Mass-analysed Ion Kinetic Energy Spectra Research Articles

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

AbstractThe 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.

Metastable ion study of fluorinated organic compounds. Part 3. α, α, α-trifluoroanisole and α, α, α-trifluorocresols

The spontaneous unimolecular dissociation reactions of the molecular ions of the C 7H 5F 3O positional isomers α, α, α-trifluoroanisole ( 1), o-α, α, α,-trifluorocresol ( 2) and m-α, α, α-trilfluorocresol ( 3), have been investigated by mass-analyzed ion kinetic energy (MIKE) spectrometry and deuterium-labellin. The results are compared with those of the non-fluorinated analogues anisole ( 4), o-cresol ( 5) and m-cresol ( 6). Ions 1 .+ and 4 .+ fragment in an analogue fashion yielding MIKE spectra which are characteristically different from those isaomers of the isomers 2 .+, 3 .+, 5 .+ and 6 .+ whose MIKE spectra are closely similar, the fluorinated analogues 2 .+ and 2 .+ have different characteristics. This is because a pronounced ortho-effect is operative in 2 .+.

Differentiation of anomeric glycosyl azides using mass spectrometric methods

AbstractElectron ionization (EI), chemical ionization and mass‐analysed ion kinetic energy (MIKE) spectra have been applied with the aim of obtaining an unambiguous differentiation between the anomeric 2,3‐O‐isopropylidene α‐and β‐D‐ribofuranosyl azide derivatives. The MIKE spectra of their metastable [M  CH3]+ ions, originating from EI, showed a decomposition pattern allowing distinction between the anomers. The origin of the ions in the EI mass spectra, supported by examination of the deuterium‐labelled derivatives, makes it possible to identify the actual azido group orientation in a particular anomer.

Influence of instrumental parameters and sample preparation on mass‐analysed ion kinetic energy spectra with fast atom bombardment exemplified with the oxonium ion of peracetylated ribopyranose

AbstractWhen mass‐analysed ion kinetic energy (MIKE) spectra are required to discriminate between isomeric ions formed under conditons of fast atom bombardment (FAB) in the ion source, severe interference may be observed. The interfering peaks in MIKE spectra obtained with a reversed‐geometry instrument can arise from different sources. Moreover, the intensity distribution of the true ions from the selected precursor ion may depend strongly on the instrument being used. This means that the FAB–MIKE or collisionally induced dissociation (CID) spectrum is not an absolute characteristic of a particular ion. The [M + H − HOAc]+ ion in the spectrum of peracetylated ribopyranose is used as an example to illustrate this and to trace and discuss the origin of the phenomena observed.

Rearrangements in the Molecular Ions of Some ortho-Substituted Schiff Bases

Under electron ionization conditions, the ortho-substituted Schiff bases N-benzylidene-o-toluidine (1a), N-(o-methylbenzylidene)aniline (1b), N-salicylideneaniline (1c) and N-(o-methoxybenzylidene)aniline (1d) give fragment ions which have been shown by collision-activated mass-analysed ion kinetic energy spectra to have the structure of the protonated molecular ions of indole (2), benzofuran (3), and 1,2-benzisoxazole (4). The molecular ion of N-(o-methylbenzylidene)-o-toluidine (1f) gives as fragment ions not only the protonated molecular ion (2) of indole and the tropylium ion but also the molecular ion of anthracene. Attempts to find supporting evidence for a mechanism for this rearrangement by deuterium labelling of a methyl group in (1b), such as (1g), have been unsuccessful.

Mass-analysed ion kinetic energy and collisionally induced dissociation mass spectra of clusters of acetylated xylo-oligosaecharides with protonated ammonia and methylannine

Per-O-acetylated methyl glycosides of D-xylan-type di- and trisaccharides were studied by mass-analysed ion kinetic energy (MIKE) and collisionally induced dissociation (CID) mass Spectrometry using protonated ammonia and methylamine, respectively, as reaction gases in chemical ionization (CI). The oligosaccharides form abundant cluster ions, [M + NH 4] + or [M + CH 3NH 3] +, and the main fragmentation of these ions in the MIKE and CID spectra is the cleavage of interglycosidic linkages. Thus, CI (NH 3) or CI (CH 3NH 2) spectra in combination with the MIKE or CID spectra allow the molecular masses, the masses of monosaccharide units and the branching point in oligosaccharides to be established. In the case of disaccharides, it is possible to distinguish the (1 2) linkage from the other types of linkages.

