Quadrupole mass spectrometric study of positive ions from RF plasmas of pure CH4, CH4/H2, and CH4/Ar systems

Quadrupole mass spectrometry has been employed to characterize the ionic species in the discharges of pure CH₄, CH₄/H₂, and CH₄/Ar systems. For pure methane, the major positive ions in the discharge at low pressure (e.g., 0.15 torr) are CH ₃ ⁺ , C₂H ₃ ⁺ , CH ₂ ⁺ , C₂H ₂ ⁺ , CH ₄ ⁺ , C₂H ₄ ⁺ , and C₂H ₅ ⁺ at high pressure (e.g., 0.5 torr) the major ions are CH ₃ ⁺ , C₂H ₃ ⁺ , C₂H ₅ ⁺ , C₃H ₃ ⁺ , C H₃H ₇ ⁺ , C₄H ₇ ⁺ , C₅H ₇ ⁺ , C₆H ₅ ⁺ , and C₇H ₇ ⁺ . The relative abundances of C₁ ions decrease with increasing pressure, whereas those of higher-order ions increase with pressure. For 5% CH₄ + 95% H₂ mixture, in addition to those sampling from the pure methane plasma at the lower pressure, H _n ⁺ ions have also been detected. For 5% CH₄ +95% Ar mixture, the principal ions are CH ₃ ⁺ , CH ₂ ⁺ , CH⁺, CH ₅ ⁺ , Ar⁺, and ArH⁺; the ions containing more than two carbon atoms are negligible. In these discharges, the CH ₃ ⁺ and C₂H ₃ ⁺ are the most important positive ions in C₁ and C₂ ions, respectively. The ions detected are believed to come from the sheath between the electrode and the luminous plasma, and have high kinetic energy. An ion-molecule reaction mechanism is proposed which can well explain the observed main features of ionic products.Died June 1, 1991.     

氯苯类污染物在高层大气环境中形成的可能性

简要地总结了大气离子与自由基反应的研究进展和以氯苯为目标产物的气相离子-分子反应的质谱研究。根据大气层状结构图、高层大气的特点以及相关的研究结果,提出了氯苯类污染物的形成不排除是大气化学反应尤其是离子反应产物的观点。对进一步研究大气中氯苯类污染物的生成机制和控制具有重要意义。     

Ion—Molecule reactions of environmentally significant organotin compounds in a triple quadrupole mass spectrometer system

Tandem MS techniques have been used to examine the formation of cluster ions derived from organotin compounds of environmental significance. The cluster formation was based on the addition of either water or methanol molecules (common HPLC solvents) to a cationic species derived from the organotin compound. For the compounds and conditions studied, cluster adducts were only observed from trisubstituted tin cations. The results show that, in high-pressure ionization methods used in the interface between HPLC and MS, the tin atom may be associated with a range of ions depending on the system parameters, and that care should be taken if selection ion monitoring (SIM) is to be used.     

一类2(1H)-喹喔啉衍生物的电子轰击质谱分析

以邻苯二胺为原料合成表征了一类喹喔啉衍生物；1-烷基-3-甲基-2（1H）-喹喔啉-2-酮（1-alkyl-3-methly-2(1H)-quinoxalin-2-one，简称AMQ，其中alky=H、CH3、Et、n-C3H5、n-C5H11、n-C16H33)。用电子轰击电离质谱（EI-MS）和核磁共振谱图（^1H-NMR)鉴定了AMQ，研究了不同取代基对这类化合物EI-MS谱图的影响，分析得到一些裂解规律。结果表明：所有样品谱图中均有芳香化合物的特征CnHn^ 系列峰m/z78，65，52，39和（或）m/z77、76、64、63、51、50、38、37。对于末取代的样品及低级烷烃取代的样品，谱图中的基峰为分子离子峰，对于取代基中碳数大于等于3的样品。谱图中的基峰不是分子离子峰，分子离子峰强度随着碳数的增加而降低。这些样品一般容易发生骨架重排，丢失一分子CO，生成1-烷基-2-甲基苯并哒唑正离子。此外，还讨论了谱图中其它的一些碎片离子峰。这对于研究和指导喹喔啉衍生物 的合成，探讨它们在药效及结构特征与生理活性的关系等方面具有重要的应用价值和理论意义。     

Cation-π Interactions between a Noble Metal and a Polyfunctional Aromatic Ligand: Ag+(benzylamine)

The structure of an isolated Ag⁺(benzylamine) complex is investigated by infrared multiple photon dissociation (IRMPD) spectroscopy complemented with quantum chemical calculations of candidate geometries and their vibrational spectra, aiming to ascertain the role of competing cation-N and cation-π interactions potentially offered by the polyfunctional ligand. The IRMPD spectrum has been recorded in the 800–1800 cm⁻¹ fingerprint range using the IR free electron laser beamline coupled with an FT-ICR mass spectrometer at the Centre Laser Infrarouge d'Orsay (CLIO). The resulting IRMPD pattern points toward a chelate coordination (N-Ag⁺-π) involving both the amino nitrogen atom and the aromatic π-system of the phenyl ring. The gas-phase reactivity of Ag⁺(benzylamine) with a neutral molecular ligand (L) possessing either an amino/aza functionality or an aryl group confirms N- and π-binding affinity and suggests an augmented silver coordination in the product adduct ion .     

