The reactions of H3O+, NO+ and O with several flavourant esters studied using selected ion flow tube mass spectrometry

The reactions of H₃O⁺, NO⁺, and O with nineteen ester compounds occurring naturally in plants, and having important flavourant properties, were examined using selected ion flow tube mass spectrometry (SIFT‐MS). The H₃O⁺ reactions primarily generate [R₁COOR₂·H]⁺, and may also produce [R₂]⁺ fragment ions and/or fragmentation within the ester linkage. Collisional association/adduct ions, [R₁COOR₂·NO]⁺, are the main products formed in the NO⁺ reactions, although the carboxyl fragment ion is also detected frequently. The identification of the parent compound may be made more easily in the H₃O⁺ and NO⁺ reactions. The inclusion of O reactions in the analysis provides additional information, which may be applied when the identity of a parent compound cannot be determined solely from the H₃O⁺ and NO⁺ analysis. Consideration of the product ions generated with the three precursors suggests that SIFT‐MS can differentiate between many of the esters investigated, including isomers, although the product ions generated in the reactions with some esters are too similar to allow independent quantification. Our data therefore suggest that SIFT‐MS may be a useful tool to rapidly analyse and quantify flavourant esters in complex gas mixtures. Copyright © 2010 John Wiley & Sons, Ltd.     

A selected ion flow tube study of the reactions of H3O+, NO+, and O2+ with saturated and unsaturated aldehydes and subsequent hydration of the product ions

We have carried out a selected ion flow tube (SIFT) study of the reactions of H₃O⁺, NO⁺, and O₂⁺ ions with several saturated and unsaturated aldehydes. This study is mainly directed toward providing the essential data for a projected SIFT mass spectrometry (SIFTMS) study of the volatile emissions from cooked meats, which always include aldehydes. Thus, it is necessary to know the rate coefficients and the product ions of the reactions of the above-mentioned ions, used as the precursor ions for SIFTMS analyses, with the aldehydes, if proper identification and quantification of the emitted species are to be achieved. The results of this study show that the reactions of H₃O⁺ with the aldehydes, M, result in the protonated molecules MH⁺ and for the saturated aldehydes also in (M - OH)⁺ ions resulting from the loss of a H₂O molecule from the nascent MH⁺ ion. The NO⁺ reactions invariably proceed via the process of hydride ion, H⁻, transfer producing (M - H)⁺ ions, but parallel minor association product ions NO⁺ · M are observed for some of the unsaturated aldehyde reactions. The O₂⁺ reactions proceed by way of charge transfer producing nascent M⁺ ions that partially dissociate producing fragment ions. Because water vapour is invariably present in real samples analysed by SIFTMS, the current experiments were also carried out following the introduction of humid laboratory air into the helium carrier gas of the SIFT. Thus, the reactions of the product ions that form hydrates were also studied as a prelude to future SIFTMS studies of the (humid) emissions from cooked meats.     

Selected ion flow tube studies of the reactions of H3O+, NO+ and O2+ with the anaesthetic gases halothane,isoflurane and sevoflurane

We have carried out a study of the reactions of H(3)O(+), NO(+) and O(2) (+), the commonly used precursor ions for selected ion flow tube mass spectrometry (SIFT-MS), with three anaesthetic gases, halothane, isoflurane and sevoflurane. The motivation for this study was to provide the necessary kinetic data that would allow the quantification of these anaesthetic gases in operating theatre air and in the breath of theatre staff and post-operative patients. A clear negative result from these experiments is that NO(+), although undergoing the simplest chemistry, is unsuitable for this SIFT-MS application. However, although the ion chemistry of H(3)O(+) and O(2) (+) with these compounds is very complex, there being several product ions in each reaction, many of which react rapidly with water molecules, monitor ions have been identified for all three anaesthetic gases when using H(3)O(+) and O(2) (+) as precursor ions. The detailed ion chemistry is discussed and the specific monitor ions are indicated. Hence, the feasibility of on-line breath monitoring is demonstrated by simple examples. These studies have opened the way to measurements in the clinical environment.     

