Electron Impact Fragmentation Patterns Research Articles

Comprehensive Screening of Polycyclic Aromatic Hydrocarbons and Similar Compounds Using GC-APLI-TIMS-TOFMS/GC-EI-MS.

In the present work, a novel workflow based on complementary gas-phase separations for the identification of isomeric PAHs from complex mixtures is described. This is the first report on the coupling of gas chromatography (GC), atmospheric pressure laser ionization (APLI), and trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) for the characterization of polycyclic aromatic hydrocarbons. Over a hundred known unknowns are uniquely identified based on the molecular ion retention indices I (5%), mobility (RSD < 0.6% and R = 50-90 with Sr = 0.18 V/ms), mobility-based theoretical candidate assignment (<3%), accurate mass chemical formula assignment (<2 ppm), and electron impact fragmentation pattern and database search. The advantages of theoretical modeling of PAHs and similar compounds were evaluated using candidate structures ranked by retention indices and fragmentation pattern from GC-EI-MS data sets. Over 20 PAH isomeric and deuterated standards were utilized for the GC-APLI-TIMS-TOF MS workflow validation. Noteworthy is the analytical capability for untargeted screening of isomeric and isobaric compounds with additional characterization metrics not available in traditional GC-EI-MSn workflows.

Isolation and identification of impurities in the natural vitamin E

Two impurities in the natural vitamin E extracted from oil deodorizer distillate were separated and characterized by high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS) and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). The impurities were purified and collected by normal-phase HPLC. The accurate masses were determined using FTICR-MS and fragmentation behavior was studied by GC-MS. The results showed that the impurities had identical MS spectra and similar electron impact (EI) fragmentation patterns. Based on the spectra, the structures of the two impurities were proposed as the enantiomers of sesamin. The presented method is rapid and effective, and can be applied for the food safety to the vitamin E manufacturing industry.

The Characterization of 3,4‐Dimethylmethcathinone (3,4‐DMMC)

Analogs and derivatives of traditional illicit drugs are ever increasing in variety and creativity. Staying abreast of the new developments is a constant challenge for every forensic laboratory. Recently, a seizure from Australian Customs Service presented our laboratory with the designer cathinone 3,4-dimethylmethcathinone (3,4-DMMC). Gas chromatography/mass spectrometry (GC/MS), liquid chromatography/mass spectrometry (LC/MS), nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and ultraviolet (UV) spectrophotometry were employed to analyze the spectroscopic characteristics of this cathinone. As an analog, 3,4-DMMC exhibits similar if not identical IR and UV profiles to mephedrone (4-MMC) and methcathinone; however, the retention time from GC is unique as expected, and the electron impact fragmentation pattern is consistent with the fragmentation pattern of other cathinones. The chemical shifts of the carbons and hydrogens were assigned by both one- and two-dimensional NMR techniques, while the molecular weight was confirmed by LC/MS.

Production of dimethyl triselenide and dimethyl diselenenyl sulfide in the headspace of metalloid-resistant Bacillus species grown in the presence of selenium oxyanions

A Bacillus species harvested from the environment is metalloid resistant and, when grown anaerobically in complex growth medium and amended with the selenium oxyanion selenate, selenite, or selenocyanate, produces volatile organoselenium compounds in bacterial culture headspace. Two novel compounds so far undetected in bacterial culture headspace, CH3Se2SCH3 and CH3SeSeSeCH3, are produced and can be detected using solid-phase microextraction and gas chromatography with either fluorine-induced chemiluminescence or mass spectrometric detection. Differences in the electron impact fragmentation pattern of the mixed sulfur/selenide compounds allow the tentative differentiation between the symmetric and asymmetric isomers in this bacterium’s headspace in favor of the asymmetric CH3SeSeSCH3 isomer.

