Origin of the tailing signal on the low-energy side of the main beam in mass-analyzed ion kinetic energy spectra

The tailing signal on the low-energy side of the precursor ion signal observed during fast atom bombardment (FAB) mass-analyzed ion kinetic energy spectrometric (MIKES) analyses is due largely to ions of higher m/z value than the chosen precursor. The majority of these ions are independent, unfragmented species that emerge from the ion source with less than the full amount of kinetic energy predicted by the source potential. The tailing precursor ion signal observed under helium collision-activated decomposition conditions is too short to account for the protracted MIKES tail (as judged from mass-to-charge ratio-deconvoluted MIKES analyses performed on a BEqQ hybrid instrument), and a tailing precursor signal is not observed under unimolecular decomposition conditions. Measurements of the mass-to-charge ratios of the ionic species comprising the MIKES tail demonstrated that ions higher in mass-to-charge ratio than the chosen precursor are present throughout the tail, with the mass-to-charge ratio increasing as kinetic energy decreases. These ions possess the same momentum as the chosen precursor, and thus were formed prior to the magnetic field. The existence of intact, source-formed [M + H]⁺ ions with reduced kinetic energy was demonstrated through several types of tandem mass spectrometric experiments. These [M + H]⁺ ions with reduced kinetic energy do not appear to have undergone collisional deceleration, because they do not possess increased internal energy (as judged by observation of their fragmentation patterns). The kinetic energy profiles of unfragmented FAB-desorbed ions were determined and found to exhibit a tailing character similar in appearance to that of the MIKES tail. The population of ions emerging from the source under FAB conditions thus incorporates the characteristics necessary to account for the MIKES tail, namely, the presence of ions of a mass-to-charge ratio higher than the chosen precursor (due to matrix and other background ions), which possess reduced kinetic energy such that their momentum is identical to that of the selected precursor. These ions may arise via desolvation and declustering processes in the acceleration region of the ion source, or via FAB or chemical ionization processes in regions removed from the FAB target.     

Structural determination of glucosylceramides isolated from marine sponge by fast atom bombardment collision‐induced dissociation linked scan at constant B/E

Five glucosylceramides (GlcCers) were isolated by reversed phase high‐performance liquid chromatography from the MeOH extracts of a marine sponge, Haliclona (Reniera) sp., collected from the coast of Ulleung Island, Korea, and analyzed by fast atom bombardment mass spectrometry (FAB–MS) in positive‐ion mode. FAB‐mass spectra of these compounds included protonated molecules [M + H]⁺ and abundant sodiated molecules [M + Na]⁺ from a mixture of m‐NBA and NaI. The structures of these GlcCers, which were similar, were elucidated by FAB‐linked scan at constant B/E. To find diagnostic ions for their characterization, the GlcCers were analyzed by collision‐induced dissociation (CID) linked scan at constant B/E. The CID‐linked scan at constant B/E of [M + H]⁺ and [M + Na]⁺ precursor ions resulted in the formation of numerous characteristic product ions via a series of dissociative processes. The product ions formed by charge‐remote fragmentation provided important information for the characterization of the fatty N‐acyl chain moiety and the sphingoid base, commonly referred to as the long‐chain base. The product ions at m/z 203 and 502 were diagnostic for the presence of a sodiated sugar ring and β‐D ‐glucosylsphinganine, respectively. For further confirmation of the structure of the fatty N‐acyl chain moiety in each GlcCer, fatty acid methyl esters were obtained from the five GlcCers by methanolysis and analyzed by FAB–MS in positive‐ion mode. On the basis of these dissociation patterns, the structures of the five GlcCers from marine sponge were elucidated. In addition, the accurate mass measurement was performed to obtain the elemental composition of the GlcCers isolated from marine sponge. Copyright © 2009 John Wiley & Sons, Ltd.     

