Characterization by three-dimensional representation of sequential transitions of metastable ions in a double focusing mass spectrometer

The ion currents due to products of selected ion reactions of toluene and 3-chlorophenol, occurring in the source or flight tube of a double focusing mass spectrometer for which the electric sector precedes the magnetic sector, have been represented by plotting them against two variables that reflect independently the effects of the sectors. The daughter ion produced from the molecular ion of 3-chlorophenol that decays by sequential losses of CO in the field free region preceding the electric sector and Cl˙ in the field free region preceding the magnetic sector gives rise to a peak with a distinctive shape on the resultant three-dimensional surface. A peak due to the daughter ion arising from the corresponding sequential losses of H˙ and C₂H₂ from the molecular ion of toluene could not be detected. A portion of the ion kinetic energy spectrum which would be recorded by a detector at the energy resolving slit is derived from the surface for 3-chlorophenol and is in satisfactory agreement with a published spectrum.     

Time‐of‐flight mass spectrometry for time‐resolved measurements: Some developments and applications

In this paper, the time resolution for kinetic studies of reactions with mass spectrometric detection is characterized in detail, and it is shown how this allows faster kinetic processes to be determined. The time‐resolved technique used pulsed laser photolysis to initiate reaction and a time‐of‐flight mass spectrometer (TOFMS) to monitor progress, where the reactant gas was sampled by a sampling orifice and photoionized using pulsed, laser vacuum ultraviolet light before being analyzed by the TOFMS. Characterization of this setup has been carried out to identify the parameters that affect the time for “sampling,” which limits the fastest reactions that can be measured. A simple mathematical equation has been developed to correct for “sampling” delays (k_sampling～25, 000 s^?1), which extends the range of rate coefficients to be measured in a kinetic mass spectrometry reactor to k′ < 7000 s^?1. This method could be applied to any other kinetic mass spectrometry system where k_sampling can be measured; an important advantage since it allows the study of reactions over a wider range of conditions (e.g., larger concentrations of reagents/products can be used to minimize the contribution from wall losses). The system can produce reliable kinetic data whether monitoring reactant decay or product growth even when the reaction and sampling processes are occurring on a similar timescale (k′ < 7000 s^?1). Reproducible and reliable kinetic data have been obtained for the following reactions: SO + NO₂ → products (R1), ClSO + NO₂ → products (R2), where SO and ClSO were monitored under pseudo‐first‐order conditions, and HCO + O₂ → CO + HO₂ (R3), where CO was monitored by a [1+1] resonance enhanced ionization multiphoton ionization (REMPI) scheme with HCO reacting under pseudo–first‐order conditions. The limitations and potential developments of this setup are described. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 532–545, 2012     

Stopped-flow electrospray ionization mass spectrometry: a new method for studying chemical reaction kinetics in solution

In this work a new mass spectrometry based method for monitoring the kinetics of chemical reactions in solution is described. A stopped-flow mixing instrument is coupled to an electrospray ionization (ESI) mass spectrometer via a novel type of interface. Chemical reactions are initiated by rapid mixing of two reactant solutions. The mixture is instantaneously transferred to a reaction tube where the kinetics can be monitored in real-time by ESI mass spectrometry. With the current setup, a time window from 2.5 to 36 seconds after mixing of the reactants can be monitored. The experimental setup is used to study the kinetics of acetylcholine hydrolysis under alkaline conditions as a function of pH. The intensities of reactant (acetylcholine) and product (choline) ions are monitored simultaneously as a function of time. The reaction is carried out under pseudo-first-order conditions and the intensity-time curves are well described by single exponentials. The rate constants determined from these fits compare favorably with previous data from the literature.     

Gas-phase ion chemistry and organic chemistry-the story of a hybrid six sector mass spectrometer--the "AutoSpec 6F"

The AutoSpec 6F mass spectrometer is a large, floor standing instrument comprising a pair of commercial EBE geometry (AutoSpec) mass spectrometers coupled in series to provide an hybrid EBE-EBE configuration, (E and B being respectively electrostatic and magnetic sectors.) It was designed in close collaboration between Professor R. Flammang and VG Analytical in Manchester, UK. It was equipped with five collision cells and allowed the recording of high energy CID (collision induced dissociation), MIKES (mass analyzed ion kinetic energy spectrometry) and NRMS (neutralization re-ionization mass spectrometry) data as well as consecutive MSn analyses. The field-free regions between sectors allowed the study of unimolecular decomposition products from long-lived metastable ions. The mass spectrometer became even more versatile when an RF-only quadrupole collision cell was installed between the second and the third electric sector. This allowed the study of associative ion/molecule reactions in the low kinetic energy regime. Bimolecular chemical reactions were performed inside the quadrupole cell when a neutral reagent was introduced and the reaction products were analyzed by high energy CID in the downstream sectors. This paper tells the history and summarizes the capabilities of this versatile instrument.     

