Coincidence Mass Spectrometry Research Articles

Coincidence mass spectrometry study of double ionization of pyrene by 70 eV electron impact.

We have performed coincidence mass spectrometry of fragmentation of pyrene molecules by 70 eV electron impact. Ionized fragments have been mass selected using a reflectron time-of-flight mass spectrometer, and a field programmable gate array has been used for the timing of the electron and ion extraction pulses and for the event-by-event detection of the ions. Double ionization results in a number of prominent fragmentations producing two singly-ionized fragments with kinetic energies of up to a few eV. A number of fragmentations produce ions with four or more carbon atoms, which can only be formed by the breaking of at least three C-C bonds.

Electron ionisation of cyanoacetylene: ionisation cross sections and dication formation

Cyanoacetylene (HC3N) is an important trace species in the atmosphere of Titan. We report, for the first time, absolute partial electron ionisation cross sections and absolute precursor-specific partial electron ionisation cross sections for cyanoacetylene, following an experimental and computational investigation. Our methodology involves using 2D ion–ion coincidence mass spectrometry to generate relative cross sections, over the electron energy range 50–200 eV. These relative values are then normalised to an absolute scale, using a binary encounter-Bethe (BEB) calculation of the total ionisation cross section. The BEB calculation agrees well with previous determinations in the literature. The mass spectrometric observations of HC2N+ and HCN+, ions with a connectivity markedly different to that of the neutral molecule, point towards a rich cationic energy landscape possessing several local minima. Indeed, [HC3N]2+ minima involving a variety of cyclic configurations are revealed by a preliminary computational investigation, along with two minima with linear and bent geometries involving H atom migration (CCCNH2+). Determination of the energy of a transition state between these local minima indicates that the dication is able to explore the majority of this rich conformational landscape at our experimental energies. This investigation of the energetics also determines an adiabatic double ionisation energy of 30.3 eV for the lowest lying singlet state of HCCCN2+, and 30.1 eV for the lowest-lying triplet state. The bulk of the cation pairs detected in the coincidence experiment appear to originate from markedly excited dication states, not the ground state. We observe 5 two-body dissociations of HCCCN2+, and subsequent decay of one of the ions generated in such two-body processes accounts for the majority of three-body dissociations we observe.

Chemistry deriving from OOQOOH radicals in alkane low-temperature oxidation: A first combined theoretical and electron-ion coincidence mass spectrometry study

While there is consensus on the fact that OOQOOH radicals, produced by two oxygen additions from alkyl radicals, are the heart of the low-temperature oxidation of alkanes, the determination of the isomeric distribution and the quantification of their derived products (ketohydroperoxides and diones) are still a challenge. For the first time, heavy oxygenated products produced during alkane oxidation have been investigated using electron/ion coincidence mass spectrometry. The investigated prototype reaction is n-pentane oxidation carried out in a jet-stirred reactor (temperatures from 585 to 665 K, pressure of 1.1 bar, lean mixture). Identification attempts were made for m/z 100 and 118 species using coincident mass-tagged Slow PhotoElectron Spectra obtained by electron-ion coincidence mass spectrometry combined with first principle computations, consisting in the determination of their adiabatic ionization energies and the Franck-Condon envelope of the photoionization spectra. 4-hydroperoxypentan-2-one has been confirmed as the dominant obtained ketohydroperoxide, as predicted by up-to-date kinetic models. However, difficulties due to fragmentation has made impossible the identification of the ketohydroperoxides present in lower amounts. In parallel, C5H8O2 isomers were identified, showing the possible formation, in addition to diones, of species with a ketone and an enol function. In addition, we provide new information on the first steps of the fragmentation pathways of C5 ketohydroperoxides. From the shape of their corresponding peaks on mass spectra and the energy and temperature dependence of their signal, ions at m/z 43, 57 and 85 have been identified as fragments from ketohydroperoxides. Taking into account these fragmentations lowers, by more than a factor of 10, the previously observed deviation between experiments and modeling for ketohydroperoxide mole fractions. The formation of the C1-C2 carboxylic acids, predicted from Korcek decomposition, was also observed, but with a favored formation of acetic acid versus formic acid that what was predicted for propane.

Coincidence time-of-flight mass spectrometry of electron impact on anthracene using a field programmable gate array

SynopsisA new data acquisition system for electron-impact time-of-flight mass spectrometry has been set up, in which a field programmable gate array is used for the timing and the recording of mass spectra on an event by event basis. Fragmentation studies of anthracene have shown clear evidence for double ionization above 21 eV electron impact, which provides the motivation to look for coincident fragments. The first results for coincidence mass spectrometry of anthracene will be presented at the conference.

Understanding the formation of metastable furan dication in collisions with ions

SynopsisThis work relies on complementary theoretical and experimental studies of the processes induced by ion-furan collisions. Results of the Molecular Dynamics simulations and exploration of the energy profiles combined with coincidence mass spectrometry provide complete picture of the fragmentation of furan dication.

Quantitative label-free characterization of avidin-biotin assemblies on silanized glass.

