the cyclohexane-fused heterocycles could be distinguished based on their FAB/CID spectra.     

Direct evidence for degradation of esperamicin A1 with thiol confirmed by fast-atom bombardment mass spectrometry.

A degradation of esperamicin A1, which is a potent antitumor antibiotic having DNA-cleaving activity, was studied by fast-atom bombardment (FAB) mass spectrometry. It was proved that the degradation easily occurs via the thiol group(s) in the matrix solution and not by a FAB-induced reaction. It was concluded that from the FAB mass spectra of all non-volatile compounds, the molecular weight should be determined by using at least two chemically different matrices.     

Structural characterization of reactive dyes using liquid secondary ion mass spectrometry/tandem mass spectrometry

Reactive Blue 19 (RB 19), its reactive form (RB 19-VS) and its hydrolyzed form (RB 19-OH) were examined using liquid secondary ion mass spectrometry/tandem mass spectrometry (LSIMS/MS/MS) in the negative-ion mode under low-energy collision conditions (240–300 eV). Structurally characteristic fragment ions were obtained, none of which has been previously reported for these reactive dyes. Among the ions obtained were SO₃^? ions, ions due to central amino cleavage and reactive group cleavage, and ions due to loss of SO₃ and SO₂. Possible pathways for the formation of product ions are proposed. The structural information obtained should help to characterize and identify reactive dyes better.     

Fast-atom bombardment of the cyclic acetals: Evidence indicating the predominant involvement of condensed-phase processes in ionization

A series of cyclic acetals, the 2-phenyl-l,3 dioxolanes, and their deuterated analogues were studied by electron ionization (EI), chemical ionization (CI), and fast-atom bombardment (FAB) mass spectrometry to gain insight into the primary ionization processes for these compounds in FAB/liquid secondary ion mass spectrometry. Comparison of EI and CI data with that of FAB led to the conclusion that the predominant [M - H]+ ion observed in FAB for the nondeuterated cyclic acetals cannot to a large extent be rationalized in thermodynamic terms by known gas-phase ion-molecule reactions. Instead, a condensed-phase model in which the multicharged transition state for hydride abstraction is better solvated than the transition state for proton transfer appears to be a plausible explanation for the FAB data obtained for the nonlabeled cyclic acetals; however, this explanation is not entirely sufficient to rationalize the FAB data for the deuterated cyclic acetals. For these compounds, a dramatic time dependence of protonation versus hydride abstraction is observed that suggests that beam-induced reactive species are responsible for hydride abstraction in the condensed phase. This time dependence can be interpreted in terms of a buildup of highly reactive beam-induced species in the bulk of solution. Comparison of the results obtained for deuterated acetals with different surface activities support this hypothesis. (J Am Sot Mass     

Structural characterisation of flavonoids and flavonoid-O-glycosides extracted from Genista tenera by fast-atom bombardment tandem mass spectrometry

Genista tenera is a plant native to the Madeira Island (Portugal). From the ethanol extract of its powdered aerial parts, two flavones, three isoflavones and one 7-O-glucosyl isoflavone were isolated. A mass spectrometric study of these compounds was performed using liquid secondary ion mass spectrometry (LSIMS) in combination with high-energy collision-induced dissociation (CID) and tandem mass spectrometry (MS/MS). Characteristic fragmentation patterns were observed in all the investigated compounds; the loss of small neutral species from the protonated molecules was useful for identifying the presence of specific functional groups in the A- and B-rings. In order to help to establish the proposed structures, NMR and UV studies were also performed.     

Identification of hemes and related cyclic tetrapyrroles by matrix-assisted laser desorption/ionization and liquid secondary ion mass spectrometry

Mass spectrometry has proven to be a powerful technique applicable on trace amounts for the identification of known hemes and cyclic tetrapyrroles, and for providing critical information for the structure of new and novel versions. This report describes investigations of the practical limits of detection for such bioinorganic prosthetic groups, primarily by liquid secondary ion mass spectrometry (LSIMS) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), including a survey of the utility of common matrices. The lower limit of detection under favorable conditions extends to low picomole amounts. Certain derivatization techniques, such as methyl esterification and chelation to zinc, both increase the sensitivity of analyses and provide spectroscopic signatures that enable heme/cyclic tetrapyrrole ions to be identified in the presence of contaminants.     