Characterization of Polymers by Direct Pyrolysis/Chemical Ionization Mass Spectrometry

The characterization of polymers by pyrolysis directly in the ion source of a double focusing magnetic sector mass spectrometer, operating in the chemical ionization mode, is described. Pyrolysis is achieved by two different probe techniques. A low temperature, slow heating rate direct insertion probe (DIP) is used at 400°C, and a specifically constructed high temperature, fast heating rate, high temperature pyrolysis (HTP) probe is used at 1000°C. This probe is capable of achieving pyrolysis temperatures of 1200°C at controlled heating rates up to 20,000°C/s. The mass spectrometric analysis of the pyrolysis products was achieved under chemical ionization (CI) conditions utilizing methane, isobutane, and ammonia as reagent gases. Under CI conditions the molecular ions formed in the mass spectrometer show little tendency to fragment. The CI mass pyrograms are very simple, with each peak in the spectra ascribable to a particular component in the pyrolysis product mixture. The results of the two probe pyrolysis techniques are compared and the utility of each technique for the characterization of polymers is demonstrated using the vinyl polymers polymethyl methacrylate, polyvinyl chloride, and polystyrene.     

Deconvolution of composite metastable peaks: A new method for the determination of metastable transitions occurring in the first field free region

The masses of daughter ions resulting from metastable transitions in the first field free region of a “reversed geometry” mass spectrometer can be accurately determined through a mass analysed ion kinetic energy scan at the apparent mass at which these daughter ions travel through the magnetic sector. A combination of these mass analysed ion kinetic scans with scanning the magnetic flux allows the resolution of overlapping metastable diffuse peaks.     

Renaissance of gas chromatography-time-of-flight mass spectrometry. Meeting the challenge of capillary columns with a beam deflection instrument and time array detection

This report describes the use of a unique beam deflection time-of-flight mass spectrometer to address some of the demands made on mass spectrometry by new developments in high-resolution capillary column gas chromatography. An integrating transient recorder is used in combination with this beam deflection time-of-flight instrument to apply the concept of time array detection in capturing all of the mass spectral information available from the ion source, thereby greatly enhancing the signal-to-noise ratio quality of the mass spectral data. The applicability of the time array detection approach to gas chromatography-mass spectrometry is demonstrated in the context of an analysis of the standard Grob mixture for assessing performance of capillary column chromatography. During analysis of the Grob mixture by gas chromatography-mass spectrometry, mass spectra were recorded at a rate of 20 scan files per second. The data indicate that this rate of mass spectral scan file generation is adequate to provide a suitable data base for reconstruction of the chromatographic profile. In addition, the effective scan rate is high enough that there is no distortion in the relative peak intensities throughout the individual mass spectra of components regardless of the relatively high dynamic changes in partial pressure of the analyte as reflected by the sharp peaks in the chromatographic profile. The experimental results indicate that the beam deflection time-of-flight mass spectrometer can provide mass spectra at a scan file generation rate much higher than that possible with the conventional quadrupole or magnetic sector mass spectrometer, but at comparable detection limits.     

The effect of N2O on the isotopic composition of air-CO2 samples

The ionisation efficiencies of N2O vs. CO2 as well as their ratios were measured in detail introducing clean N2O and CO2 into the electron impact ion source of an isotope ratio mass spectrometer. Changes in the ionisation efficiency ratio (IER) were found for different electron energy settings and compared with the ratios of literature ionisation cross-section values for pure N2O and CO2. To establish the influence of mixtures of N2O and CO2 in a mass spectrometer, artificial air mixtures were prepared by mixing different amounts of N2O and CO2 from well-calibrated spike cylinders with CO2-free air. The mixing ratios varied from 8-512 ppb for N2O and from 328-744 ppm for CO2. With these mixtures the effects of varying N2O concentrations on apparent CO2 isotope ratios in air samples were determined. After applying a mass balance correction the delta13C results were consistent within small error margins. The data seemed almost independent from a particular choice for the IER of N2O vs. CO2 in the correction algorithm. For delta18O a small effect of the ionisation efficiency ratio of N2O vs. CO2 was found. Several sets of calculations were made varying the IER between 0.88 and 0.62. The dependence of delta18O was the smallest with an adopted IER of 0.68-0.72 in the mass balance correction equation for isotopic analysis of CO2 in air. For high-precision measurements of the CO2 stable isotope ratios in air samples a careful assessment of the mass spectrometer performance is necessary. Different ion sources, even different ion source settings, alter the IER of N2O vs. CO2 which is used in the N2O correction algorithm. Preferably, the specific mass spectrometric behaviour should be established with clean N2O/CO2 mixtures or with air mixtures covering a larger range of N2O concentrations.     