Xenon-nitrogen chemistry: gas-phase generation and theoretical investigation of the xenon-difluoronitrenium ion F2N-Xe+

The xenon–difluoronitrenium ion F₂N? Xe⁺, a novel xenon–nitrogen species, was obtained in the gas phase by the nucleophilic displacement of HF from protonated NF₃ by Xe. According to Møller–Plesset (MP2) and CCSD(T) theoretical calculations, the enthalpy and Gibbs energy changes (ΔH and ΔG) of this process are predicted to be ?3 kcal mol^?1. The conceivable alternative formation of the inserted isomers FN? XeF⁺ is instead endothermic by approximately 40–60 kcal mol^?1 and is not attainable under the employed ion‐trap mass spectrometric conditions. F₂N? Xe⁺ is theoretically characterized as a weak electrostatic complex between NF₂⁺ and Xe, with a Xe? N bond length of 2.4–2.5 Å, and a dissociation enthalpy and free energy into its constituting fragments of 15 and 8 kcal mol^?1, respectively. F₂N? Xe⁺ is more fragile than the xenon–nitrenium ions (FO₂S)₂NXe⁺, F₅SN(H)Xe⁺, and F₅TeN(H)Xe⁺ observed in the condensed phase, but it is still stable enough to be observed in the gas phase. Other otherwise elusive xenon–nitrogen species could be obtained under these experimental conditions.     

Gas‐Phase Reaction of CeV2O7+ with C2H4: Activation of CC and CH Bonds

The reactivity of metal oxide clusters toward hydrocarbon molecules can be changed, tuned, or controlled by doping. Cerium‐doped vanadium cluster cations CeV₂O₇⁺ are generated by laser ablation, mass‐selected by a quadrupole mass filter, and then reacted with C₂H₄ in a linear ion trap reactor. The reaction is characterized by a reflectron time‐of‐flight mass spectrometer. Three types of reaction channels are observed: 1) single oxygen‐atom transfer , 2) double oxygen‐atom transfer , and 3) C?C bond cleavage. This study provides the first bimetallic oxide cluster ion, CeV₂O₇⁺, which gives rise to C?C bond cleavage of ethene. Neither Ce_xO_y^± nor V_xO_y^± alone possess the necessary topological and electronic properties to bring about such a reaction.     

Prompt and Slow Electron‐Detachment‐Dissociation/Electron‐Photodetachment‐Dissociation of a 21‐Mer Peptide

Electron detachment dissociation (EDD) and electron photodetachment dissociation (EPD) are relatively new dissociation methods that involve electron detachment followed by radical‐driven dissociation from multiply deprotonated species. EDD yields prompt dissociation whereas only electron detachment is obtained by EPD; subsequent vibrational activation of the charge‐reduced radical anion is required to obtain the product ions. Herein, the fragmentation patterns that were obtained by EDD and by vibrational activation of the charge‐reduced radical anions that were produced through EDD or EPD (activated‐EDD and activated‐EPD) were compared. The observed differences were related to the dissociation kinetics and/or the contribution of electron‐induced dissociation (EID). Time‐resolved double‐resonance experiments were performed to measure the dissociation rate constants of the EDD product ions. Differences in the formation kinetics were revealed between the classical EDD/EPD ′a^._i/′′x_j complementary ions and some ′a^._i/c_i/′′′z^._j product ions, which were produced with slower dissociation rate constants, owing to the presence of specific neighbouring side chains. A new fragmentation pathway is proposed for the formation of the slow‐kinetics ′a^._i ions.     

C alpha-C backbone fragmentation dominates in electron detachment dissociation of gas-phase polypeptide polyanions

Fragmentation of peptide polyanions by electron detachment dissociation (EDD) has been induced by electron irradiation of deprotonated polypeptides [M-nH](n-) with >10 eV electrons. EDD has been found to lead preferentially to a* and x fragment ions (C(alpha)-C backbone cleavage) arising from the dissociation of oxidized radical anions [M-nH]((n-1)-*. We demonstrate that C(alpha)-C cleavages, which are otherwise rarely observed in tandem mass spectrometry, can account for most of the backbone fragmentation, with even-electron x fragments dominating over radical a* ions. Ab initio calculations at the B3 LYP level of theory with the 6-311+G(2 p,2 d)//6-31+G(d,p) basis set suggested a unidirectional mechanism for EDD (cleavage always N-terminal to the radical site), with a*, x formation being favored over a, x* fragmentation by 74.2 kJ mol(-1). Thus, backbone C(alpha)-C bonds N-terminal to proline residues should be immune to EDD, in agreement with the observations. EDD may find application in mass spectrometry for such tasks as peptide sequencing and localization of labile post-translational modifications, for example, those introduced by sulfation and phosphorylation. EDD can now be performed not only in Fourier transform mass spectrometry, but also in far more widely used quadrupole (Paul) ion traps.     

