Dissociative Electron Capture Research Articles

Capillary Gas Chromatographic Introduction of Environmental Compounds into a Trochoidal Electron Monochromator/Mass Spectrometer

A gas chromatograph was interfaced to an electron monochromator/quadrupole mass spectrometer. The new system was tested for the analysis of environmental compounds. Detection sensitivity for hexachlorobenzene (HCB) through the gas chromatograph was 5 pg or better, and a mass-resolved molecular ion cluster for this compound on the fly was achieved with 10.8 ng of sample. An ion chromatogram was obtained using 45 ng of Aroclor 1254, and the extract from a trout muscle sample recently collected in the Arctic yielded a chromatographic profile similar to that observed using negative ion chemical ionization mass spectrometry. A mixture of HCB and 2,4,6-trinitrotoluene (TNT) were shown to be distinguishable at 2.4-eV electron energy. The energetics of regioselective fragmentation of [4-15NO2]TNT by dissociative electron capture can be determined on the fly. Complete negative ion gas chromatography/mass spectrometry (GC/MS) spectra were obtained for hexafluorobenzene and TNT by simultaneously ramping the electron energy from -2 to 15 eV and scanning the mass over a 200-Da range.

One-electron oxidation and reduction reactions of vitamin C derivatives: 6-bromo- and 6-chloro-6-deoxy-ascorbic acid.

6-Bromo- and 6-chloro-6-deoxy derivatives of ascorbate anion are able to transfer an electron to the oxidizing radicals .OH, Br2.- and RS. with the same rate constants as the ascorbate anion itself. The resulting radicals also show the same kinetic stabilities and optical absorption spectra as the well-characterized ascorbate radical anion (lambda max = 360 nm; epsilon 360 = 330 m2 mol-1). This proves that there is no influence of the structural changes in the side chain on the antioxidant capacity of the ascorbate moiety. In contrast, measured reduction of the 6-halo-6-deoxy derivatives occurs significantly faster (up to one order of magnitude) than the reduction of unsubstituted ascorbate. For example, absolute rate constants of 4.6 x 10(9) and 2.2 x 10(7) dm3 mol-1 s-1 have been measured for the reactions of bromo-derivative with eaq- and (CH3)2COH respectively. These radical-induced reductions proceed via dissociative electron capture and, under cleavage of the C-halogen bond, yield C-6 carbon-centered radicals. In the presence of oxygen the corresponding peroxyl radical is readily formed. This radical is again able to oxidize the ascorbate moiety (rate constant 2 x 10(7) dm3 mol-1 s-1). Results are discussed in terms of biological relevance of the investigated compounds regarding their ability to act as efficient antioxidants and bioreductive antitumour agents simultaneously.

The Use of Mobile Phase Additives in the Determination of 55 (Polar) Pesticides by Column Liquid Chromatography-Thermospray-Mass Spectrometry

The effect of various additives (ammonium acetate, ammonium formate, triethyl- and tripropylammonium formate and chloroacetonitrile) on the sensitivity and selectivity in liquid chromatography-thermospray-mass spectrometry (positive and negative ion mode) is studied for a group of 55 pesticides including phenoxy and carboxylic acids, hydroxy-keto-lactones, substituted amides, triazines, (thio)carbamates, phenylureas, substituted phenols, organophosphorus compounds, thiocyanates and anilines. In the positive ion mode the base peaks correspond to [M + H] + and [M + NH 4 ] + for the majority of the test compounds. Fragmentation is observed in some isolated cases only. In the negative ion mode processes like (dissociative) electron capture and ion-molecule reactions take place. Fragment ions such as [M − H - CONCH 3 ] − for the carbamates, [M − C 3 H 6 O] − for the hydroxy-keto-lactones and [M − H] − for the chlorinated phenoxy acids are observed. Depending on the additive used, adduct ions such as [M + CH 3 COO] − , [M + HCOO] − and [M + CI] − are formed. Adducts with fragment ions are also observed quite often. The use of triethylammonium formate and tripropylammonium formate instead of ammonium formate increases the sensitivity and selectivity of the method in the NI mode, because their high proton affinity enhances deprotonation

Electron addition to trimetyl phosphite induced by ionising radiation: an electron spin resonance study