Transient and steady state CO oxidation kinetics on nanolithographically prepared supported Pd model catalysts: experiments and simulations

Applying molecular-beam methods to a nanolithographically prepared planar PdSiO2 model catalyst, we have performed a detailed study of the kinetics of CO oxidation. The model catalyst was prepared by electron-beam lithography, allowing individual control of particle size and position. The sample was structurally characterized by atomic force microscopy and scanning electron microscopy before and after reaction. In the kinetic experiments, the O-rich and CO-rich regimes were investigated systematically with respect to their transient and steady-state behaviors, both under bistable and monostable reaction conditions. Separate molecular beams were used in order to supply the reactants, allowing individual control over the reactant fluxes. The desorbing CO2 was detected by both angle-resolved and angle-integrated mass spectrometries. The experimental results were analyzed using different types of microkinetic models, including a detailed reaction-diffusion model, which takes into account the structural parameters of the catalyst as well as scattering of the reactants from the support. The model quantitatively reproduces the results as a function of the reactant fluxes and the surface temperature. Various kinetic effects observed are discussed in detail on the basis of the model. Specifically, it is shown that under conditions of limited oxygen mobility, the switching behavior between the kinetic regimes is largely driven by the surface mobility of CO.     

CO2 dynamic adsorption/desorption on zeolite 5A studied by 13C magnetic resonance imaging

We present the first 13C magnetic resonance imaging study of CO2 transient adsorption/desorption processes in a zeolite 5A column. CO2 transient concentration profiles were measured with a centric scan spin-echo single point imaging technique. The adsorption wave profiles were determined under flow conditions, with the results analyzed by the Bohart-Adams model. The model adequately accounts for the spatial and the temporal behavior of CO2 in the column. CO2 adsorption rate constants were calculated from the fit. Desorption profiles were acquired by blowing a helium stream through a zeolite 5A column saturated with CO2. An asymmetry between the adsorption and desorption profiles is readily apparent. A linear relationship between the CO2 condensed phase concentration and square root of time was observed.     

High resolution measurements of kinetic energy release distributions of neon, argon, and krypton cluster ions using a three sector field mass spectrometer

Using a newly constructed three sector field mass spectrometer (resulting in a BE1E2 field configuration) we have measured the kinetic energy release distributions of neon, argon, and krypton cluster ions. In the present study we used the first two sectors, B and E1, constituting a high resolution mass spectrometer, to select the parent ions in terms of mass, charge, and energy, and studied the decay of those ions in the third field free region. Due to the improved mass resolution we were able to extend earlier studies carried out with a two sector field machine, where an upper size limit arose from the fact that several isotopomers contribute to a decaying parent ion beam when the cluster size exceeds a certain value. Furthermore we developed a new data analysis. It allows us to model also fragment ion peaks that are a superposition of different decay reactions and thus we can determine the average kinetic energy release for all decay reactions of a given cluster ion. In a further step we used these results to determine the binding energies of cluster ions Rg(n) (n> or =10) by applying finite heat bath theory. The smaller sizes have not been included in this analysis, because the validity of finite heat bath theory becomes questionable below n approximately 10. The present average kinetic energy releases and binding energies are compared with other experiments and various calculations.     

Electron transfer and ligand addition to atomic mercury cations in the gas phase: kinetic and equilibrium studies at 295 k

Results are reported for experimental measurements of the room-temperature chemical reactions between ground-state Hg*+ ions and 16 important environmental and biological gases: SF6, CO, CO2, N2O, D2O, CH4, CH3F, O2, CH3Cl, OCS, CS2, NH3, C6F6, NO2, NO*, and C6H6. The inductively coupled plasma/selected-ion flow tube tandem mass spectrometer used for these measurements has provided both rate and equilibrium constants. Efficient electron transfer (>19%) is observed with CS2, NH3, C6F6, NO2, NO*, and C6H6, molecular addition occurs with D2O, CH4, CH3F, CH3Cl, and OCS, and SF6, CO, CO2, N2O, and O2 showed no measurable reactivity with Hg*+. Theory is used to explore the stabilities and structures of both the observed and unobserved molecular adducts of Hg*+, and reasonable agreement is obtained with experimental observations, given the uncertainties of the theory and experiments. A correlation is reported between the Hg*+ and proton affinities of the ligands investigated. Solvation of Hg*+ with formic acid was observed to increase the rate of electron transfer from NO* by more than 20%.     