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 + M]+ 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.

Analysis of tetrabenzyl N‐giucosidic, N‐mannosidic and N‐galactosidic Isomers using fast atom bombardment/mass‐analysed ion kinetic energy spectrometry

AbstractA series of new synthetic tetrabenzyl N‐glucosidic, N‐mannosidic and N‐galactosidic isomers were investigated by fast atom bombardment (FAB)/mass‐analysed ion kinetic energy (MIKE) spectrometry. The [M + H]+ ions were obtained with high abundance in the FAB spectra when using 3‐nitrobenzyl alcohol as the matrix. The FAB/MIKE spectra provide characteristic daughter ions fragmented from selected molecular parent ions, allowing these isomers to be differentiated. In addition, an interesting rearrangement was found from the MIKE spectra, indicating that the benzyl (Bzl) group on the sugar ring is rearranged on to the N atom of the base (R) group to form [R + Bzl + H]+ and [R+ 2Bzl]+ ions.

Electron impact studies of alkanesulfonyl chlorides. An investigation of the sulfur dioxide radical cation

Electron impact mass spectrometry was used to study the fragmentation of a number of alkanesulfonyl chlorides. Mass-analyzed ion kinetic energy (MIKE) spectra of the ionic species obtained after the chlorine atom loss, i.e. [RO 2S] +, showed that the direction of further fragmentation depends on the molecular weight, leading to R + and/or SO + 2 cations. Based on experimental evidence an explanation for this phenomenon is advanced. The electronic excited states of SO + 2 so produced as well as its dissociation to SO + and S + were investigated by means of translational energy (TE) and MIKE spectroscopies respectively, and compared with the analogous data obtained from the SO 2 molecular ion. The TES data pertaining to SO + 2 support the existence of a long-lived non-dissociative electronic excited state at 2.5 ± 0.3 eV above its ground state.

Differentiation of anomericC-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.

Fragmentation ofo-nitrodiarylamines on electron impact: Formation of carbazole radical cations

The HO· fragment eliminated on electron impact (EI) from the molecular ions of o-nitrodiarylamines contains either the amine or the aryl hydrogen. Moreover, 2-phenylamino-3-nitropyridine loses H· from the phenyl group. As confirmed by collision-activated dissociation mass-analysed ion kinetic energy spectra, the fragmentation of such compounds involves the formation of carbazole radical cations. Such a process was not observed for the corresponding o-aminodiarylamines.

Study of selected ions in the ammonia desorption chemical ionization mass spectra of peracetylated gentiobiose and two isotopically labelled peracetylated gentiobioses

AbstractThe ammonia desorption chemical ionization (NH3‐DCI) mass spectra of peracetylated gentiobiose (1) and two isotopically labelled gentiobioses (2 and 3) were examined. Compound 2 is labelled with trideuteroacetyl groups in the non‐reducing moiety and 3 with trideuteroacetyl groups in the reducing moiety. It is shown that the [M + NH4 – 42]+ ion is not formed direct from [M + NH4]+ by loss of ketene but appears to be formed by way of a nucleophilic acyl substitution reaction resulting in a neutral species which complexes with NH4+. The disaccharides undergo cleavage at either side of the glycosidic oxygen joining the two sugar residues, a process which is accompanied by addition of H or CH3CO to afford neutral species which complex with NH4+. The structures of the ions resulting from H transfer have been inferred by comparison of their mass‐analysed ion kinetic energy (MIKE) spectra with MIKE spectra of the [M + NH4]+ ions of compounds of established structure. A ring fragmentation reaction of 1, 2 and 3 is reported.