Fast atom bombardment (FAB) mass spectrometry of piperidine N-oxide derivatives and free radical quenching under FAB conditions

Mechanisms are proposed for the formation of M⁺, [M + 2H]⁺ and [M + 3H]⁺ ions in the fast atom bombardment (FAB) mass spectra of 4-(2,2,6,6-tetramethyl-1-oxyl)-piperidol and its carboxylates. Free radical quenching induced by the fast atom beam has been observed. The effects of temperature on the radical quenching and of acid on the FAB mass spectra are discussed. The experiment showed that the volatile liquid samples with vapour pressures higher than that for glycerol produced M⁺ even-electron molecular ions, and the FAB mass spectra were similar to the corresponding electron ionization mass spectra. For the solid samples, it was found that the free radicals were quenched during the FAB process so that the mononitroxide and dinitroxide compounds produced [M + 2H]⁺ and [M + 3H]⁺ ions, respectively. Further experiments showed that the intensities and stabilities of [M + 2H]⁺ and [M + 3H]⁺ ions could be improved by addition of acids.     

Differentiation of anomeric C-glycosides by mass spectrometry using fast atom bombardment,mass-analysed ion kinetic energy and collision-activated dissociation

Positive-ion fast atom bombardment mass spectrometry appears to be a useful method for the differentiation of anomeric C-glycosides. The mass-analysed ion kinetic energy (MIKE) and collision-activated dissociation (CAD) MIKE spectra of selected positive ions can be used as fingerprints of the α- or β-anomers. The main fragmentation routes and particularly the formation of the [M ? H]⁺ ion and the [M + H ? PhCH₂OH]⁺ ion were traced for each anomer.     

Fast atom bombardment mass spectrometric study of some unsaturated aryl C-glycosides

Fast atom bombardment (FAB), FAB mass-analysed ion kinetic energy (FAB MIKE) and collision-activated dissociation (FAB CAD-MIKE) mass spectra were obtained for two series of unsaturated anomeric aryl C-glycosides. These tandem mass spectrometric techniques allowed the differentiation of the anomers by analysing either the [M + H]⁺ ion or the [M + met]⁺ ion (met=Li, Na).     

A 15N labelling,FD and FAB study of ammonia loss from protonated arginine molecules

The positive ion field desorption (FD) spectrum of arginine taken at the best anode temperature only contains a peak due to [M+H]⁺ ions. At higher emitter temperatures a considerable amount of fragmentation is induced and the [M+H NH₃]⁺ ions become most abundant. Specific ¹⁵N labelling reveals that the eliminated ammonia molecule, exclusively, contains one of the terminal nitrogen atoms of the guanidyl group. This also applies to the ammonia loss from metastably decomposing [M+H]⁺ ions. The positive ion fast atom bombardment (FAB) spectrum shows more fragmentation than the FD spectrum. In contrast with the FD results, the [M+H]⁺ ions generated upon FAB with ion lifetimes <10⁻⁶ s eliminate both ammonia containing one of the terminal nitrogen atoms of the guanidyl group and ammonia containing the α-amino group in the ratio of 1.35, as found by ¹⁵N labelling. The metastably decomposing [M+H]⁺ ions, however, eliminate only the former ammonia molecule. In the negative ion FD and FAB spectra no other peak than that corresponding to the [M H]⁻ ion is observed. Some attention has been paid to the thermal degradation of arginine on the basis of a few Curie-point pyrolysis experiments.     

Ion-molecule reactions observed for nitroaromatic compounds in negative ion fast atom bombardment mass spectrometry

Analyses of a series of nitroaromatic compounds using fast atom bombardment (FAB) mass spectrometry are discussed. An interesting ion-molecule reaction leading to [M + O ? H]^? ions is observed in the negative ion FAB spectra. Evidence from linked-scan and collision-induced dissociation spectra proved that [M + O ? H]^? ions are produced by the following reaction: M + NO₂^? → [M + NO₂]^? → [M + O ? H]^?. These experiments also showed that M^?˙ ions are produced in part by the exchange of an electron between M and NO₂^? species. All samples showed M^?˙, [M ? H]^? or both ions in their negative ion FAB spectra. Not all analytes studied showed either [M + H]⁺ and/or M⁺˙ in the positive ion FAB spectra. No M⁺˙ ions were observed for ions having ionization energies above ～9 eV.     