Quality control in quantification of volatile organic compounds analysed by thermal desorption–gas chromatography–mass spectrometry

This paper presents a detailed study on the calibration of a thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS)-based methodology for quantification of volatile organic compounds (VOCs) in gaseous and liquid samples. For the first time, it is documented to what extent three widely encountered problems affect precise and accurate quantification, and solutions to improve calibration are proposed. The first issue deals with the limited precision in MS quantification, as exemplified by high relative standard deviations (up to 40%, n = 5) on response factors of a set of 69 selected VOCs in a volatility range from 16 Pa to 85 kPa at 298 K. The addition of [ 2H 8]toluene as an internal standard, in gaseous or liquid phase, improves this imprecision by a factor of 5. Second, the matrix in which the standard is dissolved is shown to be highly important towards calibration. Quantification of gaseous VOCs loaded on a sorbent tube using response factors obtained with liquid standards results in systematic deviations of 40–80%. Relative response factors determined by the analysis of sorbent tubes loaded with both analytes and [ 2H 8]toluene from liquid phase are shown to offer a reliable alternative for quantification of airborne VOCs, without need for expensive and often hardly available gaseous standards. Third, a strategy is proposed involving the determination of a relative response factor being representative for a group of analytes with similar functionalities and electron impact fragmentation patterns. This group method approach indicates to be useful (RSD ≈ 10%) for quantifying analytes belonging to that class but having no standards available.

Mass spectral studies of a series of N,N‐dialkyl aminoethyl‐2‐chlorides and trimethyl silyl ethers of N,N‐dialkyl aminoethane‐2‐ols under electron impact conditions

The electron impact (EI) mass spectra of a series of N,N-dialkyl-aminoethyl-2-chlorides, N(R(1))(R(2))-CH(2)-CH(2)Cl and trimethylsilyl ethers of N,N-dialkyl aminoethane-2-ols, N(R(1))(R(2))-CH(2)-CH(2)-O-Si(CH(3))(3), where R(1) and R(2) = methyl, ethyl, propyl and isopropyl, which are precursors of VX type of compounds, are studied. All the compounds (1-20) show abundant molecular ions, in addition to a weak [M - H](+) ion, except the N,N-diisopropyl group containing compounds (8 and 18). A general EI fragmentation pattern for the above two series of compounds is discussed. The observed fragment ions are due to simple homolytic cleavages, and they are distinct to allow the identification of the compounds unequivocally including those of isomeric compounds. The primary fragmentation of compounds 1-20 is beta-cleavage, i.e. homolytic cleavage of C-C bond, which is linked to the nitrogen atom. Three types of beta-cleavages are possible for these compounds, in which the abundance of beta-cleavage product ions is found to depend on the size and structure of the alkyl group attached to nitrogen. The alpha-cleavage fragment ions are found only for N,N-dialkyl aminoethyl-2-chlorides but are absent in the corresponding trimethylsilyl ethers of N,N-dialkyl aminoethane-2-ols. The retention indices are calculated for all the studied compounds (1-20) and are in the ranges of 750.38-1079.24 for 1-10 and 905.23-1190.25 for 11-20.

A comparison of the NMR, mass spectral, and molecular structure data for [R 2AlN(CH 2Ph) 2] 2, where R=Me, Et, Pr n, Bu n, and Bu i

Aminoalane dimers of the formula [R 2AlN(CH 2Ph) 2] 2, where R=Pr n , Bu n , and Bu i , were synthesized by thermolysis of the corresponding 1:1 mixtures of the trialkylaluminum and dibenzylamine at 120 °C. The 1H and 13C NMR spectra of the series [R 2AlN(CH 2Ph) 2] 2, where R=Me ( 1), Et ( 2), Pr n ( 3), Bu n ( 4), and Bu i ( 5), were fully assigned using a combination of 1-D and 2-D NMR techniques. This series was also characterized by electron impact (EI) mass spectrometry and fragmentation patterns are discussed. The single-crystal X-ray structural data of [Et 2AlN(CH 2Ph) 2] 2 and [Bu n 2AlN(CH 2Ph) 2] 2 were determined and the data compared with those of [Me 2AlN(CH 2Ph) 2] 2 to identify any structural trends in the series with respect to R. The structural data for these three compounds are also compared with data for [MeAlN(H)CH 2Ph] 2, [RAlN(CH 2Ph)-μ-(CH 2C 6H 4)] 2, and Me 3Al·HN(CH 2Ph) 2. Finally, the molecular structure data of [Me 2MN(CH 2Ph) 2] 2 and Me 3M·HN(CH 2Ph) 2, where M=Al, Ga and In, are discussed in terms of the influence of the Group 13 metal.