Determination of the glycosidic linkage in peracetylated disaccharides comprised of d‐glucopyranose units by use ofr desorption electron‐ionization mass spectrometry

An oxonium ion at m/z317 is present in the desorption electron ionization and ammonia desorption chemical ionization mass spectra of peracetylated disaccharides, comprised of glucopyranose units linked (1-->2), (1-->3), (1-->4) and (1-->6), but is absent in the spectra of the (1-->1)-linked isomer. The ion at m/z317, which is derived from the reducing moiety, has an O-formyl group at the position of linkage to the non-reducing moiety, and O-acetyl groups at each of the remaining positions. The isomeric monoformyl, triacetyl oxonium ions (at m/z317), derived from the (1-->2)-, (1-->3)-, (1-->4)- and (1-->6)-linked disaccharides, give distinctly different mass-analysed ion kinetic energy spectra, thereby enabling the linkage position to be assigned unambiguously.

Mass-analysed ion kinetic energy spectra and collisional activation spectra of cluster ions of methly 2,3,4,6-tetra-o-methly-d-hexopyranosides with ammonia and methylamine

Mass-analysed ion kinetic energy spectra and collisional activation spectra of cluster ions of methly 2,3,4,6-tetra-o-methly-d-hexopyranosides with ammonia and methylamine

Determination of enzyme kinetic parameters of cyclic CMP‐specific phosphodiesterase by quantitative fast atom bombardment tandem mass spectrometry

AbstractThe determination of cytidine 3′,5′‐cyclic monophosphate‐specific phosphodiesterase activity by means of fast‐atom bombardment (FAB) mass Spectrometry with mass‐analysed ion kinetic energy (MIKE) spectrum scanning is described. Initial efforts to determine the activity of the enzyme by this method were unsuccessful owing to the obfuscation of sample‐related peaks by peaks emanating from the incubation buffer and cation adducts; dilution of buffer and a desalting procedure overcame these difficulties. In the resulting positive‐ion FAB mass spectra, characteristic peaks of the enzyme substrate and product could be readily identified and the protonated molecular ions selected for MIKE scanning. By spiking enzyme incubates with known amounts of substrate and product, and measuring peak heights in the MIKE spectra of both spiked and unspiked samples, the substrate/product ratio at the end of a series of phosphodiesterase incubations was determined. From the data obtained, the Km and Vmax of the phosphodiesterase were calculated as 6.08 mM and 11 μmol min−1 mg−1, respectively, showing good agreement with the analogous values of 8.06 mM and 5.8 μmol−1 min−1 mg−1 obtained by radioactive assay.

Revision of the amino acid sequence of the smallest bc1 complex subunit: use of fast atom bombardment mass spectrometry and mass-analysed ion kinetic energy spectrum analysis.

We have isolated the smallest bc1 complex subunit from an acidic chloroform/methanol extract of bovine cardiac muscle. The identification of the polypeptide was made possible by classical Edman degradation and amino acid analysis. The measurement of its exact molecular weight by fast atom bombardment mass spectrometry (m/z 6519.8), the characterization of a tryptophan cleavage peptide and pepsic peptides by mass measurements and by mass-analysed ion kinetic energy spectrum analysis allow rectification of the amino acid sequence of the smallest bc1 complex subunit. We found a serine residue instead of Gln 22 and tryptophan residues in place of Ser 34 and Ser 38.

Dissociation of ionized keto-enol tautomers of some 1,3-dicarbonyl compounds

Unimolecular dissociation processes of ionized keto-enol tautomers of 2,4-pentanedione ( 1), 1,1,1-trifluoro-2,4-pentanedione ( 2) and 1,1,1,3,3,3-hexafluoro-2,4-pentanedione ( 3) have been investigated by mass analyzed ion kinetic energy (MIKE) and photoionization mass spectrometry. The MIKE spectra show that the predominant fragmentation of the enol molecular ion of 1 at low internal energy is loss of CH 3, while the diketo molecular ion of 1 preferentially eliminates a CO molecule with a relatively large kinetic energy release (KER: T 0.5 = 78 meV). The metastable fragmentations of fluorinated 2 + and 3 + are characterized by loss of CF 3. The KER values for the CF 3 loss are strongly different (182 meV for 2 and 28 meV for 3). The potential energy profiles for the low energy dissociation processes of 1 + and 2 + are probed by measuring the ionization and appearance energies for the appropriate ions. The profiles provide a rationale for the fragmentation characteristics of the metastable molecular ions. The enthalpy change for the keto-enol tautomerization of 1 + ion is found to be 77 kJ mol -1. Mechanistic proposals for the losses of CH 3 and CO from 1 + and the loss of CF 3 from 2 + and 3 + are presented.