An investigation of the reactions of H3O+ and O2+ with NO, NO2, N2O and HNO2 in support of selected ion flow tube mass spectrometry

A selected ion flow tube (SIFT) experimental investigation has been carried out of the reactions of H3O+, NO+ and O2+ with NO, NO2, N2O and HNO2, in order to obtain the essential kinetic data for the analyses of these compounds in air using selected ion flow tube mass spectrometry (SIFT-MS). These investigations show that NO+ ions do not react at a significant rate with any of these NOx compounds and that H3O+ ions react only with HNO2 (product ions H2NO2+ (75%) and NO+ (25%)). O2+ ions react with NO (product ion NO+), NO2 (product ion NO2+) and HNO2 (product ions NO+ (75%), NO2+ (25%)), but not with N2O. We conclude that both NO and NO2 can be accurately quantified in air using only O2+ precursor ions and SIFT-MS when HNO2 is not present. However, when HNO2 is present it invariably co-exists with both NO and NO2 and then both H3O+ and O2+ precursor ions are needed to determine the partial pressures of NO, NO2 and HNO2 in the air mixture. We also conclude that currently N2O cannot be analysed in air using SIFT-MS.     

SIFT studies of the reactions of H3O+, NO+ and O+2 with a series of volatile carboxylic acids and esters

We report the results of a selected ion flow tube (SIFT) study of the reactions of H₃O⁺, NO⁺ and O⁺₂ with some nine carboxylic acids and eight esters. We assume that all the exothermic proton transfer reactions of H₃O⁺ with all the acid and esters molecules occur at the collisional rate, i.e. the rate coefficients, k, are equal to k_c; then it is seen that k values for most of the NO⁺ and O⁺₂ reactions also are equal to or close to k_c. The major ionic products of the H₃O⁺ reactions with both the acids and esters are the protonated parent molecules, MH⁺, but minor channels are also evident, these being the result of H₂O elimination from the excited (MH⁺)1 in some of the acid reactions and an alcohol molecule elimination (CH₃OH or C₂H₅OH) in some of the ester reactions. The NO⁺ reactions with the acids and esters result in both ion-molecule association producing NO⁺M in parallel with hydroxide ion (OH⁻) transfer with some of the acids, and parallel methoxide ion (CH₃O⁻) and ethoxide ion (C₂H₅O⁻) transfer as appropriate with some of the esters. The O⁺₂ reactions proceed by dissociative charge transfer with the production of two or more ionic fragments of the parent molecules, the different isomeric forms of both the acid and the ester molecules resulting in different product ions.     

Kinetics for the reactions of O- and O2- with O2(a1Deltag) measured in a selected ion flow tube at 300 K

The kinetics of the reactions of O- and O2- with O2(a1Deltag) have been studied at 300 K in a selected ion flow tube (SIFT). The O2(a1Deltag) concentrations have been determined using emission at 1270 nm from the O2(a1Deltag, v=0-->X3Sigmag-, v=0) transition measured with an InGaAs detector calibrated against absolute spectrally dispersed emission measurements. The rate constants measured for O- and O2- are 1.1x10(-10) and 6.6x10(-10) cm3 s-1, respectively, with uncertainties of +/-35%. The O2- reaction only produces electrons and can be described as Penning detachment, while the O- reaction has been found to produce both O2- and e-. The O2- branching fraction has a lower limit of approximately 0.30. Comparison of the present results to previous measurements found in the literature provides a resolution to a previous discrepancy in the rate constant values.     

Selected ion flow tube mass spectrometry analyses of stable isotopes in water: Isotopic composition of H3O+ and H3O+(H2O)3 ions in exchange reactions with water vapor

A new method has been developed for the determination of the isotope abundance ratios of deuterium, D, and oxygen-18, 18O, in water vapor (and water) using selected ion flow tube mass spectrometry (SIFT-MS). H3O+ ions are injected into the helium carrier gas where they associate with the H2O and HDO molecules in a sample of water introduced into the carrier gas. The D and 18O contents of the product cluster ions H8DO4+ and H9(18)OO3+ at m/e = 74 and 75, respectively, are determined by reference to the majority cluster ion H9O4+ at m/e = 73. Allowance is made for the contribution of the H8(17)OO3+ ions to the m/z = 74 ions. Absolute isotopic ratios are measured within seconds without the need for precalibration of the SIFT-MS instrument, currently to an accuracy of better than 2%.     