In this study, a time-of-flight secondary ion mass spectrometer TOF-SIMS, operating in the event-by-event bombardment/detection mode was used to characterize avidin-biotin assemblies on silane-modified glass substrates. SIMS was used to analyze several variants of the biointerface, including avidin physically adsorbed on a monofunctional acryl silane surface and covalently attached on monofunctional (amine terminated) and bifunctional (amine and acryl terminated) silanes. The goal of these studies was to determine density of avidin and biotin layers chemically or physically adsorbed on silanized glass substrate. An individual impact of a C(60) projectile used in this study creates a hemispherical crater (∼10 nm in diameter) and emits large numbers of secondary ions from the same nanovolume. Thus, a single impact enables one to unfold distinct secondary ions that span the thickness of the assembled film. This method was used to monitor the presence of glass, silane, and protein ions and to estimate the thickness and density of the avidin layer. In addition, we employed the double coincidence mass spectrometry approach to identify ions coemitted from a specific stratum of the biointerface. This approach was used to determine density of biotin and avidin immobilization while eliminating interferences from isobaric ions that originated from other constituents on the surface. Overall, novel TOF-SIMS quantitative approaches employed here were useful for examining complex biointerfaces and determining both lateral and in depth composition of the film.

Competition of electron transfer, dissociation, and bond-forming processes in the reaction of the CO22+ dication with neutral CO2

The bimolecular reactivity of the CO(2)(2+) dication with neutral CO(2) is investigated using triple quadrupole and ion-ion coincidence mass spectrometry. Crucial for product analysis is the use of appropriate isotope labelling in the quadrupole experiments in order to distinguish the different reactive pathways. The main reaction corresponds to single-electron transfer from the neutral reagent to the dication, i.e. CO(2)(2+) + CO(2) --> 2CO(2)(+); this process is exothermic by almost 10 eV, if ground state monocations are formed. Interestingly, the results indicate that the CO(2)(+) ion formed when the dication accepts an electron dissociates far more readily than the CO(2)(+) ion formed from the neutral CO(2) molecule. This differentiation of the two CO(2)(+) products is rationalized by showing that the population of the key dissociative states of the CO(2)(+) monocation will be favoured from the CO(2)(2+) dication rather than from neutral CO(2). In addition, two bond-forming reactions are observed as minor channels, one of which leads to CO(+) and O(2)(+) as ionic products and the other affords a long-lived C(2)O(3)(2+) dication.

Unimolecular dissociation of energy-selected ethyl formate ions

The unimolecular dissociation of ethyl formate ions has been investigated using threshold photoelectron photoion coincidence mass spectrometry. The fragment ions appearing at low internal energies are C 2H 4CO + and C 2H 4 ·+, whose appearance potentials are higher than the ionization energy of the ethyl formate molecule by 0.18 eV and 0.28 eV respectively. Observed decay rates for the formation of C 2H 4CO + were found to be different to those of C 2H 4 ·+, although the direct fragmentation mechanism indicates that they should be identical. Furthermore, the rate constants predicted by RRKM/QET calculations are 10 4–10 5 times higher than the observed rate constants for both ions. The different decay rates probably result from different isomerization processes. One of the reaction channels is a stepwise McLafferty rearrangement involving a distonic intermediate, with charge retention on the olefin.

Fragmentation of acetic acid ions with selected internal energies

The unimolecular dissociation of acetic acid ion in the photon energy range 10.5–17.0 eV was studied using threshold photoelectron photoion coincidence mass spectrometry. The detailed breakdown graph was obtained and the fragmentation pathways were elucidated. The breakdown graph calculated using statistical theories was found to be consistent with the experimental data up to a photon energy of about 12.5 eV. The average kinetic energy release observed is higher than that calculated on the basis of quasi-equilibrium theory for the formation of COOH + while it seems to be statistical for the formation of CH 3CO +. The origin of kinetic energy release accompanying the formation of these two ions is discussed. The structure of [COH 3] + ion ( m/z 31) is determined to be hydroxymethyl cation, CH 2OH +, which could be formed by a two-step rearrangement prior to dissociation.

Unimolecular kinetics of pyridine ion fragmentation

The fragmentation of the pyridine ion to form C 4H 4 + and HCN has been studied by means of photoelectron—photoion coincidence mass spectrometry with variable ion-source residence time. A detailed analysis of the time-dependent breakdown curves leads to a T = 0 K fragmentation threshold of 12.15 ± 0.02 eV, Δ H f 0 0(C 4H 4 +) = 1195 ± 12 kJ mol −1, somewhat higher than the value reported by Eland et al. The fragmentation process requires a tight transition state, in agreement with earlier conclusions of Eland et al. There is some disagreement with earlier work on the energy-dependence of the fragmentation rate. It is suggested that this may be due to distortion of the thermal vibrational population distribution of the ion after vertical ionization.