A range of solutions containing trimethyl phosphite have been exposed to 60 CO γ-rays at 77K and studied by electron spin resonance (ESR) spectroscopy in the hope of obtaining unambiguous evidence for the formation of [·P(OMe) 3 ] − radical anions, and of establishing their preferred structure. It is suggested that one of two phosphorus centered π-type radicals detected in this study is the parent radical anion, having a planar T structure, the other being ·P(OMe) 2 formed by dissociative electron capture. Using an aprotic glassy medium (methyltetrahydrofuran), the only significant electron adduct had A ‖ = 267 G with a small A ⊥ value. This is assigned to the primary radical anion having a planar T structure, with SOMO that is large 3p on phosphorus, normal to the radical plane. For methanolic solutions the same radical anion was formed, but extensive protonation also occurred giving a phosphoranyl-type radical with large 1 H and 31 P hyperfine splittings. Another phosphorus centre with an even larger hyperfine splitting is thought to be ·P(OMe) 4 , formed by addition of methanol to the radical cation or by attack of MeO· radicals on P(OMe) 3 . In concentrated methanol solutions, the cation dimer, (MeO) 3 P∸P(OMe) + 3 was clearly detected by ESR spectroscopy. In CD 3 OD glasses, the 2 H adduct gave well defined triplets, thus confirming the above assignment. Methyl radical signals were also clearly defined. A second π-type phosphorus centred radical is thought to be ·P(OMe) 2 .

Quantum chemical estimation of the possibility of the formation of chloro- and bromomethane radical anions and their fragmentation paths in the gas phase and in solution by MNDO and AM1 techniques

By semiempirical MNDO and AM1 methods it was shown that electron transfer on the chloro-and bromomethane molecules of the general formula CHnHal4−n (n=1−3) results either in a kinetically independent particle, i.e., a radical anion, or in C-Hal-bond cleavage with the formation of Hal− and the respective radical. The enthalpy, activation energy of the reactions, and data on the geometry of the radical anion obtained show that the increasing the number of halogen atoms in the initial molecule and decreasing the solvent polarity favor radical anion stabilization. It was established that the cleavage of the C-H-bond in the radical anion is not favored energetically. Fragmentation at the C-H-bond can proceed according to the mechanism of dissociative electron capture by halomethane molecule only with additional factors favoring this reaction.

Qualitative and quantitative response characteristics of a capillary gas chromatograph/ion mobility spectrometer to halogenated compounds

The response of a capillary column gas chromatograph/ion mobility spectrometer (GC/IMS) system to chlorinated and brominated alkanes and alkenes in nitrogen, air and nitrogen/CO 2 mixtures was studied. Product ions were formed through dissociative electron capture processes or charge transfer reactions, yielding chloride or bromide ions. The quantitative response was shown to depend on the composition of the carrier gas in which atmospheric pressure ionization processes occurred as well as on the IMS cell temperature. Large variations in the quantitative response of the GC/IMS to different halogenated compounds were observed. The results were compared to those reported for the doped electron capture detector (ECD). Practical implications of the GC/IMS system as a monitor and detector for halogenated aliphatic compounds are discussed.

Rydberg electron transfer to hydrogen iodide: Dissociative and nondissociative electron capture

Rydberg electron transfer (RET) to jet-cooled hydrogen iodide (HI) molecules has been studied for alkali atoms excited to ns and nd Rydberg levels (9<n<40). I− ions produced by dissociative electron capture are observed for all Rydberg levels studied, even though dissociative RET becomes endothermic for n<52. For n≳23, rate constants for I− formation are in good agreement with previous results for low energy free electron attachment. For lower values of n, the RET results deviate from the free electron results due to electrostatic interactions between the nascent positive (Rydberg core) and negative ions. A simple calculation based upon the ‘‘free electron model’’ agrees quantitatively with the rate constant data for n≳13. For Rydberg levels with n<13, long-lived HI− (and DI−) ions are observed which appear to be stable with respect to both dissociation and autodetachment. These observations indicate that a bound and stable anion state exists, correlating with the lowest H(D)+I− limit, with a potential well which supports at least one vibrational level for both HI− and DI−.