Pulsed Plasma Polymerization of Cyclic Ethers: Production of Biologically Nonfouling Surfaces

Polymerization of low molecular weight cyclic ethers was investigated under pulsed plasma conditions. Film formation conditions were adjusted to optimize retention of ethylene oxide (EO) content of the monomers in the resultant plasma generated polymers. To a large extent this goal was achieved with the 12-crown-4 and 15-crown-5 monomers, but not with dioxane. Films obtained from the 12-crown-4 monomer under ultra low power inputs are shown to be highly resistant to protein adsorption, while exhibiting good chemical compositional stability and adhesion during prolonged immersion in aqueous solutions. The dramatic differences observed in contrasting polymer film compositions from 12-crown-4 and dioxane are believed to arise from distinctive differences in the low electron impact fragmentation patterns of these two compounds, as discussed in this report.

Influence of surface charge field on the electron impact fragmentation pattern of KCl molecules in the free surface vaporization of single crystals

A mass-spectrometric method was used to investigate the temperature dependence of the electron impact fragmentation pattern of KCl molecules [ion current ratio I(K +)/ I(KCl +)] vaporizing from a free surface of a potassium chloride single crystal. In the temperature range 766–945 K a very good correlation is observed between the variation in fragmentation pattern with temperature and the pattern of the curve E versus T, where E is the strength of surface charge field. It is concluded that molecules desorbing from an open crystal surface may possess excessive vibrational and rotational energy compared to that characteristic of thermal excitation.

Gas chromatography-mass spectrometry identification of a novel N 3-methylated metabolite of 5′-deoxy-5-fluorouridine in plasma of cancer patients undergoing chemotherapy

Evidence of in vivo biomethylation of the anticancer pro-drug 5′-deoxy-5-fluorouridine (5′-dFUR) is presented for the first time. Biomethylation seems to occur specifically at the N 3 site on the pyrimidine ring. This novel metabolic product was detected by gas chromatography-mass spectrometry of the trimethylsilylated extract of plasma samples from cancer patients undergoing doxifluridine chemotherapy. Considering the observed electron impact fragmentation pattern, the metabolic product was tentatively identified as {ce:inline-formula}N 3-Me-5′-dFUR{/ce:inline-formula}. Definite confirmation of the proposed structure was achieved by comparison of the mass spectra and chromatographic characteristics of the suspected metabolite with those of a synthetically prepared reference standard.

Cluster chemical ionization and deuterium exchange mass spectrometry in supersonic molecular Beams

A cluster-based chemical ionization method has been developed that produces protonated molecular ions from molecules introduced through a supersonic molecular beam interface. Mixed clusters of the analyte and a clustering agent (water or methanol) are produced in the expansion region of the beam, and are subsequently ionized by "fly through" electron impact (EI) ionization, which results in a mass spectrum that is a combination of protonated molecular ion peaks together with the conventional EI fragmentation pattern. The technique is presented and discussed as a tool complementary to electron impact ionization in supersonic molecular beams. Surface-induced dissociation on a rhenium oxide surface is also applied to simplify the mass spectra of clusters and reveal the analyte spectrum. The high gas flow rates involved with the supersonic molecular beam interface that enable the easy introduction of the clustering agents also have been used to introduce deuterating agents. An easy-to-use, fast, and routine on-line deuterium exchange method was developed to exchange active hydrogens (NH, OH). This method, combined with electron impact ionization, is demonstrated and discussed in terms of the unique information available through the EI fragmentation patterns, its ability to help in isomer identification, and possible applications with fast gas chromatography-mass spectrometry in supersonic molecular beams.

Oxidation of dicyclopentadiene in surface water

Electron impact (EI) and chemical ionization gas chromatography/mass spectrometry were used to identify four oxygenated derivatives and characterize a fifth derivative of dicyclopentadiene in methylene chloride extracts of wastewater and ground water from the Rocky Mountain Arsenal near Denver, Colorado. Based on their EI fragmentation patterns and resistance to trimethylsilylation, putative structures of these derivatives were proposed, and authentic standards were synthesized. For structural confirmation, aliquots of each water extract were spiked with a synthetic standard, and coelution of each of four environmental dicyclopentadiene derivatives with one of the standards was demonstrated on three different capillary gas chromatography columns. In addition, the EI mass spectra of each of the four environmental derivatives and the corresponding synthetic standards were found to be essentially identical. Two monoepoxides, one ketone and a monoepoxide-ketone derivative of dicyclopentadiene were identified in wastewater from a sewage treatment facility that had been contaminated by ground water. An extract of activated charcoal that had been used to remove dicyclopentadiene wastes from contaminated ground water was found to contain the same ketone derivative, dicyclopentadien-1-one, but no epoxide derivatives. Next, it was demonstrated in vitro that the three monooxygenated dicyclopentadiene derivatives could be formed by light-catalyzed autoxidation independently of microbial metabolism.

