Electron impact,chemical ionization and negative chemical ionization mass spectra,and mass-analysed ion kinetic energy spectrometry–collision-induced dissociation fragmentation pathways of some deuterated 2,4,6-trinitrotoluene (TNT) derivatives

Mass-analysed ion kinetic energy spectrometry (MIKES) with collision-induced dissociation (CID) has been used to study the fragmentation processes of a series of deuterated 2,4,6-trinitrotoluene (TNT) and deuterated 2,4,6-trinitrobenzylchloride (TNTCI) derivatives. Typical fragment ions observed in both groups were due to loss of OR′ (R′ = H or D) and NO. In TNT, additional fragment ibns are due to the loss of R₂′O and 3NO₂, whilst in TNTCI fragment ions are formed by the loss of OCI and R₂′OCI. The TNTCI derivatives did not produce molecular ions. In chemical ionization (Cl) of both groups. MH⁺ ions were observed, with [M – OR′]⁺ fragments in TNT and [M – OCI]⁺ fragments in TNTCI. In negative chemical ionization (NCI) TNT derivatives produced M^?′, [M–R′]^?, [M–OR′]^? and [M–NO]^? ions, while TNTCI derivatives produced [M–R]^?, [M–Cl]^? and [M – NO₂]^? fragment ions without a molecular ion.     

Distinction between 17-epimeric hydroxy steroids of the 3,17-dioxygenated androstane series by chemical ionization

The distinction between 17-epimeric 3,17-dioxygenated hydroxyandrostanes has been made by comparison of both their methane or ammonia positive and OH^? negative chemical ionization (CI) mass spectra. In the methane or ammonia positive CI, the 17α-configuration in the eight stereoisomeric 5ξ-androstane-3ξ,17ξ-diols can be determined by the relative abundances of the ion [MH? 2 H₂O]⁺. In the ammonia CI spectra, the ion [M+NH₄? H₂O]⁺ possesses only a low abundance, but a comparison of the relative rates of the loss of water v. the loss of ammonia from [M.NH₄]⁺ in the second field-free region allows a clear distinction to be made between the 17α- and 17β-series. In the OH^? negative CI mass spectra, the 5ξ-androstane-3-one-17ξ-ols produce an intense ion [M? H? H₂O]^? in the 17α-series only.     

Ion-Neutral complexes from protonated propyl phenyl ethers. Gas-phase solvolysis versus elimination-readdition

Ion-neutral complexes, well attested as intermediates in the expulsion of alkenes from M⁺? and MH⁺ ions from primary alkyl phenyl ethers, are shown to intervene in the decomposition of the MH⁺ ion of a secondary alkyl phenyl ether, (CD₃)₂CHOPh. Chemical ionization (CI) (methane reagent gas)-mass-analysed ion kinetic energy spectroscopy (MIKES) shows ions of both m/z 96 and 97, indicating that the proton deposited by the CI reagent exchanges with the methyl deuterium atoms. The ratio of daughter ion intensities, as well as the proportions of ions of m/z 95, 96 and 97 from the MH⁺ of CD₃CH₂CD₂OPh, agree with predictions based on the gas-phase solvolysis mechanism, in which [i-Pr⁺ PhOH] complexes form from the protonated parent via simple bond heterolysis. An alternative mechanism, elimination-readdition, would proceed via [propene PhOHD⁺] complexes. This latter mechanism predicts a ratio of daughter ion intensities that is very different from gas-phase solvolysis and which disagrees with experiment. The elimination-readdition pathway is effectively ruled out, while the gas-phase solvolysis mechanism is reinforced.     

Electron ionization and chemical ionization mass spectra of pyridinium and isoquinolinium ylides

Electron ionization (EI) spectra and both positive and negative chemical ionization (CI) spectra have been obtained for four isoquinolinium ylides and two pyridinium ylides. Electron transfer reactions dominate the CI mass specra. The base peak in negative chemical ionization is the [M]^?· ion, formed by electron capture. In the positive methane CI spectra the molecular ion, [M]^+·, is relatively more intense than [MH]⁺ showing electron transfer to be the main positive ionization process. In the positive ammonia CI spectra, proton transfer to give [MH]⁺ is the main ionization process, but electron transfer is also observed. The EI spectra show fragmentations in which the aromatic nitrogen moiety retains the charge and fragmentation is by loss of radicals or small neutral molecules from the side-chains. Radical driven reactions are proposed to explain these spectra.     