Dissociative electron capture of diazoquinones: matrix isolation ESR study

It is postulated that the electron beam exposure process of the AZ-type resists (those containing diazoquinones as the labile component) is initiated by dissociative electron capture producing a phenoxy radical anion and N 2 . Na atoms and model diazoquinone molecules, 1-oxo-2-diazonaphthoquinone, 2-oxo-1-diazonaphthoquinone, and o-diazo-benzoquinone, were cocondensed in argon matrices, and the Na-to-diazoquinone electron transfer was induced by mild radiation (λ>580 nm). ESR examination of the matrices revealed that the three diazoquinones all readily captured low-energy electrons and dissociated to yield the respective phenoxy anion radicals resulting from N 2 cleavage as postulated

Gas Chromatography-Negative Ion Chemical Ionization Mass Spectrometry for the Determination and Identification of Planar Polychlorinated Biphenyls in Biological Samples

Abstract Planar and mono-ortho substituted polychlorinated biphenyls (PCBs) have been identified and quantified at very low concentrations in biological samples (herring and polecat samples) using an off-line combination of column liquid chromatography and gas chromatography-negative ion chemical ionization mass spectrometry (GC-NCIMS). The PCBs generate good mass spectra dominated by the molecular ion as the base peak. The dissociative electron capture mechanism is more pronounced for the higher chlorinated PCBs. The limits of detection of the planar and mono-ortho substituted PCBs under selected ion monitoring (SIM) conditions are 40–100 fg.

Radiation induced reactions in the ethanol-monochlorobenzene dosimeter solution

Using gamma radiolysis, electron pulse radiolysis, laser flash photolysis and endproduct measuring techniques the basic radiation chemical processes taking place in the ethanol-chlorobenzene dosimeter (composition: 24 vol% chlorobenzene, 4 vol% water, 0.04 vol% acetone, 0.04 vol% benzene and 71.92 vol% ethanol) were investigated. The HCl, used to measure the absorbed dose, forms by dissociative electron capture of chlorobenzene yielding Cl - ions (90%) and by Cl atom elimination from neutral excited chlorobenzene molecules (10%). In both reactions phenyl radicals are left behind, which predominantly abstract H atom from ethanol molecules producing α-hydroxy-ethyl radicals and benzene. The yield of the dimer, tetramer, etc. chlorobenzene molecules is small, which is important from the aspect of the linearity of the dose-HCl concentration relation.

Electron paramagnetic resonance studies of irradiated D-glucose-6-phosphate ions: relevance to DNA

Exposure of disodium D-glucose-6-phosphate to 60Co γ-rays at 77 K gave a major centre identified by its EPR spectrum as the parent phosphoranyl radical, formed by electron addition to phosphorus. Several carbon-centred radicals were also detected and their structures discussed. The expected primary electron-loss centre, (RO)PO˙–3 was not detected, and probably reacted by hydrogen-atom abstraction with a neighbouring sugar unit. On annealing, the phosphoranyl radicals were converted into two other phosphorus-centred radicals, one of which is identified as the ˙PO2–3 radical formed by loss of RO–. The other species, formed in comparable yields is probably also a phosphoryl radical centre.Dilute solutions in methanol or CD3OD gave only dissociative electron capture, giving R˙ radicals, as detected by EPR spectroscopy, plus phosphate ions. There was no evidence for phosphoranyl or phosphoryl radical formation. This switch in mechanism of electron capture between the disodium salt and methanolic solutions illustrates the major control exerted by the medium on reaction mechanism in such systems. These results are of significance with respect to processes of radiation damage to DNA. In particular, they confirm that electron addition to phosphate does not occur as a process of significance in irradiated DNA. Reasons for this are discussed.

Dichotomy in dissociative electron capture: electron paramagnetic resonance detection of iodine atoms in methanol glasses

Exposure of dilute solutions of N-iodosuccinimide and other iodo-derivatives (RI) in methanol to 60Co γ-rays at 77 K gives features assigned to σ* radical anions, in which the excess electron is accommodated in the R–I σ* orbital. On annealing, these features gave way to a new set of features exhibiting large hyperfine coupling to iodine and large g-shifts which were the same for all substrates. It is postulated that this centre is an iodine atom forming a weak three-electron bond to methanol. This was strongly supported by the observation that UV photolysis of solutions of potassium iodide in methanol gave the same electron paramagnetic resonance (EPR) features.The results are discussed in terms of the polarity of the R–I bonds, and the effects of solvation. Attempts to prepare a similar bromine atom centre using bromo-analogues have not yet been successful.