Electron impact fragmentation and primary vacuum pyrolysis products of isomeric perfluorocarbons of composition C6F10 and their hydrocarbon analogs C6H10

1. A pronounced temperature dependence has been observed for the mass spectra of perfluorocarbons of composition C6F10, involving sequential decreases in the intensities of their molecular ions and increasing fragment ion intensities in the temperature range 20–700°C at low ionizing potential (14 eV). 2. The absence of any correlation between the thermal decomposition mechanism of isomeric C6H10 hydrocarbons and their electron impact fragmentation pattern has been demonstrated, and is a consequence of the known rearrangement of their molecular ions. 3. The similarity or overlap noted between the principal fragment ions present in the mass spectra of C6F10 perfluorocarbons and their primary vacuum pyrolysis products is indicative of the absence of C6F10 molecular ion rearrangement and makes it possible to predict the primary act in their thermal dissociation pathways based on their mass spectra, obtained at low ionizing voltages.

Mass spectral characteristics of derivatized metabolites of benzo [A] pyrene.

The electron impact (EI) mass spectra of the permethyl, peracetyl and per(trifluoroacetyl) derivatives of hydroxylated benzo[a]pyrene (B[a]P) metabolites were determined and the fragmentation chemistry producing the spectra elucidated. The metabolites investigated were: 3-hydroxy-B[a]P; 7,8-dihydro-7,8-dihydroxy-B[a]P; 7,8,9,10-tetrahydro-7,8,9-trihydroxy-B[a]P; and 7,8,9,10-tetrahydro-7,8,9,10-tetrahydroxy-B[a]P. In addition, the positive and negative methane chemical ionization spectra were determined for the derivatives of the BP-tetrol. The EI fragmentation patterns of the methylated metabolites that contained partially saturated rings were complex and, in the case of the di- and trimethoxy compounds, included apparent violations of the even-electron rule. The permethylated triol and tetrol cleaved through a retro-Diels-Alder reaction. The EI spectra of the peracetates were dominated by losses of acetic acid and ketene. The per(trifluoroacetyl) species fragmented by losing elements of trifluoroacetic acid, trifluoracetate radical and trifluoroacetyl. The spectra obtained from the permethylated tetrol permitted accurate prediction of the corresponding permethylated derivatives of tetrol metabolites of chrysene and benz[a]anthracene. The ability to predict spectra may be useful in trace analysis of hydrocarbon metabolites in biological samples.

Thromboxane synthase catalyses hydroxylations of prostaglandin H2 analogs in the presence of iodosylbenzene

Human platelet microsomes supplemented with iodosylbenzene converted the stable prostaglandin H2 analog 15(S)-hydroxy-11 alpha,9 alpha-epoxymethano-5(Z),13(E)-prostadienoic acid (U46619) into three metabolites (17.5% yield) which were not formed in the presence of specific thromboxane synthase inhibitors. The same three products were also formed among others by incubation of U46619 with liver microsomes from phenobarbital-pretreated rats with NADPH/O2 or with iodosylbenzene. The NADPH-supported metabolism of U46619 was suppressed in the presence of carbon monoxide. Combined gas chromatography/negative-ion chemical ionization mass spectrometry analysis revealed for all three compounds the incorporation of one oxygen atom which according to the electron impact fragmentation pattern had to be introduced either at the 9-methylene group or at the cyclopentane ring. The identification of these metabolites as 9 beta,15(S)-dihydroxy-11 alpha,9 alpha-epoxymethano-5(Z),13(E)-prostadienoic acid and the R and S isomer of 15(S)-hydroxy-11 alpha,9 alpha-(C-hydroxy-epoxymethano)-5(Z),13(E)-prostadienoic acid is only tentative since no reference compounds were available, but clearly thromboxane synthase was acting as an oxene transferase in this reaction. In contrast to U46619, its 9,11-epoxymethano isomer U44069 was found to be only a poor substrate for the oxene transferase activity of thromboxane synthase (1% yield) which indicates a preference for the 9-methylene group of U46619 which is orientated close to the heme iron of thromboxane synthase as evidenced by spectroscopic studies. Low-level chemiluminescence detected following incubation of iodosylbenzene with partly purified thromboxane synthase is in agreement with the formation of an activated (FeO)3+ oxygen species. In summary, these results point to a common role of the thiolate ligand in the oxygen activation mechanism by thromboxane and prostacyclin synthase and liver cytochrome P-450 monooxygenases.