A fast atom bombardment and field ionization/field desorption study of some isomeric unsaturated dicarboxylic acids

Geometrically isomeric dicarboxylic acids, such as maleic and fumaric acid and their methyl homologues, and the isomeric phthalic acids, have been investigated using fast atom bombardment, field ionization and field desorption mass spectrometry. The most intense peak in the positive ion fast atom bombardment spectra corresponds with the [M + H]⁺ ion. This ion, when derived from the E -acids, tragments either by successive loss of water and carbon monoxide or by elimination of carbon dioxide. In the case of the Z -acids only elimination of water from the [M + H]⁺ ions is observed to occur to a significant extent. The same is true for the [M + H]⁺ ions of the isomeric phthalic acids, that is the [M + H] ions derived from iso- and terephthalic acid exhibit more fragmentation than those of phthalic acid. All these acids undergo much less fragmentation upon field ionization, where not only abundant [M + H]⁺ ions, but also abundant [M]^+· ions, are observed. Upon field desorption only the [M + H]⁺ and [M + Na]⁺ ions are observed under the measuring conditions. Negative ion fast atom bombardment spectra of the acids mentioned have also been recorded. In addition to the most abundant [M? H]^? ions relatively intense peaks are observed, which correspond with the [M]^?˙ ions. The fragmentations observed for these ions appear to be quite different from those reported in an earlier electron impact study and in a recent atmospheric pressure ionization investigation.     

Use of fast atom bombardment mass spectrometry for the characterization of nitroaromatic isomers

Positive and negative ion fast atom bombardment (FAB) mass spectra of some monosubstituted nitroaromatic isomers are reported. Generally ions carresponding to [M + H]⁺ and M⁺ are observed in the positive ion FAB spectra; ions such as [M ? H] ^? and M^?˙ are observed in the negative ion FAB spectra. The use of FAB mass spectra to distinguish the isomers is discussed. Comparisons of FAB, chemical ionization and electron impact mass spectra of the same isomers (wherever possible) are reported. The structural information obtained in the negative ion FAB spectra complement those obtained in the positive ion FAB spectra.     

Structure and further fragmentation of significant [a3 + Na − H]+ ions from sodium‐cationized peptides

A good understanding of gas‐phase fragmentation chemistry of peptides is important for accurate protein identification. Additional product ions obtained by sodiated peptides can provide useful sequence information supplementary to protonated peptides and improve protein identification. In this work, we first demonstrate that the sodiated a₃ ions are abundant in the tandem mass spectra of sodium‐cationized peptides although observations of a₃ ions have rarely been reported in protonated peptides. Quantum chemical calculations combined with tandem mass spectrometry are used to investigate this phenomenon by using a model tetrapeptide GGAG. Our results reveal that the most stable [a₃ + Na ? H]⁺ ion is present as a bidentate linear structure in which the sodium cation coordinates to the two backbone carbonyl oxygen atoms. Due to structural inflexibility, further fragmentation of the [a₃ + Na ? H]⁺ ion needs to overcome several relatively high energetic barriers to form [b₂ + Na ? H]⁺ ion with a diketopiperazine structure. As a result, low abundance of [b₂ + Na ? H]⁺ ion is detected at relatively high collision energy. In addition, our computational data also indicate that the common oxazolone pathway to generate [b₂ + Na ? H]⁺ from the [a₃ + Na ? H]⁺ ion is unlikely. The present work provides a mechanistic insight into how a sodium ion affects the fragmentation behaviors of peptides. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterization of sodiated glycerol phosphatidylcholine phospholipids by mass spectrometry

