Rod S. Mason

Glow discharge mass spectrometry of some organic compounds

Abstract A simple glow discharge ion source is described which can be adapted to fit any magnetic deflection mass spectrometer, but which is here used with an updated MS9 (AEI) instrument. It is shown to operate with great sensitivity towards both metal analysis, and inorganic solutions. Accurate mass measurement by “high resolution” peak matching was used to identify trace elements when there was a high density of isobaric ion peaks, such as occurs in an unrefined matrix of oil. An investigation was made into the suitability of the technique for the analysis of difficult organic samples deposited directly onto the cathode probe. Spectra were obtained which are very similar to rapid desorption chemical ionisation spectra, with ample molecular and structurally significant fragment ion peaks. Evidence is presented to show that although sputtering of the organic sample does occur, the principal desorption process for relatively large (⩾ 1 μg) sample sizes is caused by rapid sputter heating of the probe tip and subsequent evaporation. The effect of adding the protonating gases methane and ammonia to the discharge gas is reported.

The addition of H2 to an Ar plasma studied by fast flow glow discharge mass spectrometry (FFGD-MS): mechanism and relative sensitivities

Changes in steel and copper cathode ion intensities from the afterglow of a fast flow dc glow discharge (FFGD) have been studied as a function of the amount of H2 added downstream from the active discharge region. The signals from the majority of different elements increased, although some analytes were actually quenched. In general, the data were qualitatively similar to previous ‘hydrogen effect’ studies using conventional ‘static gas’ GD source designs. Here variations in the size of the effect are reported as hydrogen concentration, plasma density, interaction time and ion extraction voltage were changed independently. It was not possible to rationalise the data obtained using a mechanism based on ion–molecule reactions. The results also indicated that Penning ionisation involving the Ar 3P2,0 states is probably not the primary mechanism for the ions detected in this kind of flowing plasma. On the other hand, a mechanism based on the Rydberg gas model of the flowing plasma [R. S. Mason, P. D. Miller, I. P. Mortimer, D. J. Mitchell and N. A. Dash, Phys. Rev. E, 2003, 68, 016408] fits well with all the changes observed.

Thermodynamics of some proton-transfer reactions. Dynamic ion structures and the measurement of entropies of ‘internal translation’

Entropies well in excess of expected values have been measured for some protonated aromatic compounds (benzene, halogenobenzenes, halogeno-toluenes and xylenes). It is proposed that this occurs only when very facile proton migration is possible and that the large excess arises out of rapid ‘internal translation’ of the proton across a broad potential well within the molecule. An attempt has been made to model these systems, using a ‘particle-in-a-box’ approach to estimate the molecular partition functions. It was possible to generate sections of the potential-energy surfaces involved by using ab initio calculations at the 4–31G basis set level. The ideas presented are consistent with independent experimental evidence not based on thermodynamic measurements. In particular, the measurements and calculations involving protonated benzene in the gas phase can be compared with a low-temperature study in the liquid phase, using n.m.r. spectroscopy. Making some assumptions with regard to the total excess entropy available in the dynamic protonated species, it is possible to estimate barrier heights for proton migration. The general implications are that such barriers are probably somewhat lower than those currently predicted by the best ab initio methods and that the relative proton affinities measured are not ground-state values and may therefore appear to be significantly lower than expected. Although the idea of ‘internal translation’ has been used in the past in the context of the transition-state theory, the concept of a stable, but dynamic ion structure, as suggested here, is new.