Formation of an unusual MH− ion in the methane negative-ion chemical ionization mass spectra of chlorprothixene and aromatic sulfur compounds

The methane negative-ion chemical ionization (NCI) mass spectrum of chlorprothixene shows an unusual MH^? ion. This ion can be accounted for by electron capture followed by H˙ transfer from the reagent gas. The most probable site of electron attachment was concluded to be related to the sulfur atom of the thioxanthene ring based on the observation of analogous ions for structurally related compounds, all containing a heterocyclic sulfur. The MH^? ion observed with methane as the reagent gas was shifted to MD^? when tetradeuteromethane was used in place of methane. The ratio of [M ? H]^? to MH^? did not change with emission current suggesting that the process is independent of the radical concentration in the CI plasma. Consistent with this observation is the lack of CH₃˙ or C₂H₅˙ adduct ions in the NCI mass spectrum and the fact that gold-plating the ion source did not decrease the proportion of MH^?. Also, this mechanism is consistent with thermochemical considerations of reactions of a phenyl radical with various alkanes and observations of ions formed by methane NCI from model compounds. Therefore, unlike other MH^? ions observed in methane NCI mass spectra, the mechanism of formation does not appear to involve a hydrogen radical addition followed by electron capture.     

High energy collisional spectroscopy of [RCOO]− anions from negative ion chemical ionization of fatty acid methyl esters

Saturated and mono-unsaturated fatty acid methyl esters as well as corresponding underivatized acids gave abundant carboxylate anions [RCOO]^? under negative ion chemical ionization (OH^? and NH₂ ^?) and electron attachment conditions. B/E or mass-analysed ion kinetic energy spectra of fragments arising from high energy collision activated dissociation (CAD) of these [RCOO]^? species contained decisive information on the original structure of the neutrals. From an analytical point of view, the method can utilize the gas chromatographic mass spectrometric combination and, in the B/E mode, can be used in practice on any double-focusing mass spectrometer.     

Structural identification of fatty acid methyl esters by collision spectra of their [M ? H]− species

Negative ion chemical ionization linked to collisionally activated decomposition (CAD) experiments has proved to be an efficient analytical tool in mass spectrometric characterization of fatty acids. The CAD mass-analysed ion kinetic energy spectra of [M ? H]^? species, obtained by OH^? reaction with a selection of six C₁₈ fatty acid methyl esters, reveal useful correlations with the original structure of the neutrals, giving evidence of both chain branching and double bond positions.     

[M + 11]+˙ and [M + 11]−˙ ions in the nitrogen positive and negative ion chemical ionization mass spectra of aromatic amines

The N₂ negative ion chemical ionization (NICI) mass spectra of aniline, aminonaphthalenes, aminobiphenyls and aminoanthracenes show an unexpected addition appearing at [M + 11]^?˙. This addition is also observed in the N₂ positive chemical ionization (PCI) mass spectra. An ion at [M – 15]^? is found in the NICI spectra of aminoaromatics such as aniline, 1- and 2-aminonaphthalene and 1- and 2-aminoanthracene. Ion formation was studied using labeled reagents, variation of ion source pressure and temperature and examination of ion chromatograms. These experiments indicate that the [M + 11]^?˙, [M – 15]^?˙ and [M + 11]^+˙ ions result from the ionization of analytes altered by surface-assisted reactions. Experiments with ¹⁵N₂, [¹⁵N] aniline, [2,3,4,5,6-²H₅] aniline and [¹³C₆] aniline show that the [M + 11]^?˙ ion corresponds to [M + N – 3H]^?˙. The added nitrogen originates from the N₂ buffer gas and the addition occurs with loss of one ring and two amino group hydrogens. Fragmentation patterns in the N₂ PCI mass spectrum of aniline suggest that the neutral product of the surface-assisted reaction is 1,4-dicyanobuta-1,3-diene. Experiments with diamino-substituted aromatics show analogous reactions resulting in the formation of [M – 4H]^?˙ ions for aromatics with ortho-amino groups. Experiments with methylsubstituted aminoaromatics indicate that unsubstituted sites ortho to the amino group facilitate nitrogen addition, and that methyl groups provide additional sites for nitrogen addition.     