Particle beam liquid chromatography—mass spectrometry on a double-focusing sector instrument

A commercially available particle beam interface developed by the Vestec Corporation has been coupled to a double-focusing high-resolution mass spectrometer (VG ZAB 2F) via a momentum separator. An ultraviolet detector was in-line with the interface and allowed monitoring of the eluting materials. With this instrumentation, complete electron-impact (EI) mass spectra on synthetic mixtures of steroids, nucleosides, and several derivatives of amino acids were recorded. Amongst these the EI mass spectra of 2,4-dinitrophenyl, 9-fluorenylmethoxycarbonyl and phenylthiohydantoin derivatives were obtained. The standard LC conditions used here (250 mm × 4.6 mm I.D., reversed-phase C 18 column, 1.0 or 1.5 ml/min flow-rate, isocratic or gradient programming) allowed separation of the mixtures. The resulting mass spectra were compared with those obtained using a direct insertion probe. The agreement was excellent in most cases. Sensitivity measurements were performed on cholesterol and caffeine. A complete low-resolution mass spectrum can be obtained on 100 ng of cholesterol. Single-ion monitoring of the M + of 200 pg of caffeine gave a 4:1 signal-to-noise ratio at m/z 194. Complete high-resolution mass spectra were obtained on 5 μg of cholesterol (loop injection) and on every peak of a five-steroid mixture of 5 μg each. The accuracy of the mass measurements were better than 5 m.m.u. for most cases. Three to four scans could be obtained for each liquid chromatographic peak. Mass-analyzed ion kinetic energy spectra were recorded on the M + of 2.5 μg of cholesterol at m/z 386. Similarly the B/E linked scan of the same ion was recorded on 2.5 μg and 500 ng.

Gas chromatography—mass spectrometry and gas chromatography—tandem mass spectrometry of cyclic fatty acid monomers isolated from heated fats

The analysis of hydrogenated cyclic fatty acid monomers isolated from heated linseed and sunflower oils is achieved by gas chromatography—tandem mass spectrometry of their pentafluorobenzyl esters. Collisionally activated dissociation of the carboxylate anions produced by electron-capture ionization shows remote charge-site fragmentation that allows location of cyclopentane and cyclohexane rings by examining the resulting mass-analysed ion kinetic energy spectra. Oxidative ozonolysis of the methyl esters of the unsaturated cyclic fatty acid monomers allows location of some double bonds. However, preliminary results obtained with remote charge fragmentation of synthetic unsaturated models make this approach an alternative for double bond location in the cyclic fatty acid monomers isolated from heated fats.

Metastable ion study of organosilicon compounds. Part IV. 1,1,1,3,3,3-Hexamethyldisilazane and 1,1,3,3-tetramethyldisilazane.

The unimolecular fragmentations of 1,1,1,3,3,3-hexamethyldisilazane (1), and 1,1,3,3-tetramethyldisilazane (2) were investigated by mass-analyzed ion kinetic energy (MIKE) spectra. D-Labelling studies and high resolution data were also included. One of the characteristic fragmentations of these compounds was the loss of CH4 after the expulsion of CH3 or H from the molecular ions.In the case of 1, D-labelling study indicated that the hydrogen atom bonded to nitrogen atom concerned this CH4 loss. Almost complete scrambling of CH3 groups occurs prior to loss of CH4. An unusual NH3 elimination was found in the MIKE spectra of the [M-CH3-CH4]+ ion for 1 and of the [M-CH3]+ ion for 2.The fragmentations of 2 were compared with those of its carbon analogue, diisopropylamine, 3.