Fast atom bombardment mass spectrometry in the positive mode was used for the characterization of sodiated glycerol phosphatidylcholines. The relative abundance (RA) of the protonated species is similar to the RA of the sodiated molecular species. The sodiated fragment ion, [M + Na - 59](+), corresponding to the loss of trimethylamine, and other sodiated fragment ions, were also observed. The decomposition of the sodiated molecule is very similar for all the studied glycerol phosphatidylcholines, in which the most abundant ion corresponds to a neutral loss of 59 Da. Upon collision-induced dissociation (CID) of the [M + Na](+) ion informative ions are formed by the losses of the fatty acids in the sn-1 and sn-2 positions. Other major fragment ions of the sodiated molecule result from loss of non-sodiated and sodiated choline phosphate, [M + Na - 183](+), [M + Na - 184](+.) and [M + Na - 205](+), respectively. The main CID fragmentation pathway of the [M + Na - 59](+) ion yields the [M + Na - 183](+) ion, also observed in the CID spectra of the [M + Na](+) molecular ion. Other major fragment ions are [M + Na - 205](+) and the fragment ion at m/z 147. Collisional activation of [M + Na - 205](+) results in charge site remote fragmentation of both fatty acid alkyl chains. The terminal ions of these series of charge remote fragmentations result from loss of part of the R(1) or R(2) alkyl chain. Other major informative ions correspond to acylium ions.     

The renaissance of high-energy CID for structural elucidation of complex lipids: MALDI-TOF/RTOF-MS of alkali cationized triacylglycerols

Triacylglycerols were analyzed as cationized species (Li⁺, Na⁺, K⁺) by high-energy CID at 20 keV collisions utilizing MALDI-TOF/RTOF mass spectrometry. Precursor ions, based on [M+Li]⁺-adduct ions exhibited incomplete fragmentation in the high and low m/z region whereas [M+K]⁺-adducts did not show useful fragmentation. Only sodiated precursor ions yielded product ion spectra with structurally diagnostic product ions across the whole m/z range. The high m/z region of the CID spectra is dominated by abundant charge-remote fragmentation of the fatty acid substituents. In favorable cases also positions of double bonds or of hydroxy groups of the fatty acid alkyl chains could be determined. A-type product ions represent the end products of these charge-remote fragmentations. B- and C-type product ions yield the fatty acid composition of individual triacylglycerol species based on loss of either one neutral fatty acid or one sodium carboxylate residue, respectively. Product ions allowing fatty acid substituent positional determination were present in the low m/z range enabling identification of either the sn-1/sn-3 substituents (E-, F-, and G-type ions) or the sn-2 substituent (J-type ion). These findings were demonstrated with synthetic triacylglycerols and plant oils such as cocoa butter, olive oil, and castor bean oil. Typical features of 20 keV CID spectra of sodiated triacylglycerols obtained by MALDI-TOF/RTOF MS were an even distribution of product ions over the entire m/z range and a mass accuracy of ±0.1 to 0.2 u. One limitation of the application of this technique is mainly the insufficient precursor ion gating after MS1 (gating window at 4 u) of species separated by 2 u.     