Gas phase prototropic equilibrium studies of 2,4-dihydroxyquinoline by cndo/2 calculations, mass spectrometry and derivatization

The tautomerism of 2,4-dihydroxyquinoline in the gas phase is discussed in terms of its electron impact fragmentation pattern and of quantum chemical calculations by the LCAO-MO method in the CNDO/2 approximation. Dihydroxyquinoline methylated at position 4 can only exist in two tautomeric forms. Thus, comparison of mass spectrometric data of this compound with dihydroxyquinoline provided additional evidence for the gas phase equilibrium of the latter. The equilibria of these compounds in the gas phase were found to be displaced towards the hydroxyl tautomers.

Influence of the metal–adsorbate interaction on secondary ion emission from UHV-prepared amino acid overlayers on metals

Amino acid overlayers with controlled coverages were produced in situ using a molecular beam technique. The metal–adsorbate interaction was examined by monitoring the secondary ion emission and the desorption of neutral species during temperature-programmed target heating. Identification of the desorbing neutrals was possible by comparing the thermal desorption mass spectrum with the electron impact fragmentation pattern in the molecular beam measured in a cross-beam arrangement. Considering Au and Ni substrates, different binding sites could be distinguished, which are correlated with the emission of specific molecular secondary ions. The interaction between glycine and coadsorbed D2 was examined. Only on Pt significant hydrogen exchange rates were observed.

Aminotetryls: Synthesis, spectral characterization, thermal decomposition and explosive properties

AbstractThis paper describes the synthesis and spectral investigations of two amino derivatives of N‐methyl‐N‐(2,4,6‐trinitrophenyl)nitramine (tetryl). Also discussed are the results from thermal decomposition studies on the three explosives, viz. tetryl, 3‐aminotetryl (3 AT) and 3,5‐diaminotetryl (3,5 DAT) and preliminary work on the explosive properties of the last two compounds. The aminotetryls have been prepared by the amination of the corresponding chlorotetryls. The yield was 87% for 3 AT, but was only 33% for 3,5 DAT, probably due to steric crowding around the benzene nucleus. The mass spectra show interesting differences in the electron impact fragmentation patterns of the three tetryls with the M+ ion relative intensities following the order 3,5 DAT &gt; 3 AT &gt; tetryl, which could be due to (a) resonance stabilization and (b) hydrogen bonding effects. Evidence for the latter is also found in the infrared spectra of these compounds. Arrhenius kinetic parameters derived from thermal decomposition studies of the three compounds are presented and show that 3,5 DAT is thermally less stable than 3 AT. Explosive sensitiveness tests indicate that the diamino compound is the most sensitive, the trend being 3,5 DAT &gt; 3 AT &gt; tetryl. This is contrary to the generally found desensitizing influence of NH2 groups on the thermal stability and explosive sensitiveness of trinitroaromatic energetic molecules. Mechanisms to account for the observed thermal decomposition behaviour and explosive sensitiveness patterns are discussed.

Conjugated Schiff's bases 16—Substituent effect on electron impact fragmentation of some 1‐oxa‐4‐azabutadienes1

AbstractThe electron impact fragmentation patterns of some 1‐oxa‐4‐azabutadienes have been determined and supported by the corresponding metastables. The effect of substituents on this fragmentation has been investigated. Satisfactory correlation of intensity ratios of the peaks characterizing the primary fragmentation versus the Brown and Okamoto σ+ substituent parameters has been found. The relationship between the substituent parameters and the dissociation energy ED,C(3)‐C(5), estimated with the help of appearance energies has been considered.

Determination of the electron impact fragmentation pattern of methamidophos via stable isotope enrichment

Determination of the electron impact fragmentation pattern of methamidophos via stable isotope enrichment