Kinetics, equilibria and diffusion of ions produced in N2, CO and CO2, studied as a function of temperature using a high-pressure pulsed mass spectrometer source

Gas phase ion–molecule association reactions of the type A++ A [graphic omitted] A˙ A+ have been studied as a function of temperature in a pulsed electron beam high-pressure mass spectrometer source. Unlike previous studies of this type, the effect of ionic diffusion on the measured pseudo-first-order ion decay is measured and accounted for when estimating kf. Experiments were performed in the ranges 67–399 N m–2 and 365–530 K. At 300 K values of kf were found to be 4.52 × 10–29 and 1.33 × 10–28 molecules–2 cm6 s–1 for N+˙4 formation in N2 and (CO)+˙2 formation in CO, respectively. When expressed in the form kf=CTm, the values of m were found to be –3.8 ± 0.3 and –3.3 ± 0.2, a factor of two larger than the most recent literature values in each case.The equilibrium CO+˙2+ 2CO2rlarr;(CO2)+˙2+ CO2 was also observed for which the values of ΔH=–66.1 ± 2.9 kJ mol–1 and ΔS=–95.4 ± 4.6 J K–1 mol–1 were found, in good agreement with other literature values, leading to ΔHf[(CO2)+˙2]= 475.3 ± 2.5 kJ mol–1.The diffusion coefficients of N+˙2 in N2 and CO+˙ in CO were found to be almost identical, as were those of N+˙4 and (CO)+˙2 over the range of temperatures studied. The fact that the dimer ions have larger coefficients than the monomer is consistent with the role played by resonance charge transfer in the transport of ions. Temperature dependences are, on the whole, lower than expected from theory.

Experimental and theoretical proton affinity of limonene

Gas-phase basicity (GB) and proton affinity (PA) of limonene were derived from measurements of proton transfer equilibria carried out by high pressure pulsed electron beam source mass spectrometry. Experimental GB and PA are 842±5 kJ mol-1 and 875±5 kJ mol-1, respectively. The proton affinity of C10H16 is compared to abinitio (HF and MP2) and density functional predictions for the protonation energy. Theoretical calculations based on density functional theory are in very good agreement with experimental results. Our best theoretical predictions for the enthalpy of protonation range from 869.6 kJ mol-1 (B3PW91/6-31G*) to 873.9 kJ mol-1 (BLYP/6-31G*). Theoretical calculations also suggest an important reorganisation of the molecular structure and charge distribution of limonene after protonation.

Thermodynamic studies of gas-phase proton transfer equilibria involving benzene: a reassessment of earlier data

Temperature-variable equilibrium constant measurements have been performed for a number of proton-transfer equilibria in which benzene was a partner, using a newly built high-pressure pulsed source mass spectrometer. Entropy values obtained showed that the protons in protonated benzene are not as mobile as previously thought. Systems involving ethanol are found to give anomalous, though self-consistent results owing to the onset of thermal decomposition. In the light of this, previous data involving halotoluenes and xylenes which appear to show unusually large increases in entropy on protonation, have been reassessed. There is evidence of proton-induced isomerisation. In the reaction [4-CIC6H4CH3]H+→[3-CIC6H4CH3]H+ the free energy of activation is derived to be ca. 90 kJ mol–1 from a computer model fit to the results, consistent with the energy calculated to be needed for a proton shift from the 3 to the 4 position in the precursor. The equivalent reaction for protonated 4-fluorotoluene has a barrier which is ca. 10 kJ mol–1 higher. A kinetic scheme is presented which shows how this could account for the observed ‘thermodynamic’ behaviour, and also give rise to the ‘isokinetic effects’ previously noted. There has therefore been some readjustment of the recommended proton affinity values for some of these compounds.

Observation and lifetime of autoionising states of Ar produced in a glow discharge ion source

Abstract In experiments to measure the bombardment energies of cathodic Ar ions in a glow discharge mass spectrometer ion source, ions were observed, dependent on the experimental conditions, which had been created by autoionisation of neutral metastable Ar in the accelerating region of the mass spectrometer. These excited Ar atoms are carried into the vacuum by the discharge gas escaping through the ion exit aperture. An expression is derived to describe the expansion of the gas into the vacuum and hence the relative decrease in gas density as the gas envelope moves away from the ion exit, giving the relative pressure, at a distance x along the direction of the ion beam, Px/P0 = S2/(S2 + x2), where S is the radius of the ion exit orifice. This is very similar to the form of the measured autoionisation profile. Two classes of metastable atom can be distinguished with lifetimes of about 1 μs and > 10 μs respectively. This is consistent with previous electron spectroscopy measurements [P. Marchand and J. Cardinal, Can. J. Phys., 57 (1979) 1624] and it may explain anomalous results reported for the energy distribution of cathodic Ar+ in earlier glow discharge studies [W.D. Davies and T.A. Vanderslice, Phys. Rev., 131 (1963) 219].