Structural determination of oxofatty acids by charge-remote fragmentations

A strategy is described to locate the carbonyl position in oxofatty acids by utilizing charge-remote fragmentations of various molecular ions that are desorbed by fast atom bombardment (FAB). Oxofatty acids were cationized with alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) to form [M+2Met?H]⁺ or alkaline earth metal ions (Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺) to form [M+Met?H]⁺ in the gas phase. The cationized acids undergo charge-remote fragmentations upon high-energy activation, giving a product-ion pattern that has a gap corresponding to the oxo position and bordered by two high-intensity peaks. One of the peaks corresponds to an ion that is formed by the cleavage of the C-C bond β to the oxo position and proximal to the charge (β ion), whereas the other is formed from the cleavage of the C-C bond γ to the oxo position and distal to the charge (γ′ ion). The oxo position is easily determined by identifying the gap and the β and γ′ ions. Furthermore, there are two competing patterns of fragments in a CAD spectrum of an oxofatty acid or ester [M+Li]⁺ ion. These arise because Li⁺ attaches to either the oxo or the carboxylic end, as was confirmed by ab initio molecular orbital calculations. The results demonstrate that control of the fragmentation can be guided by an understanding of metal-ion affinities. Collisional activation of the anionic carboxylates gives results that are similar to those for positive ions, showing that the process is not related to the charge status. Collisional activation of [M+H]⁺ ions does not give structural information because the charge migrates, leading to charge-mediated fragmentations.     

Characteristic neutral loss of CH3CHO from Thr‐containing sodium‐associated peptides

A characteristic neutral loss of 44 Da is observed in the MS/MS spectra of Thr‐containing sodiated peptides. A combination of tandem mass spectrometry and quantum chemical calculations calculated at the B3LYP/6‐311G (d, p) level of ab initio theory is used to elucidate this fragmentation pathway. The high resolution mass spectrometry data indicate this neutral loss is acetaldehyde lost from the side chain of Thr rather than CO₂. The intensity of this neutral loss can be enhanced when Thr residue is far from the C‐terminus and when the C‐terminus is esterified as well. The mechanism of the acetaldehyde loss is proposed to adopt a McLafferty‐type rearrangement reaction, which involves a proton transfer from the hydroxyl of Thr side chain to its C‐terminal neighboring carbonyl oxygen inducing the cleavage of the C_a–C_β bond. This mechanism is further supported by examining the fragmentation of a [GT(tBu)G + Na]⁺ peptide derivative and by comparing the product ion spectra of [M + Na‐44]⁺ of [GTGA + Na]⁺ with [M + Na]⁺ of [GGGA + Na]⁺. A similar neutral loss of HCHO can also be detected in Ser‐containing peptides. Our computational results reveal that the most stable [GTG + Na]⁺ ion is present as a tridentate charge‐solvated structure and the dissociation leading to the 44 loss is dynamically and energetically favorable. Copyright © 2015 John Wiley & Sons, Ltd.     

Electrospray ionization for analysis of platelet-activating factor.

Platelet-activating factor (PAF) was analyzed by electrospray-ionization mass spectrometry (ESI-MS) using a single quadrupole mass spectrometer. The positive-ion spectrum was dominated by an ion corresponding to a sodiated molecule when a low potential difference between the capillary exit (nozzle) and the skimmer was employed, but when the capillary exit voltage was increased, fragmentation of PAF was observed. Initial fragmentation involved the loss of the elements of trimethylamine from the sodiated molecule to yield [M+Na-59]+. An intense ion at m/z 147, generated by the loss of trimethylamine from the sodiated phosphocholine portion of the molecule was also detected, along with a lower intensity ion at m/z 184 which is representative of a protonated phosphocholine moiety. With negative-ion detection the major molecular species was [M+Cl]-. Interpretation of the mass spectral fragments was verified by ESI tandem mass spectrometry on a triple-quadrupole tandem mass spectrometer.     

Direct evidence for preformed ions of porphyrins in the solvent matrix for fast atom bombardment mass spectrometry