Obtaining Kinetic Parameters by Modulated Thermogravimetry

A new technique, called modulated thermogravimetry, is introduced as a tool for obtaining continuous kinetic information for decomposition and volatilization reactions. The approach makes use of an oscillatory temperature program to obtain kinetic parameters during a mass loss. MTGA™ may be used under quasi-isothermal conditions to observe a single mass loss or may be combined with linear heating rate or Hi-Res™ controlled rate thermogravimetry to scan from one mass loss region to another. Results obtained are in agreement with those obtained by other kinetic methods. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental microkinetic approach of the photocatalytic oxidation of isopropyl alcohol on TiO2. Part 1. Surface elementary steps involving gaseous and adsorbed C3H(x)O species

The present study concerns an experimental microkinetic approach of the photocatalytic oxidation (PCO) of isopropyl alcohol (IPA) into acetone on a pure anatase TiO2 solid according to a procedure previously developed. Mainly, the kinetic parameters of each surface elementary step of a plausible kinetic model of PCO of IPA are experimentally determined: natures and amounts of the adsorbed species and rate constants (preexponential factor and activation energy). The kinetics parameters are obtained by using experiments in the transient regime with either a FTIR or a mass spectrometer as a detector. The deep oxidation (CO2 and H2O formation) of low concentrations of organic pollutants in air is one of the interests of the PCO. For IPA, literature data strongly suggest that acetone is the single route to CO2 and H2O and this explains that the present study is dedicated to the elementary steps involving gaseous and adsorbed C3H(x)O species. The microkinetic study shows that strongly adsorbed IPA species (two species denoted nd-IPA(sads) and d-IPA(sads) due to non- and dissociative chemisorption of IPA, respectively) are involved in the PCO of IPA. A strong competitive chemisorption between IPA(sads) and a strongly adsorbed acetone species controls the high selectivity in acetone of the PCO at a high coverage of the surface by IPA(sads). The kinetic parameters of the elementary steps determined in the present study are used in part 2 to provide a modeling of macroscopic kinetic data such as the turnover frequency (TOF in s(-1)) of the PCO using IPA/O2 gas mixtures.     

A Study of Heterogeneous Catalysis by Nanoparticle‐Embedded Paper‐Spray Ionization Mass Spectrometry

We have developed nanoparticle‐embedded paper‐spray mass spectrometry for studying three types of heterogeneously catalyzed reactions: 1) Palladium‐nanoparticle‐catalyzed Suzuki cross‐coupling reactions, 2) palladium‐ or silver‐nanoparticle‐catalyzed 4‐nitrophenol reduction, and 3) gold‐nanoparticle‐catalyzed glucose oxidation. These reactions were almost instantaneous on the nanocatalyst‐embedded paper, which subsequently transferred the transient intermediates and products to a mass spectrometer for their detection. This in situ method of capturing transient intermediates and products from heterogeneous catalysis is highly promising for investigating the mechanism of catalysis and rapidly screening catalytic activity under ambient conditions.     

Automotive catalysis studied by surface science

In this tutorial review I discuss the significant impact that surface science has had on our understanding of the catalytic phenomena associated with automobile exhaust depollution catalysis. For oxidation reactions it has generally been found that reactions are self-poisoned at low temperatures by the presence of strongly adsorbed reactants (such as molecular CO and NO), and that the rapid acceleration in rate at elevated temperatures (often called 'light-off') is due to the desorption of such adsorbates, which then frees up sites for dissociation and hence for oxidation reactions. In some circumstances such autocatalytic phenomena can then manifest themselves as oscillatory reactions which can vary in rate in both space and time. For NO reduction, the efficiency of depollution (by production of molecular nitrogen) is strongly affected by the nature of the metal used. Rh is extremely effective because it can dissociate NO much more readily than metals such as Pd and Pt, enabling oxygen removal (by reaction with CO to CO(2)) even at room temperature. Rh is also very selective in producing predominantly N(2), rather than N(2)O. NO(x) storage and reduction (NSR) is an important recent development for removal of NO(x) under the highly oxidising conditions of a lean-burn engine exhaust, and the strategy involves storing NO(x) on BaO under oxidising conditions followed by the creation of reducing conditions to de-store and reduce it to nitrogen. By the use of STM it has been shown that this storage process is extremely facile, occurring fast even under UHV conditions, and that the storage occurs on BaO in the vicinity of Pt, with most of the oxide being converted to nitrate.     

