Chris A. Mayhew

Endogenous volatile organic compounds in breath and blood of healthy volunteers: examining breath analysis as a surrogate for blood measurements.

To investigate the premise that levels of endogenous volatile organic compounds (VOC) in breath reflect those in blood, the concentration of acetone and isoprene were measured in radial arterial blood, peripheral venous blood and breath samples from ten healthy volunteers. Coefficients of repeatability as a percentage of mean are less than 30% in breath but greater than 70% in blood. The volunteer-mean ratios of arterial to venous blood concentration are 1.4 (0.9-2.1) for acetone and 0.55 (0.3-1.0) for isoprene. Concentration in breath showed a significant inter-subject correlation with concentration in arterial blood (CAB) for acetone but not for isoprene. Arterial blood/breath ratios are 580 (280-1060) for acetone and 0.47 (0.22-0.77) for isoprene. The sample-mean blood/breath ratio was used to calculate an estimate of CAB and the standard deviation of this estimate was lower than that of arterial blood measured directly. For most subjects, estimated CAB was within uncertainty limits of the actual CAB. Owing to the poor repeatability of VOC concentrations from consecutive blood samples, and the capacitive effects of the lung, this study suggests that breath VOC measurements may provide a more consistent measure than blood measurements for investigating underlying physiological function or pathology within individuals.

Proton Transfer Reaction Mass Spectrometry: Principles and Applications

Description: Proton transfer reaction mass spectrometry (PTR–MS) is a specialist technique for detecting and identifying very small quantities amounts of organic gases in air. It is used mostly in atmospheric research, but also has applications in environmental science, pollution science, food science, and medical diagnosis. It is used as a monitoring system in various industries, e.g. measuring volatile organic compounds (VOCs) emitted by waste incineration plants. This book aims to provide a comprehensive account of the technique, and includes coverage of the basic principles, experimental techniques, and a detailed account of its various applications. Written by two leading researchers, the book will be essential reading for researchers, technicians, postgraduate students and professionals in industry.

A preliminary comparison of volatile organic compounds in the headspace of cultures of Staphylococcus aureus grown in nutrient, dextrose and brain heart bovine broths measured using a proton transfer reaction mass spectrometer

The effect of different growth media on the types and intensities of volatile organic compounds (VOCs) emitted by Staphylococcus aureus has been investigated using a proton transfer reaction mass spectrometer (PTR-MS). The microbial culture was grown in three different broths (nutrient, dextrose and brain heart bovine). Sterile sampling flasks contained either actively growing S. aureus or uninoculated broth. The headspace VOCs in these flasks were sequentially sampled and analysed. From the mass spectra obtained we selected the product ions (resulting from emissions from the inoculated samples) to monitor changes in their intensities as a function of time. Whilst the VOCs emitted by S. aureus in each medium were observed to be the same, the time-dependent pattern of growth showed marked differences and intensities between the different growth media. We conclude that PTR-MS provides sufficient sensitivity for in vivo microbial diagnosis: this could lead to more rapid treatment of bloodstream infections, particularly crucial for the well-being of immunocompromised and intensive care patients.

Proton transfer reaction mass spectrometry and the unambiguous real-time detection of 2,4,6 trinitrotoluene.

Fears of terrorist attacks have led to the development of various technologies for the real-time detection of explosives, but all suffer from potential ambiguities in the assignment of threat agents. Using proton transfer reaction mass spectrometry (PTR-MS), an unusual bias dependence in the detection sensitivity of 2,4,6 trinitrotoluene (TNT) on the reduced electric field (E/N) has been observed. For protonated TNT, rather than decreasing signal intensity with increasing E/N, which is the more usual sensitivity pattern observed in PTR-MS studies, an anomalous behavior is first observed, whereby the signal intensity initially rises with increasing E/N. We relate this to unexpected ion-molecule chemistry based upon comparisons of measurements taken with related nitroaromatic compounds (1,3,5 trinitrobenzene, 1,3 dinitrobenzene, and 2,4 dinitrotoluene) and electronic structure calculations. This dependence provides an easily measurable signature that can be used to provide a rapid highly selective analytical procedure to minimize false positives for the detection of TNT. This has major implications for Homeland Security and, in addition, has the potential of making instrumentation cost-effective for use in security areas. This study shows that an understanding of fundamental ion-molecule chemistry occurring in low-pressure drift tubes is needed to exploit selectivity and sensitivity for analytical purposes.