Chemical ionization of cyclopropyl ethers II—the nature of the alkyl group fragmentationt

Oxygen-alkyl cleavage is ruled out in the methane chemical ionization- and electron mpact-induced decomposition of cyclopropyl ethers by the finding that for trans,trans-2,3-diethylmethoxycyclopropane the [M ? C₂H₅·]+ ion is more intense than the [M ? CH₃·]+ ion. The possibility for [M + H ? C₂H₆]⁺ is discounted by comparison with the methane chemical ionization nass spectrum of tran,tran-2,3-dimethylmethoxycyclopropane. The isobutane chemical ionization nass spectrum of the diethylcyclopropyl methyl ether affords nearly exclusive electrocyclic methanol fragmentation, i.e. [M + H ? CH₃OH]⁺.     

Isomer differentiation and structural characterization of penem β-lactam antibiotics by mass-analyzed ion kinetic energy spectrometry

A number of clinically significant penem β-lactams, both free acids and sodium salts, were investigated by mass-analyzed ion kinetic energy spectrometry (MIKES) following fast atom bombardment (FAB) ionization. The collisionally activated dissociation (CAD) products of [M + H] ⁺ and [M + Na]⁺ ions are described. Carbon dioxide loss was observed for some of the free acids, whereas a daughter ion generated by β-lactam ring cleavage was characteristic of the sodiated species. Other fragments included successive cleavages and rearrangements of the substituent side-chain, permitting complete characterization of these chains. The fragmentation pattern for both protonated and sodiated species were more clearly established by CAD MIKES than by normal FAB mass spectral analyses. A notable feature of this technique was its ability to differentiate between pairs of regioisomeric penems on the basis of their fragmentation patterns. These compounds could not be differentiated in the usual mass spectra.     

Mass spectral fragmentation pathways in RDX and HMX. A mass analyzed ion kinetic energy spectrometric/collisional induced dissociation study

A collisional induced dissociation study of 1,3,5-trinitro-1,3,5 triazacyclohexane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX) was carried out using mass analyzed kinetic energy spectrometry. High resolution mass spectra and mass analyzed ion kinetic energy/collisional induced dissociation spectra of RDX and HMX were recorded in the electron impact, chemical ionization and negative ion chemical ionization modes. Fragmentation pathways of the compounds investigated were determined in all three modes of ionization. It was found that a major part of the fragment ions in RDX and HMX originate from formation of the aduct ions [M+NO]⁺ and [M+NO₂]⁺ in electron impact and chemical ionization, and from [M+NO]^? and [M+NO₂]^? in negative chemical ionization, followed by dissociation.     

Negative- and positive-ion chemical ionization mass spectra of aromatic amines: Surface-assisted reactions involving oxygen

The O₂–N₂ and O₂–Ar negative-ion chemical ionization mass spectra of aromatic amines show a series of unusual ions dominated by an addition appearing at [M + 14]^?˙. Other ions are observed at [M – 12]^?˙, [M + 5]^?˙, [M + 12]^?˙, [M + 28]^?˙ and [M + 30]^?˙. Ion formation was studied using a quadrupole instrument equipped with a conventional chemical ionization source and a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. These studies, which included the examination of ion chromatograms, measurement of positive-ion chemical ionization mass spectra, variation of ion source temperature and pressure and experiments with ¹⁸O₂, indicate that the [M + 14]^?˙ ion is formed by the electron-capture ionization of analytes altered by surfaceassisted reactions involving oxygen. This conversion is also observed under low-pressure conditions following source pretreatment with O₂. Experiments with [¹⁵N]aniline, [2,3,4,5,6-²H₅] aniline and [¹³C₆]aniline show that the [M + 14]^?˙ ion corresponds to [M + O ? 2H]^?˙, resulting from conversion of the amino group to a nitroso group. Additional ions in the spectra of aromatic amines also result from surface-assisted oxidation reactions, including oxidation of the amino group to a nitro group, oxidation and cleavage of the aromatic ring and, at higher analyte concentrations, intermolecular oxidation reactions.     

Adduct ion formation by metal complexes under methane negative chemical ionization conditions

Negative chemical ionization mass spectrometry is used as a probe to examine reactions between hydrocarbon radicals and metal complexes in the gas phase. The methane negative chemical ionization mass spectra of 27 complexes of cobalt(II ), nickel(II ) and copper(II ) in the presence of O₄, O₂N₂ and N₄ donor atom sets are characterized by two dominant series of adduct ions of the form [M + C_nH_2n]^? and [M + C_nH_2n+1]^? at m/z values above the molecular ion, [M]^?. Insertion of the CH radical into the ligand followed by radical/radical recombination and electron capture is proposed as the major mechanism leading to the formation of [M + C_nH_2n]^? adduct ions. A second pathway involves ligand substitution by C_nH_2n+1 radicals concomitant with H elimination and electron capture. Oxidative addition at the metal followed by ionization is suggested as the principal pathway for the formation of [M + C_nH_2n+1]^? adduct ions.     