Reactions of hydrated electron and alcohol radicals with halogenated aromatic compounds: A pulse radiolysis study

The bimolecular rate constants for the reaction of e - aq with chlorotoluenes, chloronitrobenzenes and chloronitrotoluenes have been determined from the decay of e - aq. With chlorotoluenes, e - aq reacts by dissociative electron capture without any transient absorption band in 270–500 nm region. In case of chloronitrobenzenes and chloronitrotoluenes, e - aq forms a transient band with λ max in 290–310 nm region without any Cl - formation. Hydroxyl alkyl radicals, derived from alcohols, do not seem to react with chlorotoluenes whereas a transient species with λ max=305–315 nm region are formed on addition of hydroxy alkyl radical to —NO 2 group of chloronitrobenzenes.

An investigation of fragmentation mechanisms of doubly protonated tryptic peptides.

Peptides formed as reaction products, of specific hydrolysis of proteins by trypsin, are characterized by a basic residue (Arg or Lys) at the C-terminus, which facilitates formation of abundant [M + 2H]2+ ions under electrospray or ionspray conditions. These doubly charged ions readily dissociate upon collisional activation to y" and b fragment ions which are mass complements of one another. The suggestion that these fragments are formed by direct charge-separation dissociations must contend with the observation that the y" intensities are generally appreciably larger than those of their b counterparts. However, it is shown that this can be accounted for by a greater susceptibility of the b ions to undergo further dissociation to smaller fragments such as immonium ions. In addition no evidence could be found to support alternative mechanisms, including dissociative electron capture, for which equal intensities of the two fragment ion series are not obligatory. Initial protonation at the N-terminus was shown to be required for formation of these [M + 2H]2+ ions via its suppression by mono-acetylation at the N-terminus. These findings, and others concerning formation of [y"']2+ fragments, are consistent with extensions of published mechanisms for formation of b and of y" fragments from singly protonated peptides, via charge-site-induced cleavages and intramolecular proton transfers between nitrogen atoms, respectively.

Application of a trochoidal electron monochromator/mass spectrometer system to the study of environmental chemicals.

A trochoidal electron monochromator has been interfaced to a mass spectrometer to perform electron capture negative ion mass spectrometric (ECNIMS) analyses of environmentally relevant chemicals. The kinetic energy of the electron beam can be varied from 0.025 to 30 eV under computer control. No reagent gas is used to moderate the electron energies. An electron energy spread of +/- 0.1 to +/- 0.4 eV full width at half-maximum (fwhm) can readily be obtained at a transmitted current of 2 x 10(-6) A, improving to +/- 0.07 eV at 5 x 10(-7) A. Comparisons of ECNI results from the electron monochromator/mass spectrometer system with those from a standard instrument that uses a moderating gas show similar spectra for heptachlor but not for the s-triazine herbicides, as for example, atrazine. This compound shows numerous adduct ions by standard ECNIMS that are eliminated by using the electron monochromator to generate the mass spectra. Isomeric tetrachlorodibenzo-p-dioxins show distinct differences in the electron energies needed to produce the maximum amount of parent and fragment anions. Multiple resonance states resulting in stable radical anions (M.-) are easily observed for nitrobenzene and for polycyclic aromatic hydrocarbons. Ionic products of dissociative electron capture invariably occur from several resonance states.

Spectral and kinetic characteristics of the α-propionic acid methyl ester radical in cyclohexane and water

The α-propionic acid methyl ester radical was produced in dissociative electron capture reaction of 2-chloropropionic acid methyl ester. The absorption maxima of the radical are at 310 and 300 nm in cyclohexane and water with extinction coefficients of 440±50 and 400±50 mol−1 dm3 cm−1. The second order decay rate parameter in water is (2.3±0.5)×109 mol−1 dm3 s−1. The peroxy radicals have the characteristics: λmax=265–270 nm, emax=700–900 mol−1 dm3 and 2k=(7±2)·108 mol−1 dm3 s−1.