Gas chromatography combined with fast flow glow discharge mass spectrometry (GC-FFGD-MS)

A low power (<5 W) dc fast flow glow discharge (FFGD) ion source has been adapted for the detection of halogenated hydrocarbons using gas chromatography as the sample introduction technique. A simple and robust interface design enabled direct coupling of the ion source to a common gas chromatograph. After optimisation, detection limits for four compounds were from 0.3–66.5 pg (by monitoring the halogen ions 35Cl, 79Br and 127I). The source operating conditions can be ‘tuned’ to obtain different information, allowing both sensitive elemental detection and structure elucidation using a single instrument, with sub-pg detection limits for the bromobenzene molecular ion. The source pressure, power and discharge gas flow rate were found to have significant effects on the relative fragment ions detected. These observations can be rationalised using neutral, excited state chemistry.

CHARACTERIZATION STUDY OF RUTHENIUM DIOXIDE HYDRATE BEFORE AND AFTER THERMAL-ACTIVATION

Abstract Samples of ruthenium dioxide hydrate, before and after thermal activation, have been studied by the technique of direct mass spectrometric thermal analysis. Comparison is made with differential thermal gravimetric analysis and differential scanning calorimetry data and in combination the results of the three different analytical techniques clearly identify two types of chemically bound water of which the loss of one is associated with the process of thermal activation. In a second part of the study the headspace gases evolved during the reactions of both forms of this material with cerium(IV) ions, in a solution which was enriched in 18O, have been monitored by mass spectrometry. The results for the corrodible, non activated form of ruthenium dioxide hydrate show that two of the oxygen atoms in RuO4 come from the solvent and indicate that any oxygen evolved may also come from the solvent and not from the ruthenium dioxide hydrate as was believed initially. It is suggested that the unactivated sample is a partially hydroxylated ruthenium oxide polymer and that heat treatment causes “condensation” first at the surface and then within the bulk phase. It is further suggested that the hydroxylated surfaces of this unactivated form are unstable under strong oxidizing conditions because of the lower ruthenium atom binding energies compared with the rutile-related structure formed after condensation. Tentative reaction schemes are described which are consistent with the observations. The results for the non-corrodible, activated form of ruthenium dioxide hydrate confirmed that it was able to mediate the stoichiometric oxidation of water to oxygen by cerium(IV) ions, without undergoing corrosion.

Gas-phase proton transfer in halogenotoluene mixtures. Evidence for a ‘dynamic’ ion structure

Gas-phase proton transfer equilibria between fluoro- and chloro-toluene mixtures have been studied as a function of temperature to determine relative proton affinities and entropy changes. Some of the entropy changes are much greater than any values previously reported for proton-transfer reactions. It is suggested that this can only be explained if the ortho- and para-halogenotoluene isomers have a ‘dynamic’ protonated ion structure in which a mobile hydrogen atom is shared by two or more unsubstituted ring carbon atoms. There is also an apparent linear correlation between the proton affinities measured and the corresponding entropy of proton bonding of these species. This ‘isokinetic’ relationship has a slope of 589 ± 50 K and is consistent with the suggestion in these compounds that the internal translational movement of the proton within the plane of the aromatic ring decreases the more tightly bound it becomes. This appears to be the first time such a relationship has been observed for gas-phase proton transfer. The following proton affinities relative to Ap(p-chlorotoluene) were measured: p-chlorotoluene, 0; p-fluorotoluene, 1.2 ± 0.2; o-chlorotoluene, 16 ± 0.8; o-fluorotoluene, 28 ± 1.5; m-chlorotoluene, 33.9 ± 1.4; m-fluorotoluene, 36.9 ± 1.4 kJ mol–1.