Use of a hand-portable gas chromatograph-toroidal ion trap mass spectrometer for self-chemical ionization identification of degradation products related to O-ethyl S-(2-diisopropylaminoethyl) methyl phosphonothiolate (VX)

The chemical warfare agent O-ethyl S-(2-diisopropylaminoethyl) methyl phosphonothiolate (VX) and many related degradation products produce poorly diagnostic electron ionization (EI) mass spectra by transmission quadrupole mass spectrometry. Thus, chemical ionization (CI) is often used for these analytes. In this work, pseudomolecular ([M+H]⁺) ion formation from self-chemical ionization (self-CI) was examined for four VX degradation products containing the diisopropylamine functional group. A person-portable toroidal ion trap mass spectrometer with a gas chromatographic inlet was used with EI, and both fixed-duration and feedback-controlled ionization time. With feedback-controlled ionization, ion cooling (reaction) times and ion formation target values were varied. Evidence for protonation of analytes was observed under all conditions, except for the largest analyte, bis(diisopropylaminoethyl)disulfide which yielded [M+H]⁺ ions only with increased fixed ionization or ion cooling times. Analysis of triethylamine-d₁₅ provided evidence that [M+H]⁺ production was likely due to self-CI. Analysis of a degraded VX sample where lengthened ion storage and feedback-controlled ionization time were used resulted in detection of [M+H]⁺ ions for VX and several relevant degradation products. Dimer ions were also observed for two phosphonate compounds detected in this sample.     

Determination of elemental compositions by gas chromatography/time‐of‐flight mass spectrometry using chemical and electron ionization

Many metabolomic applications use gas chromatography/mass spectrometry (GC/MS) under standard 70 eV electron ionization (EI) parameters. However, the abundance of molecular ions is often extremely low, impeding the calculation of elemental compositions for the identification of unknown compounds. On changing the beam‐steering voltage of the ion source, the relative abundances of molecular ions at 70 eV EI were increased up to ten‐fold for alkanes, fatty acid methyl esters and trimethylsilylated metabolites, concomitant with 2‐fold absolute increases in ion intensities. We have compared the abundance, mass accuracy and isotope ratio accuracy of molecular species in EI with those in chemical ionization (CI) with methane as reagent gas under high‐mass tuning. Thirty‐three peaks of a diverse set of trimethylsilylated metabolites were analyzed in triplicate, resulting in 342 ion species ([M+H]⁺, [M–CH₃]⁺ for CI and [M]^{+ .} , [M–CH₃]^{+ .} for EI). On average, CI yielded 8‐fold more intense molecular species than EI. Using internal recalibration, average mass errors of 1.8 ± 1.6 mm/z units and isotope ratio errors of 2.3 ± 2.0% (A+1/A ratio) and 1.7 ± 1.8% (A+2/A ratio) were obtained. When constraining lists of calculated elemental compositions by chemical and heuristic rules using the Seven Golden Rules algorithm and PubChem queries, the correct formula was retrieved as top hit in 60% of the cases and within the top‐3 hits in 80% of the cases. Copyright © 2010 John Wiley & Sons, Ltd.     

On the generation and characterization of the spiro[2,5]octadienyl anion in the gas phase