Resonant electron trapping and photoelectron spectroscopy for substituted fluorinated nitrobenzenes

Mass spectra have been recorded for dissociative electron capture DEC negative ions together with photoelectron spectra for the molecules of fluorinated nitrobenzenes. Nitrobenzene DEC is governed in the main by orbitals having appreciable contributions from the nitro group atoms. Resonant states are formed in the molecular negative fluoronitrobenzene ions via the Feshbach electronically excited resonance mechanism, which involves the excitation of an electron from a series of filled MO to several vacant MO.

Applications of thermospray liquid chromatography-mass spectrometry in photochemical studies of pesticides in water

Thermospray liquid chromatography-mass spectrometry (TSP-LC-MS) in the positive- and negative-ion modes (PI and NI, respectively) was used for the characterization of the pesticides aldicarb, carbaryl, cyanazine, fenitrothion, linuron and parathion-methyl, including the corresponding photodegradation products. The LC analyses were performed on RP-18 columns using methanol-water (70:30 or 50:50) + 0.05 M ammonium acetate or 0.05 M ammonium formate. Photodegradation studies of pesticides in distilled water were performed using a suntest apparatus, except for fenitrothion, for which a high-pressure mercury lamp was used because of slow photodegradation. The main photodegradation pathways corresponded to dealkylation, deamination, dehalogenation and hydroxylation. Photodegradation experiments were carried out using solutions of the different pesticides at 10–200 ppm in distilled water. For carbaryl photolysis is very slow in distilled water, so 5% of acetone was needed as photosensitizer. When PI-mode TSP-LC-MS was employed, aldicarb sulphoxide, monolinuron, hydroxysimazine and fenitrooxon could be identified as breakdown products of aldicarb, linuron, cyanazine and fenitrothion, respectively. In NI-mode TSP-LC-MS aldicarb sulphoxide, 1-naphthol and p-nitrophenol could be identified as the main degradation products of aldicarb, carbaryl and parathion-methyl, respectively. As all the pesticides were dissolved in methanol for solubility reasons, methoxy analogues of the degradation products of cyanazine were also identified. In the PI mode of operation the base peak generally corresponded to [M + H] + for cyanazine and its photodegradation products and to [M + NH 4] + for linuron, aldicarb and fenitrothion and their corresponding degradation products. For carbaryl [M + NH 4] + was also the base peak, but its main photodegradation product, 1-naphthol, could only be identified under NI conditions. In the NI mode of operation different processes such as proton abstraction, (dissociative) electron capture and anion attachment, take place. Fragment ions such as [M] ·− for aldicarb sulphoxide, the main photodegradation product of aldicarb, [M - H] − for 1-naphthol and [M - CONHCH 3] − for carbaryl and anion attachment ions corresponding to [M + CH 3COO] − for aldicarb sulphoxide and 1-naphthol and to [M - CONHCH 3 + CH 3COOH] − for carbaryl were formed. A tentative photodegradation pathway for the different pesticides in water is postulated.

Dissociative electron capture of sulfones and sulfonates: matrix isolation ESR study

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDissociative electron capture of sulfones and sulfonates: matrix isolation ESR studyPaul H. KasaiCite this: J. Am. Chem. Soc. 1991, 113, 9, 3317–3321Publication Date (Print):April 1, 1991Publication History Published online1 May 2002Published inissue 1 April 1991https://doi.org/10.1021/ja00009a015RIGHTS & PERMISSIONSArticle Views97Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (535 KB) Get e-Alerts Get e-Alerts

Thermochemical determination of structure of negative ions on the basis of data from resonance electron capture mass spectrometry. 1. Heterocyclic analogs of cyclopentadiene

Appearance energies have been measured for negative ions formed in the gas phase upon dissociative capture of electrons by molecules of cyclopentadiene, pyrrole, selenophene, thiophene, furan, and furan derivatives. From an analysis of the enthalpies of formation of the fragmentation products, it has been established that electron capture by molecules of C4H4X (X=O, S, Se, NH, CH2) leads to the formation of ions with cyclic and acyclic structures.