Real‐time trace detection and identification of chemical warfare agent simulants using recent advances in proton transfer reaction time‐of‐flight mass spectrometry

This work demonstrates for the first time the potential of using recent developments in proton transfer reaction mass spectrometry for the rapid detection and identification of chemical warfare agents (CWAs) in real-time. A high-resolution (m/Deltam up to 8000) and high-sensitivity (approximately 50 cps/ppbv) proton transfer reaction time-of-flight mass spectrometer (PTR-TOF 8000 from Ionicon Analytik GmBH) has been successfully used to detect a number of CWA simulants at room temperature; namely dimethyl methylphosphonate, diethyl methylphosphonate, diisopropyl methylphosphonate, dipropylene glycol monomethyl ether and 2-chloroethyl ethyl sulfide. Importantly, we demonstrate in this paper the potential to identify CWAs with a high level of confidence in complex chemical environments, where multiple threat agents and interferents could also be present in trace amounts, thereby reducing the risk of false positives. Instantaneous detection and identification of trace quantities of chemical threats using proton transfer reaction mass spectrometry could form the basis for a timely warning system capability with greater precision and accuracy than is currently provided by existing analytical technologies.

A study of low energy electron attachment to trifluoromethyl sulphur pentafluoride, SF5CF3: atmospheric implications

An investigation of electron attachment to the potent greenhouse gas SF5CF3 has been carried out in atmospheric pressure nitrogen and argon buffer gases at 300 K. The experiments were conducted under nonthermal electron-swarm conditions, using an instrument that combines a drift tube with a quadrupole mass spectrometer. Electron attachment rate constants, ka, have been determined as a function of mean electron energy (e = 0.04–1.9 eV). ka decreases as e increases. The estimated thermal electron attachment rate constant is kth(SF5CF3) ≈ (7.7 ± 0.6) × 10−8 cm3 molecule−1 s−1. The only observed anion product is SF5−. Free electron attachment destroys SF5CF3. This places an upper limit on the atmospheric lifetime of SF5CF3 of the order of 1000 years.

Gas phase ionic reactions of freons and related compounds: reactions of some halogenated methanes with O− and O2−

Abstract Rate coefficients and branching ratios have been measured for the reactions of O − and O 2 − with CCl 4 , CCl 3 F, CCl 2 F 2 , CClF 3 , CF 4 , CHCl 3 , CH 2 Cl 2 , and CH 3 Cl using a selected ioin flow tube at 300 K and 0.6 Torr. Measured rate coefficients for all reactions (except for O − 2 /CClF 3 and O − 2 and O − /CF 4 which do not react) are at or close to the collisional rate. Where electron transfer is exoergic it is observed for the chlorofluoromethanes but not for the chloromethanes. It is suggested that nucleophilic attack of O − and O − 2 on both chlorine and carbon (in the case of the chlorofluoromethanes) and on chlorine, carbon and hydrogen (in the case of the chloromethanes) are required to produce the observed products. It is also suggested that, with the exception of methyl chloride, the halide ion, X − (X = Cl or F), is not formed directly, but by the decomposition of ClX − which is formed with excess vibrational energy. Following a discussion of the relevance of our results to the effects of oxygen doping upon the response of an electron capture detector (ECD) to halocarbon, it is concluded that the currently accepted mechanism to explain the observed increased sensitivity of the ECD is partially in error.

Charge transfer from neutral perfluorocarbons to various cations : long-range versus short-range reaction mechanisms

The bimolecular reactions of the high recombination energy cations Ar+, F+, and Ne+ with four fully saturated (CF4, C2F6, C3F8, and n-C4F10) and three unsaturated (C2F4, C3F6, and 2-C4F8) perfluorocarbons (PFCs) are reported. The cation branching ratios obtained from these reactions, and from the reactions with O2+, H2O+, N2O+, O+, CO2+, CO+, N+, and N2+ [reported by us, Jarvis et al., J. Phys. Chem. 100 (1996) 17166], are compared with those determined from the threshold photoelectron–photoion coincidence spectra of the PFCs at the recombination energies of the reagent cations. This comparison provides information that helps to interpret the dynamics of charge transfer, and whether it occurs via a long-range or a short-range mechanism. Energy resonance and good Franck-Condon factors connecting the ground electronic state of a reactant neutral molecule to one of its ionic states, at the recombination energy of the reagent cation, are generally considered to be sufficient criteria for long-range charge transfer to occur. However, the results from this study imply that good Franck-Condon factors are not critical in determining the efficiency of a long-range charge transfer. Instead, the results suggest that, in addition to the requirement for energy resonance, the electron taking part in the charge-transfer process must be removed from a molecular orbital which is unshielded from the approaching reagent cation. This enables the cation to exert an influence on the electron at large impact parameters.