Tandem mass spectrometry of aminated fullerene derivatives

The products of the reaction between fullerenes (C₆₀/C₇₀) and dimethylamine were investigated by fast atom bombardment (FAB) mass spectrometry and tandem mass spectrometry (MS/MS). The FAB mass spectrum shows peaks corresponding to the addition of up to eight dimethylamine species, exclusively to C₇₀. MS/MS reveals an unusual fragmentation pattern. The mass spectrum of the reaction products, together with a number of tandem mass spectra, are shown.     

Sequence-specific fragmentation from oligopeptides using plasma-desorption mass spectrometry

252Cf plasma-desorption mass spectrometry (PDMS) has been demonstrated to provide sequence-specific fragmentation for several oligopeptides. The nature of the fragment ions observed is generally similar to that observed using liquid secondary-ion mass spectrometry (LSIMS) and can be observed using less sample than LSIMS requires, but PDMS spectra are acquired at a lower resolution. In addition, the molecular weight of some of the oligopeptides studied exceeds that which is generally accepted as within the sequence range of LSIMS. The specific series of sequence ions that predominate in the PDMS spectra appear to be related to the amino acid compositions and sequences of the oligopeptides.     

Ferrichrome: Surprising stability of a cyclic peptide-FeIII complex revealed by mass spectrometry

Ferrichrome, a fungal siderophore that is also utilized by some bacterial species, was studied with liquid secondary ion mass spectrometry (LSIMS) and matrix-assisted laser desorption ionixation (MALDI) mass spectrometry. A strong ionic signal corresponding to a Fe^III complex was observed with LSIMS in the positive ion mode. Switching the polarity of the mass spectrometer did not necessarily result in reduction of ferric ion, although certain conditions led to appearance of a Fe^II complex signal as well. The results of the structural studies of the metal ion-cyclic peptide complex with collisionally induced dissociation allowed unambiguous identification of the chelation sites. The action of the siderophore on Fe^III was studied by in vitro chelation of ferric ion (from ferric citrate) by the iron-free ferrichrome. Effective chelation of ferric ion was compared to actions of the iron-free ferrichrome on other metal ions. Unlike LSIMS, desorption with MALDI did not form selectively molecular ions of intact ferrichrome: the spectra contained abundant peaks corresponding to the cyclic peptide itself and its nonspecific association with alkali metal ions.     

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.     

Is there a ‘matrix suppression effect’ under fast‐atom bombardment liquid secondary ion mass spectrometry of ionic surfactants in glycerol?

Some features of a ‘matrix suppression effect’ caused by ionic surface‐active compounds under fast‐atom bombardment (FAB) liquid secondary ion mass spectrometry (LSIMS) are being revised. It is shown that abundant transfer of the glycerol matrix molecules to the gas phase does occur under FAB‐LSIMS of ionic surfactants, contrary to popular belief. This process can be obscure because of the dependence of the charge state of the glycerol‐containing cluster ions on the type of ionic surfactant. It is revealed that, while glycerol matrix signals are really completely suppressed in the positive ion mass spectra of cationic surfactants (decamethoxinum, aethonium), abundant deprotonated glycerol and glycerol‐anion clusters are recorded in the negative ion mode. In the case of an anionic surfactant (sodium dodecyl sulfate), on the contrary, glycerol is completely suppressed in the negative ion mode, but is present in the protonated and cationized forms in the positive ion mass spectra. It is suggested that such patterns of positive and negative ion FAB‐LSIMS spectra of ionic surfactants solutions reflect the structure and composition of the electric double layer formed at the vacuum‐liquid interface by organic cations or anions and their counterions. Processes leading to the formation of the glycerol‐containing ions preferentially of positive or negative charge are discussed. The most obvious of them is efficient binding of glycerol to inorganic counterions of the salts Cl^? or Na⁺, which is confirmed by data from quantum chemical calculations. The high content of the counterions and relatively small content of glycerol in the sputtered zone may be responsible for the charge‐selective suppression of neat glycerol clusters of opposite charge to the counterions. In the case of a mixture of cationic and anionic surfactants the substitution of inorganic counterions by organic ones was observed. The dependence of the exchange rate in the surface layer is not a linear function of the bulk solution concentration, and an effect of abrupt recharging of the surface can be registered. No both positively or negatively charged pure glycerol and glycerol‐inorganic counterion clusters are recorded for the mixture. Correlations between the mass spectrometric observations and some phenomena of surface and colloid chemistry and physics are discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Influence of experimental conditions on the liquid secondary ion mass spectra of sulfonated azo dyes

