Gas phase negative ion chemistry

Dynamics of Site Selectivity in Dissociative Electron Attachment in Aromatic Molecules

Dissociative electron attachment has shown site selectivity in aliphatic molecules based on the functional groups present in them. This selectivity arises from the core excited resonances that have excited parent states localized to a specific site of the functional group. Here, we show that such site selectivity is also observed in the amine group when present in aromatic molecules. However, the proximity of the aromatic ring to the functional group under investigation has a substantial effect on the dissociation dynamics. This effect is evident in the momentum distribution of the hydride ions generated from the amine group. Our results unravel the hitherto unknown facets of the site selectivity in aromatic organic molecules.

Combined Experimental and Theoretical Studies on Electron Transfer in Potassium Collisions with CCl4

Negative ion formation in electron transfer experiments from fast neutral potassium (K) atom collisions with neutral tetrachloromethane (CCl₄) molecules has been investigated in the laboratory frame range of 8-1000 eV. Comprehensive calculations on the electronic structure were performed for CCl₄ in the presence of a potassium atom and used to help analyze the lowest unoccupied molecular orbitals participating in the collision process. Additionally, K⁺ energy loss produced in the forward direction has served to further our knowledge on the electronic state spectroscopy of CCl₄. A vertical electron affinity of -0.79 ± 0.20 eV has been obtained and assigned to a purely repulsive transition from CCl₄ ground state to the ²T₂ state of the temporary negative ion yielding Cl^- formation. Other features in the energy loss spectrum were observed for the first time and related to Cl₂^-, CCl₂^-, and CCl₃^- formation. Special attention is also given to the unresolved feature corresponding to a positive electron affinity of 0.24 ± 0.2 eV, assigned to a vibrationally hot transition from CCl₄ ground state into the triply degenerate ²T₂ excited state of the negative ion. The combined time-of-flight mass spectrometry together with K⁺ energy loss data represents the most comprehensive assignment of the tetrachloromethane anion yields and the role of CCl₄ electronic states in collision induced dissociation to date.

Analytical applications of electron monochromator-mass spectrometry

An electron monochromator (EM) produces an electron beam with a narrow energy distribution that can be utilized with mass spectrometry (MS). The history and development of the EM from an initial research design to a commercial model are reviewed along with MS research applications. An EM incorporated with a mass spectrometer showed significant improvement in sensitivity over traditional methods for negative-ion generation and selectivity for compounds with electrophilic character. Sensitivity of EM-MS has been shown to be 25 fg for hexachlorobenzene in positive-ion mode and 10 fg for nitrobenzene in negative-ion mode. Reports regarding the analysis of chlorinated compounds, explosives, pesticides, phthalates, polychlorodibenzo-p-dioxins, polycyclic aromatic hydrocarbons (PAHs), nitro-polycyclic aromatic hydrocarbons (NPAHs), antioxidants, and bacterial biomarkers are discussed. Additionally, theoretical methods to predict electron-capture properties are presented.

The use of a custom mode electron capture detector to determine mixing ratios of environmental tracers: Sulfur hexafluoride, chlorotrifluoromethane and bromotrifluoromethane

Atmospheric concentrations of anthropogenic trace gases, such as sulfur hexafluoride, SF6, chlorotrifluoromethane, CF3Cl, and bromotrifluoromethane, CF3Br, are increasing. Their long lifetimes and limited chemical reactivity make them attractive environmental tracers for hydrology and oceanography. While ambient SF6 concentrations can be readily measured using GC-ECD, the simultaneous analysis of CF3Cl and CF3Br is hampered due to their low ECD sensitivity. The response of a commercial ECD for those gases was enhanced using the resonance detection mode which is based on shifting the mean energy of electrons in the ECD detector towards the region where the electron-capture reaction reveals a distinct maximum. A custom electronic system enabled operation of a commercial ECD in the resonance detection mode. An approximately 50-fold amplification of the ECD signal was obtained for CF3Cl by application of high-frequency electric field (amplitude of 50V and frequency of 40MHz). For CF3Br, a 3.5-fold increase of the ECD signal was obtained, with a lower HF field (20-30V). In the case of SF6 the application of the HF field reduces the magnitude of ECD signal by a factor of 40. The electron-capture coefficients for SF6, CF3Cl and CF3Br were determined from 453 to 633K in the standard and the resonance modes. The electron-capture coefficients for CF3Cl and CF3Br increase with increasing temperature for both modes, while that for SF6 decreases slightly with increasing temperature. The application of the resonance detection mode to a commercial ECD provides an attractive and cost-effective alternative to GCMS for high-quality quantitative analyses of CF3Cl and CF3Br as environmental tracers.