Formation of H3+ in Collisions of H2+ with H2 Studied in a Guided Ion Beam Instrument

In order to study collisions between ions and neutrals, a new Guided Ion Beam (GIB) apparatus, called NOVion, has been assembled and tested. The primary purpose of this instrument is to measure absolute cross sections at energies relevant for technical or inter- and circumstellar plasmas. New and improved results are presented for forming H₃⁺ in collisions of H₂⁺ with H₂. Between 0.1 eV and 2 eV, our measured effective cross sections are in good overall agreement with most previous measurements. However, at higher energies, our results do not show the steep decline, recommended in the standard literature. After critical evaluation of all experimental and theoretical data, a new analytical function is proposed, describing properly the dependence of the title reaction on the collision energy up to 10 eV.     

Extensive study on physicochemical properties of polychlorinated biphenyls in a commercial ion trap mass spectrometer,relevance in analytical and environmental chemistry

Polychlorinated biphenyls (PCBs) exist as 209 congeners, consisting of biphenyl molecules, where the number and substitution positions of halogen atoms are known to affect industrial uses, environmental transport mechanisms, distribution, fate, and toxicity. The complexity of the problem requires accurate physicochemical studies of an increasing number of congeners in order to understand the environmental and biological processes at play. This work presents a systematic study on the thermodynamic and kinetic properties of PCBs by quadrupole ion trap mass spectrometry. A clear relationship between structure and behavior of PCBs in mass spectrometry experiments has been observed. Overall data demonstrate that di‐ortho congeners show lower thermodynamic stability and higher fragmentation rate than non/mono‐ortho. Congeners follow different fragmentation mechanisms according to the number of chlorine atoms in ortho position of the biphenyl system. Experimental kinetic curves of mono/non‐ortho and di‐ortho congeners show a strong similarity with classical first‐order kinetics curves; in particular, di‐ortho congeners follow a first‐order consecutive reaction, while mono/non‐ortho follow a first‐order parallel reaction. For each studied congener, the kinetic constant of reaction (fragmentation) has been determined. Data support environmental levels and biochemical transformations described in literature. The general picture of the PCB behavior inside a quadrupole ion trap provides the basis for the development of reliable and cost‐effective analytical methods to the determination of ultra‐low level trace of PCB congeners.     

Rapid Autoxidation Forms Highly Oxidized RO2 Radicals in the Atmosphere

Gas‐phase oxidation routes of biogenic emissions, mainly isoprene and monoterpenes, in the atmosphere are still the subject of intensive research with special attention being paid to the formation of aerosol constituents. This laboratory study shows that the most abundant monoterpenes (limonene and α‐pinene) form highly oxidized RO₂ radicals with up to 12 O atoms, along with related closed‐shell products, within a few seconds after the initial attack of ozone or OH radicals. The overall process, an intramolecular ROO→QOOH reaction and subsequent O₂ addition generating a next R′OO radical, is similar to the well‐known autoxidation processes in the liquid phase (QOOH stands for a hydroperoxyalkyl radical). Field measurements show the relevance of this process to atmospheric chemistry. Thus, the well‐known reaction principle of autoxidation is also applicable to the atmospheric gas‐phase oxidation of hydrocarbons leading to extremely low‐volatility products which contribute to organic aerosol mass and hence influence the aerosol–cloud–climate system.     

Dominant reaction channels and the mechanism of silane decomposition in a H2-Si(s)-SiH4 glow discharge

Chemical relaxation mass spectrometry has been used to study the kinetics and mechanism in the silane-hydrogen-solid silicon system under conditions of glow discharge. The emphasis was on the main processes related to the deposition of amorphous and nanocrystalline silicon thin films. It is shown that under conditions of the deposition of a-Si and nc-Si the dominant reaction channel is the electron impact induced fragmentation of silane into molecular hydrogen and SiH₂ radical. The role of other processes, such as hydrogen abstraction, is discussed.     

Absolute partial and total electron ionization cross sections for CCl4 from threshold up to 180 eV

Electron impact ionization of carbon tetrachloride was studied as a function of electron energy from threshold up to 180 eV. A double-focusing mass spectrometer system in combination with an improved electron impact ion source was used, alleviating the problems of ion extraction from the source and the transmission of the extracted ions through the mass spectrometer system. Absolute partial ionization cross sections for the occurrence of CCl ₃ ⁺ , CCl ₂ ⁺ , CCl⁺, Cl ₂ ⁺ , Cl⁺, C⁺, CCl ₃ ²⁺ , and CCl ₂ ²⁺ have been determined. In addition, the total ionization cross-section function of CCl₄ is reported and compared with theoretical predictions based on a classical binary encounter approximation. Using nth root extrapolation the following ionization energies of the doubly ionized fragment ions have been derived: AE(CCl ₃ ²⁺ )=30.4±0.3 eV; and AE (CCl ₂ ²⁺ )=31.8±0.3 eV. In accordance with theoretical predictions and previous results, no stable CCl ₄ ⁺ has been detected, however, metastable dissociation processes CCl ₄ ⁺ CCl ₃ ⁺ have been observed.