Removal of NO in NO/N2, NO/N2/O2, NO/CH4/N2, and NO/CH4/O2/N2 systems by flowing microwave discharges

In this paper, continuing previous work, we report on experiments carried out to investigate the removal of NO from simulated flue gas in nonthermal plasmas. The plasma-induced decomposition of small concentrations of NO in N2 used as the carrier gas and O2 and CH4 as minority components has been studied in a surface wave discharge induced with a surfatron launcher. The reaction products and efficiency have been monitored by mass spectrometry as a function of the composition of the mixture. NO is effectively decomposed into N2 and O2 even in the presence of O2, provided always that enough CH4 is also present in the mixture. Other majority products of the plasma reactions under these conditions are NH3, CO, and H2. In the absence of O2, decomposition of NO also occurs, although in that case HCN accompanies the other reaction products as a majority component. The plasma for the different reaction mixtures has been characterized by optical emission spectroscopy. Intermediate excited species of NO*, C*, CN*, NH*, and CH* have been monitored depending on the gas mixture. The type of species detected and their evolution with the gas composition are in agreement with the reaction products detected in each case. The observations by mass spectrometry and optical emission spectroscopy are in agreement with the kinetic reaction models available in literature for simple plasma reactions in simple reaction mixtures.     

Chelation-controlled radical chain reactions studied by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) is a novel tool for the investigation of chemical reactions in solution and for the direct detection and identification of reactive intermediates. The tributyltin hydride mediated addition of tert-butyl iodide to dimethyl 2-cyclohexyl-4-methyleneglutarate (2) in the presence of Lewis acids was investigated by ESI-MS using a microreactor coupled on-line to an ESI mass spectrometer. For the first time we have been able to show that transient radicals in radical chain reactions can be detected unambiguously under steady-state conditions in the reaction solution and can be characterized by ESI-MS/MS and accurate mass determination. The detection of different heterodimer radical complexes by ESI-MS/MS has provided new insights into the mechanism of Lewis acid controlled radical chain reactions. Dimeric chelate complexes of glutarates, such as 2 and 3, and Lewis acids, like Sc(OTf)3, MgBr2OEt2 and LiClO4, were observed as well as higher aggregates with additional equivalents of Lewis acid. Evidence for a dynamic equilibrium of the complexes in solution was found by NMR spectroscopy. The ESI-MS investigation of the chelation of glutarate 2 with various Lewis acids has led to the conclusion that the tendency for Lewis acids to form dimeric chelate complexes and higher aggregates has an important effect on the stereoselective outcome of the radical reactions.     

CO氢化合成乙醇反应的过渡应答研究——Ⅱ. 改性Rh/Al2O3催化剂上的反应动力学研究

本文用过渡应答技术研究CO在改性Rh/Al_2O_3催化剂上常压氢化合成乙醇的表面反应机理。结果表明: 1. 在Rh/Al_2O_3催化剂中添加K元素能显著地增加合成乙醇反应的活性和选择性; 2. 乙醇的生成取决于表面上相邻两个含氧中间体的密度而与气相中的H_2或CO浓度无关。反应倾向于ANDERSON的缩聚机理。在此反应机理的基础上, 由应答曲线的数据, 根据MARQUARDT非线性优化技术以及RUNG-KUTTA-GILL数值积分法计算了所有基元步骤的反应速率常数。     

Thermochemistry of the activation of N2 on iron cluster cations: Guided ion beam studies of the reactions of Fe(n)+ (n = 1-19) with N2

The kinetic energy dependences of the reactions of Fe(n)+ (n = 1-19) with N2 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-15 eV. In addition to collision-induced dissociation forming Fe(m)+ ions, which dominate the product spectra, a variety of Fe(m)N2+ and Fe(m)N+ product ions, where m < or = n, is observed. All processes are observed to exhibit thresholds. Fe(m)+ - N and Fe(m)+ - 2N bond energies as a function of cluster size are derived from the threshold analysis of the kinetic energy dependences of the endothermic reactions. The trends in this thermochemistry are compared to the isoelectronic D0(Fe(n)+ - CH), and to bulk phase values. A fairly uniform barrier of 0.48+/-0.03 eV at 0 K is observed for formation of the Fe(n)N2+ product ions (n = 12, 15-19) and can be related to the rate-limiting step in the Haber process for catalytic ammonia production.