Bond and site selectivity in dissociative electron attachment to gas phase and condensed phase ethanol and trifluoroethanol

The formation of negative ions following electron impact to ethanol (CH(3)CH(2)OH) and trifluoroethanol (CF(3)CH(2)OH) is studied in the gas phase by means of a crossed electron-molecular beam experiment and in the condensed phase via Electron Stimulated Desorption (ESD) of fragment ions from the corresponding molecular films under UHV conditions. Gas phase ethanol exhibits two pronounced resonances, located at 5.5 eV and 8.2 eV, associated with a remarkable selectivity in the decomposition of the precursor ion. While the low energy resonance exclusively decomposes into O(-), that at higher energy generates OH(-) and a comparatively small signal of [CH(3)CH(2)O](-) due to the loss of a neutral hydrogen. CF(3)CH(2)OH shows a completely different behaviour, as now an intense feature at 1.7 eV appears associated with the loss of a neutral hydrogen atom exclusively occurring at the O site. The H(-) formation from the gas phase compounds is below the detection limit of the present experiment, while in ESD from 3 MonoLayer (ML) films of CH(3)CH(2)OH and CF(3)CH(2)OH the most intense fragment is H(-), appearing from a broad resonant feature between 7 and 12 eV. With CF(3)CH(2)OH, by using the isotopically-labelled analogues CF(3)CD(2)OH and CF(3)CH(2)OD it can be shown that this feature consists of two resonances, one located at 8 eV leading to H(-)/D(-) loss from the O site and a second resonance located at 10 eV leading to the loss of H(-)/D(-) from the CH(2) site.

Dissociative electron attachment to phosphoric acid esters: the direct mechanism for single strand breaks in DNA

We use dibutyl phosphate to simulate the behavior of the phosphate group in DNA towards the attack of low energy electrons. We find that the compound undergoes effective dissociative electron attachment within a low energy resonant feature at 1 eV and a further resonance peaking at 8 eV. The dissociative electron attachment (DEA) reactions are associated with the direct cleavage of the C-O and the P-O bond but also the excision of the PO-, PO3-, H2PO3- units. For the phosphate group coupled in the DNA network these reactions represent single strand breaks. We hence propose that the most direct mechanism of single strand breaks occurring in DNA at subexcitation energies (< 4 eV) is due to DEA directly to the phosphate group.

Photon-stimulated desorption of F(-) ions from CF(3)Cl adsorbed on Si(111)-7x7

We report the photon-stimulated desorption of negative ions induced by direct dipolar dissociation and dissociative electron attachment. The photon-stimulated desorption of F(-) ions from CF(3)Cl physisorbed on a Si(111)-7x7 surface at 30 K in the photon energy range 12-35 eV was studied. The F(-) ion yield exhibits four resonances, at 12.8, 16.2, 19.5, and 22.3 eV, quite unlike the gas phase photodissociation cross section. The intensities of these resonances depend strongly on the CF(3)Cl coverage in a manner which varies from peak to peak. The resonances at 19.5 and 22.3 eV, which have a significant enhancement in the monolayer regime, are due to electron mediated dipolar dissociation of adsorbed CF(3)Cl molecules. The enhancement is attributed to surface electron attachment following molecular excitation. A significant enhancement in the monolayer regime has also been observed for the resonances at 12.8 and 16.2 eV. These two resonances are ascribable to a combination of electron mediated dipolar dissociation and dissociative electron attachment driven by photoelectrons generated in the neighboring molecules.

Factors influencing the analytical performance of an atmospheric sampling glow discharge ionization source as revealed via ionization dynamics modeling

A kinetic model is developed for the dynamic events occurring within an atmospheric sampling glow discharge that affect its performance as an ion source for analytical mass spectrometry. The differential equations incorporate secondary electron generation and thermalization, reagent and analyte ion formation via electron capture and ion-molecule reactions, ion loss via recombination processes, diffusion, and ion-molecule reactions with matrix components, and the sampling and pumping parameters of the source. Because the ion source has a flow-through configuration, the number densities of selected species can be estimated by applying the steady-state assumption. However, understanding of its operation is aided by knowledge of the dynamic behavior, so numerical methods are applied to examine the time dependence of those species as well. As in other plasma ionization sources, the ionization efficiency is essentially determined by the ratio of the relevant ion formation and recombination rates. Although thermal electron and positive reagent ion number densities are comparable, the electron capture/ion-molecule reaction rate coefficient ratio is normally quite large and the ion-electron recombination rate coefficient is about an order of magnitude greater than that for ion-ion recombination. Consequently, the efficiency for negative analyte ion formation via electron capture is generally superior to that for positive analyte ion generation via ion-molecule reaction. However, the efficiency for positive analyte ion formation should be equal to or better than that for negative analyte ions when both ionization processes occur via ion-molecule reaction processes (with comparable rate coefficients), since the negative reagent ion density is considerably less than that for positive reagent ions. Furthermore, the particularly high number densities of thermal electrons and reagent ions leads to a large dynamic range of linear response for the source. Simulation results also suggest that analyte ion number densities might be enhanced by modification of the standard physical and operating parameters of the source.

LC-electron capture-APCI(−)-MS determination of nitrobenzoxadiazole derivatives

Nitrobenzoxadiazole (NBD) derivatives are determined with limits of detection ranging down to 20 nmol l(-1) using liquid chromatography-mass spectrometry (LC-MS) with electron capture (EC) ionisation. An atmospheric pressure chemical ionisation (APCI) interface operated in the negative ion mode is used as ionisation source. Amine derivatives of 4-chloro-7-nitro-2,1,3-benzoxadiazole (NBDCl) as well as the isocyanate derivatives of 4-nitro-7-piperazino-2,1,3-nitrobenzoxadiazole (NBDPZ) have been analysed using this technique. The parameters favouring electron capture mechanisms have been investigated thoroughly under consideration of the competing mechanism of deprotonation to allow a better understanding of the electron capture process and to improve selectivity of the analysis.