Two monosulfonated and eight disulfonated azo dyes of varying relative molecular mass were examined by liquid secondary ion mass spectrometry (LSIMS). The effects of matrix, concentration, primary beam energy, and mode of operation were addressed in order to optimize sample ionization, whilst minimizing interference from matrix ions. Seven matrices were investigated: glycerol, thioglycerol, 3-nitrobenzyl alcohol, diethanolamine, 2-hydroxyethyl disulfide, a 1:1 (v/v) mixture of 2-hydroxyethyl disulfide and thioglycerol, and a 1 : 3 (v/v) mixture of dithioerythritol and dithiothreitol. Of these matrices, 3-nitrobenzyl alcohol produced LSIMS spectra that exhibited the most intense sample ions and the least inteiference from matrix ions. Minimum concentrations of 0.4 μg/μl and 4 μg/μl (dye in matrix) were necessary to produce useful full-scan spectra for monosulfonated azo dyes and disulfonated azo dyes, respectively; maximum sample ion intensities were obtained with concentrations ranging from 20 μg/μl to 60 μg/μl. A primary ion beam (cesium) of 10 to 15 kV produced the greatest secondary ionization efficiency, and a negative-ion analysis mode produced more useful spectra than those obtained in the positive-ion mode.     

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.     

Characterization of an opioid peptide-containing protein and of bovine α-lactalbumin by electrospray ionization and liquid secondary ion mass spectrometry

Mass spectrometry methods have been used to characterize two proteins: an opioid peptide-containing protein extracted from bovine pituitary, and bovine α-lactalbumin (BAL). A protein that contains β-endorphin was found in bovine pituitary, and that protein was characterized with electrospray ionization mass spectrometry (ESIMS), gel permeation chromatography, reversed-phase high performance liquid chromatography (RP-HPLC), radioimmunoassay, trypsinolysis, and liquid secondary ion mass spectrometry (LSIMS). BAL is a protein that was used as a model to develop analytical methods to study opioid peptide-containing proteins. Commercial BAL was purified by RP-HPLC, and its molecular weight (M.W.) was determined by ESIMS. The shift in mass observed following dithiothreitol (DTT) reduction estimated the number of disulfide bonds. For all of the data obtained for BAL with or without RP-HPLC separation, ESIMS determined the M.W. of the peptides produced by trypsin treatment of BAL, and LSIMS selected a precursor ion, the protonated molecule ion [M + H]⁺, of a tryptic peptide, which was analyzed by tandem mass spectrometry. Following DTT reduction, ESIMS and LSIMS detected each peptide that contained disulfide bonds in that mixture of tryptic peptides.     

Analysis of an egyptian mummy resin by mass spectrometry

Fast atom bombardment combined with mass spectrometry (FAB/MS), high resolution FAB/MS, FAB tandem mass spectrometry (MS/MS), and gas chromatography/mass spectrometry (GC/MS) were used to determine the composition of the resinous material recovered from the wrappings of an Egyptian mummy believed to be from the Roman period (100–350 A.D.). FAB/MS and MS/MS studies identified several oxidation products of abietic acid as the principal resin components, indicating that one or more species of coniferous trees were used by the Egyptians as a source of the resin. GC/MS studies also identified several n-alkanes with carbon numbers from C₁₉ to C₃₃ in the sample. The relative amounts of these n-alkanes, along with characteristic trace metals, indicate that bitumen, an asphalt native to the region, was added to the resin. The presence of this ancient source of carbon in this sample explains the inconsistent date assigned to the mummy by carbon-14 analysis.