Three routes have been explored in both a high-pressure chemical ionization (CI) source and a low-pressure Fourier transform ion cyclotron resonance (FT-ICR) cell to generate the spiro[2,5]octadienyl anion in the gas phase: (i) proton abstraction from spiro[2,5]octa-4,6-diene; (ii) expulsion of trimethysilyl fluoride by phenyl ring participation following fluoride anion attack upon the silicon centre of 2-phenylethyl trimethylsilane; and (iii) collisionally induced dissociation (CID) of the carboxylate anion of 3-phenylpropanoic acid via carbon dioxide loss. From comparison of the CID spectra of various reference [C₈H₉]^? ions with those of the [C₈H₉]^? ions which could be generated via the routes (i) and (iii) in the CI source it can be concluded that only the third route yields a [C₈H₉]^?ion whose CID spectrum is not inconsistent with the one expected for the spiro[2,5]octadienyl anion. In the FT-ICR cell [C₈H₉]^? ions are generated along all three routes; their structures have been identified by specific ion-molecule reactions and appear to be different. Route (i) yields an α-methyl benzyl anion, probably due to isomerization within the ion-molecule complex formed. An ortho-ethylphenyl anion is formed along route (ii), presumably due to an intramolecular ortho proton abstraction in the generated trimethylsilyl fluoride solvated 2-phenylethyl primary carbanion. The [C₈H₉]^? ion formed along route (iii) shows reactions similar to those of the 1,1-dimethylcyclohexadienyl anion which is structurally related to the spiro[2,5]octadienyl anion. Furthermore, the [C₈H₉]^? ion generated via route (iii) reacts with hexafluorobenzene under expulsion of only one hydrogen fluoride molecule which contains exclusively one of the original phenyl ring hydrogen atoms. On the basis of all these observations it is therefore quite likely that the spiro[2,5]octadienyl anion is formed by collisionally induced decarboxylation of the 3-phenylpropanoic acid carboxylate anion and can be a long-lived and stable species in the gas phase.     

Dimer formation in methane chemical ionization mass spectrometry of organic functionalities,including 2-azaadamantane derivatives

The methane chemical ionization mass spectra of 1-hydroxy-2-azaadamantane, its lactolactam, and 1-chloro-2-azaadamantanes exhibit high intensity dimeric species. These correspond to [2M+H]⁺ or stable fragments formed via gas phase chemical reactions, such as dehydration (aminol case) or dehydrohalogenation (chloroazaadamantanes). The tert-amino chloroazaadamantane forms a stable [2M+C₂H₅]⁺ ion with much higher intensity than the [2M+H]⁺ ion. Similar dimeric species were observed with simple functionalities, including amide, amine, alcohol and alkene. Pressure, temperature and, apparently, steric effects play important roles. Structures are proposed for the various dimeric ions.     

Oxirane as a chemical ionization gas: Reactivity towards different classes of compounds

Oxirane chemical ionization (CI) gives numerous ions, including C₂H₃O⁺ and C₂H₅O⁺. These ions react with organic molecules through various specific ion–molecule reactions such as hydride abstraction, protonation, additions or cycloadditions. Oxirane CI allows discrimination between unsaturated compounds with [M + 43]⁺ and [M + 57]⁺ adduct ions and heteroatom functions with [M + 45]⁺ adduct ion. All are diagnostic ions. Oxirane CI permits selectivity during the ionization process of a mixture and discrimination of isomers.     

Positive electron impact and chemical ionization mass spectra of some nitramine nitrates

The positive electron impact (EI) and isobutane chemical ionization (CI) mass spectra of six nitramine nitrates were studied with the aid of some accurate mass measurements. In the EI spectra, β fission relative to both the nitramine and nitrate ester is important. In the CI spectra a major ion occurs at [MH – 45]⁺ and was found to be mainly due to [M + 2H ? NO₂]⁺. All of the compounds except N-(2 hydroxyethyl)-N-(2′,4′,6′-trinitrophenyl)nitramine nitrate gave an [MH]⁺ ion. The [MH – 45]⁺ ion in the isobutane CI mass spectra of tetryl is also due to [M + 2H ? NO₂]⁺.     

Mass spectrometric behaviour of 1-imino-substituted pyrazolo[1,2-a]-[1,2,4]triazoles under electron and chemical ionization

The spectra of five pharmacologically interesting substituted pyrazolo[1,2-a][1,2,4]triazole hydroiodides were measured under electron and chemical ionization. In the electron ionization spectra, in addition to the intense molecular ion peak of the free base (M⁺*), there was also a relatively intense molecular ion peak of the hydroiodide form, which is unusual since the hydroiodides are rarely so stable. The phenylimino and phenylamino substituents of the triazole ring affected the fragmentation behaviour of the compounds very much. The chemical ionization reagent gases used in this work were methane, isobutane, deuterated ammonia and acetone. In all the cases practically only [M+H]⁺ ions were observed, the only exception being acetone which also gave rise to intense [M+C₂H₃O]⁺ and [M⁺C₃H₇O]⁺ adduct ions. None of the reagent gases used was able to cause any fragmentation.