The observation that protonated molecules are present in solvents utilized for fast atom bombardment (FAB) mass spectrometric studies has been demonstrated using visible absorption spectrometry. Addition of porphyrins to thioglycerol, a solvent used for FAB analyses, results in partial protonation of the molecule. This reaction can be monitored by observing the shift in visible absorption maxima associated with the molecular transition from free base to protonated structure. A good correlation is observed between the degree of protonation indicated by the appropriate absorption bands and the abundance of the [M + H]⁺ ion in the FAB spectrum of the corresponding solution. Addition of certain non-polar porphyrin molecules to thioglycerol does not result in the protonation of the molecule in solution; in these cases, analyses of the corresponding solutions by FAB do not yield [M + H]⁺ ions. Subsequent addition of trifluoroacetic acid to the solvent has proved sufficient to protonate the analyte molecule, as indicated by the visible absorption spectrum; FAB analyses of these non-polar porphyins in acidified solvent result in the observation of [M + H]⁺ ions. These experiments demonstrate that analyses of these analyte molecules requires that they be present as ions in solution prior to analysis by FAB. This study provides experimental evidence for the presence of ions in solutions employed for FAB analysis, suggesting that these ions are essential for the generation of the protonated molecules observed during FAB mass spectrometric analyses.     

A mass spectrometric investigation on some 4,7-dioxo-4,5,6,7-tetrahydroindole derivatives

The mass spectrometric behaviour of a series of 2-aryl substituted 4,7-dioxo-4,5,6,7-tetrahydroindoles has been studied in different ionization conditions (Electron Ionization and Fast Atom Bombardment), with the aid of the metastable ion studies. In electron ionization conditions all the compounds exhibit a highly favoured, primary H₂ loss giving rise to the corresponding indole-4,7-diones; in the usual spectra no evidence for the molecular ions in the enolic form was found, while the OH* loss observed in the MIKE (mass analyzed ion kinetic energy) spectra of molecular ions suggests that species at low internal energy content isomerize to the corresponding tautomeric enolic form. FAB mass spectra show easy formation of an unusual [M + 2H]⁺ species, together with abundant [M + H]⁺ and M⁺ cations.     

Odd-electron molecular ion and loss of toluene in fast atom bombardment mass spectra of some carotenoids

Positive-ion fast atom bombardment (FAB) and B/E linked scan FAB mass spectra of seven carotenoids are reported. In all cases the M ^+· ions are observed, and the [M + H]⁺ ions are absent in the hydrocarbons and weak in the oxygenated compounds. The usefulness of B/E linked scan FAB mass spectra to distinguish isomers and to attribute the loss of toluene from the M ^+· to an ionic fragmentation and not to a thermal process is discussed.     

Ion formation in fast atom bombardment mass spectrometry: a divided probe investigation of gas-phase reactions

Some ion-formation processes during fast atom bombardment (FAB) are discussed, especially the possibility of reactions in the gas phase. Divided (two halves) FAB probe tips were used for introducing two different samples into the source at the same time. Our results showed [M + A]⁺ ions (where M = crown ethers and A = alkali metal ions), can be produced, at least in part, in the gas phase when crown ethers and sources of alkali metal ion are placed on two halves of the FAB probe tip. The extent of this ion formation depends on the volatility of the crown ether and on steric factors. Cluster ions such as (M + LiCl)Li⁺, (2M + LiCl)Li⁺, [2M + K]⁺ and [2M + Na]⁺ are also observed to form in the gas phase. Unimolecular decompositions contribute to some ions detected in FAB. When the alkali ion salt and the crown ether are mixed together the probability of [M + A]⁺ ion formation increases significantly, regardless of the volatility of the crown ether.     

Charge remote fragmentation of fatty acids cationized with alkaline earth metal ions

Fatty acids can be collisionally activated as [M ? H + Cat]⁺, where Cat is an alkaline earth metal, by using tandem mass spectrometry. High-energy collisional activation induces charge remote fragmentation to give structural information. In the full scan mass spectra molecular ions are easily identified, particularly when barium is used as a cationizing agent; ions are shifted to a higher mass, lower chemical noise region of the mass spectrum. Moreover, the isotopic pattern of barium is characteristic, and the high mass defect of barium allows an easy separation of the cationized analyte from any remaining interfering ions (chemical noise), provided medium mass-resolving power is available. An additional advantage is that most of the ion current is localized in [M ? H + Cat]⁺ species. Structural analysis of fatty acids can be performed when the sample size is as low as 1 ng.