A study of the reactions of trifluoromethyl sulfur pentafluoride, SF5CF3, with several positive ions of atmospheric interest

An investigation of the bimolecular reactions of several positive ions of atmospheric interest, H3O+, NO+, NO2+, O2+, H2O+, N2O+, O+, CO2+, CO+, N+ and N2+, with the greenhouse gas SF5CF3 is reported. The thermal rate coefficients and ion product distributions have been determined at 300 K using a selected ion flow tube. H3O+, NO2+ and NO+ are found to be unreactive. The reaction with O2+ proceeds with a rate coefficient significantly less than the capture value ia chemical routes, in which bonds are broken and formed. The other reagent ions, H2O+, N2O+, O+, CO2+, CO+, N+ and N2+ react with SF5CF3 with reaction rate coefficients close to or at the capture values. With the exception of the reaction with H2O+, all these efficient reactions occur by dissociative charge transfer, with CF3+ and SF3+ being the dominant product ions. CF3+ forms by direct cleavage of the S–C bond in SF5CF3+, and SF3+ probably from dissociation of (SF4+)* following intramolecular rearrangement within the lifetime of (SF5CF3+)*. For H2O+, the observed ion branching ratios suggest that the reaction proceeds ia a chemical pathway. The reactions of SF5CF3 with cations will destroy this molecule in the upper atmosphere.

Investigations of low energy electron attachment to ground state group 6B hexafluorides (SF6, SeF6, and TeF6) using an electron-swarm mass spectrometric technique

Abstract Studies of low energy electron attachment to SF6, SeF6, and TeF6 have been carried out in an atmospheric pressure nitrogen buffer gas (number density N) at 300 K. The experiments are conducted under nonthermal electron-swarm conditions, using an instrument that combines an atmospheric pressure drift tube, with a quadrupole mass spectrometer. Details of the design, construction and operation of the drift tube and the associated fast electron gate are presented. Electron drift times can be measured, and mean electron drift velocities in N2 as a function of the density reduced electric field strength E/N are reported. Density normalised electron attachment coefficients, α, and electron attachment rate constants, ka, together with product anion branching ratios (for SeF6 and TeF6) are determined as a function of E/N. The studies presented here cover the range E/N = (0.4–17) × 10−18 V cm2, corresponding to mean electron energies of 0.04–0.6 eV. For all three molecules, ka decreases as E/N increases. SF6 attaches electrons much more rapidly than either SeF6 or TeF6. The ratios ka(SF6):ka(SeF6):ka(TeF6) ≈ 3000:10:1 are found not to vary with E/N. The estimated thermal (300 K) electron attachment rate constants are kth(SF6) ≈ (2.5 ± 0.3) × 10−7 cm3 s−1, kth(SeF6) ≈ (8.0 ± 1.2) × 10−10 cm3 s−1, and kth(TeF6) ≈ (8.2 ± 1.1) × 10−11 cm3 s−1. For all three molecules, attachment is dominated by the capture of near-zero-energy electrons. In each case the dominant anion product is XF6− (X = S, Se, Te), accompanied by XF5−. No other anion products directly arising from electron attachment to XF6 are observed. Extrapolation of the relative product anion intensities to zero attaching gas concentration yields the following branching ratios for attachment under swarm conditions: SeF6–SeF5− (20%), SeF6− (80%); and TeF6–TeF5− (3%), TeF6− (97%). These ratios are found to be independent of E/N. The observation of SeF6− and TeF6− as the dominant anions from SeF6 and TeF6 is ascribed to stabilisation of the initial anion formed by electron capture through collisions with the nitrogen buffer gas. For SF6, the observed proportion of SF5− decreases from 8% to 1% over the E/N range of this study, whereas an increase in the SF5− branching ratio with E/N is anticipated from previous low-pressure, electron beam investigations.