Photoelectron–photoion coincidence study of the bromobenzene ion

The technique of variable time photoelectron–photoion coincidence mass spectrometry has been applied to the fragmentation of bromobenzene ion producing a phenyl ion. A detailed analysis of the variation of the breakdown curve with parent ion residence time was performed. The results lead to ΔH °f0 (phenyl ion)=270 kcal/mole in close agreement with recalculated results from an earlier study on chlorobenzene. This, combined with other photoionization results leads to ΔH °f0 (phenyl radical)=83±3 kcal/mole, slightly higher than the value 80.9±2 kcal/mole obtained from neutral kinetics. The analysis leads to a rate-energy dependence for the fragmentation process and an equivalent 1000 K Arrhenius pre-exponential factor of ∼9.4×1014 sec−1, which may be compared to the value 2×1015 sec−1 for the analogous neutral process. The possible contribution of spin orbit splitting is discussed.

The fragmentation of C 2F 6+

The fragmentation Of C 2F 6 + has been studied by threshold photoelectron—photoion coincidence mass spectrometry and field-ionization mass spectrometry. Four types of data were obtained with the coincidence apparatus: threshold photoelectron spectra, fragmentation breakdown curves, time-dependent breakdown curves and kinetic energy release on fragmentation. The results indicate that the CF 3 + and C 2F 5 + fragments are formed independently, probably from separate electronic states which do not internally convert to the ground electronic state. The field-ionization data show that these fragments are formed in less than 5 × 10 −13 s. Quasi-equilibrium calculations could not reproduce the experimental results.

The fragmentation of propane and deuteropropane molecular ions

Fragmentation of molecular ions as a function of internal energy has been studied for the propane molecules, C3H8, CH3CD2CH3, CD3CH2CD3, and C3D8 using threshold photoelectron–photoion coincidence mass spectrometry. Within the energy range covered (0–6 eV internal energy) the experimental fragmentation curves are compared with those calculated using the quasiequilibrium theory. The general features of the experimental curves are reproduced by the calculations but significant deviations from exact calculations are apparent. The experimental curves for the d2 and d6 propanes show that hydrogen scrambling processes occur and are of sufficient importance that they must be included in future calculations.

ChemInform Abstract: THRESHOLD PHOTOELECTRON‐PHOTOION COINCIDENCE MASS SPECTROMETRIC STUDY OF ETHYLENE AND ETHYLENE‐D4

Experimental curves have been obtained for the fragmentation of ethylene and ethylene‐d4 ions as a function of the internal energy of those ions using threshold photoelectron–photoion coincidence mass spectrometry. The results are compared with the previous results of photoionization mass spectrometry, Hei photoelectron–photoion coicidence, charge exchange experiments, and with quasiequilibrium theory (QET) calculations. The discrepancies between results of these previous experiments and QET calculations do not appear in the present data. It is suggested that ion–molecule reactions competing with charge exchange has led to erroneous conclusions in the interpretation of the charge exchange data. It is concluded that QET does describe the fragmentation of ethylene and ethylene‐d4 within the limits of the data and calculations available. The secondary ion fragmentation C2H4+ → C2H3++H → C2H2++2H is discussed in detail with regard to the C2H3+ fragment ion internal energy distribution.

Threshold photoelectron–photoion coincidence mass spectrometric study of ethylene and ethylene-d4

Experimental curves have been obtained for the fragmentation of ethylene and ethylene-d4 ions as a function of the internal energy of those ions using threshold photoelectron–photoion coincidence mass spectrometry. The results are compared with the previous results of photoionization mass spectrometry, Hei photoelectron–photoion coicidence, charge exchange experiments, and with quasiequilibrium theory (QET) calculations. The discrepancies between results of these previous experiments and QET calculations do not appear in the present data. It is suggested that ion–molecule reactions competing with charge exchange has led to erroneous conclusions in the interpretation of the charge exchange data. It is concluded that QET does describe the fragmentation of ethylene and ethylene-d4 within the limits of the data and calculations available. The secondary ion fragmentation C2H4+ → C2H3++H → C2H2++2H is discussed in detail with regard to the C2H3+ fragment ion internal energy distribution.

Threshold electron-photoion coincidence mass spectrometric study of CH4, CD4, C2H6, and C2D6

An experiment is reported which directly determines the kinetics of dissociation of a molecular ion as a function of the internal energy of that ion. The experiment involves photoionization, followed by detection of only those mass analyzed ions which are produced in coincidence with photoelectrons of zero initial kinetic energy. Under these conditions the internal energy of a parent ion is equal to the photon energy minus the adiabatic ionization potential of the molecule. With this technique, in contrast to previously used techniques, the kinetics of dissociation can be studied independently of assumptions concerning threshold laws, i.e., variation in cross section above threshold, and autoionization. Fragmentation as a function of internal energy is reported for CH4+, CD4+, C2H6+, and C2D6+. The results are compared with previous photoionization, charge exchange and photoelectron-photoion coincidence mass spectrometry, and with fragmentation calculations based on quasiequilibrium theory of unimolecular dissociation. The major discrepancies of earlier experiments with the predictions of the quasiequilibrium theory are shown to be in error. The new experimental results are in good qualitative agreement with the predictions of the theory. Although this experiment was not designed to determine the average kinetic energy release in unimolecular reactions, it is sensitive to this energy. An analysis of the instrument and the data indicate average fragmentation energies which are of the same order as those obtained by other workers. The implications of these data on the thermochemistry of the molecules studied is discussed.