Ion-molecule reactions of several ions with ethylene oxide and propenal in a selected ion flow tube

The selected ion flow tube (SIFT) technique has been used to investigate the ion-molecule reactions of several ions with the neutral molecules ethylene oxide, CH(2)OCH(2)-c, and propenal, CH(2)CHCHO. Both molecules have been identified in hot-core star forming regions [] and have significance to astrochemical models of the interstellar (ISM) and circumstellar medium (CSM). Moreover, the molecules contain functional groups, such as the epoxide group (ethylene oxide) and an aldehyde group, which are part of a conjugated pi-electron system (propenal) whose reactivities have not been studied in detail in gas-phase ion-molecule reactions. The larger recombination energy ions, Ar(+) and N(2)(+), were reacted with the neutrals to give insight into general fragmentation tendencies. These reactions proceeded via dissociative charge-transfer yielding major fragmentation products of CH(3)(+) and HCO(+) for ethylene oxide and CH(2)CH(+) and HCO(+) for propenal. The amino acids glycine and alanine are of particular interest to astrobiology, especially if they can be synthesized in the gas phase. In an attempt to synthesize amino acid precursors, ethylene oxide and propenal were reacted with NH(n)(+) (n = 1-4) and HCNH(+). As might be expected from the proton detachment energies, NH(+), NH(2)(+), and HCNH(+) reacted via proton transfer. NH(3)(+) reacted with each molecule via H-atom abstraction to produce NH(4)(+), and NH(4)(+) reacted via a ternary association. All binary reactions proceeded near the gas kinetic rate. Several associated molecule switching reactions were performed and implications of these reactions to the structures of the association products are discussed Ikeda et al. and Hollis et al.     

A shock tube study of the reactions of NH with NO,O2, and O

The reactions of NH(X³Σ⁻) with NO, O₂, and O have been studied in reflected and incident shock wave experiments. The source of NH in all the experiments was the thermal dissociation of isocyanic acid, HNCO. Time-histories of the NH(X³Σ) and OH(X²Π) radicals were measured behind the shock waves using cw, narrow-linewidth laser absorption at 336 nm and 307 nm, respectively. The second-order rate coefficients of the reactions: were determined to be: and cm³ mol⁻¹ s⁻¹, where ƒ and F define the lower and upper uncertainty limits, respectively. The branching fraction of channel defined as k_3b/k_3total, was determined to be 0.19 ± 0.10 over the temperature range of 2940 K to 3040 K.     

Gas-phase reactions of azulene with OH and NO3 radicals and O3 at 298 ± 2 K

Azulene, which is isomeric with naphthalene, was studied to determine whether it behaves like a polycyclic aromatic hydrocarbon or an alkene in its gas-phase reactions with OH and NO₃ radicals and O₃. Using relative rate methods, rate constants for the reactions of azulene with OH and NO₃ radicals and O₃ of (2.73 ± 0.56) × 10^?10 cm³ molecule^?1 s^?1, (3.9) × 10^?10 cm³ molecule^?1 s^?1, and <7 × 10^?17 cm³ molecule^?1 s^?1, respectively, were obtained at 298 ± 2 K. The observation that the NO₃ radical reaction did not involve NO₂ in the rate determining step indicates that azulene behaves more like an alkene than a polycyclic aromatic hydrocarbon in this reaction. No conclusive evidence for the formation of nitroazulene(s) from either the OH or NO₃ radical-initiated reaction of azulene (in the presence of NO_x) was obtained.     

Alcohol in breath and blood: a selected ion flow tube mass spectrometric study

In this paper we compare the amounts of ethanol in breath and in blood after ingestion of whisky using analysis by selected ion flow tube mass spectrometry (SIFT-MS). Blood ethanol concentrations were also obtained using standard hospital forensic procedures for blood alcohol analyses. We demonstrate the quantitative nature of SIFT-MS analysis by correlating the observed alcohol content of the headspace above 5-mL amounts of venous blood and aqueous solution to which known trace amounts of alcohol have been added. This procedure provides a Henry's Law coefficient for ethanol in aqueous solution at 298 +/- 3 K of 209 +/- 7 mol/kg*bar. We also demonstrate that measurement of the ethanol concentration in the alveolar portion of a single breath using the SIFT-MS technique gives an accurate measure of blood alcohol and could obviate the need for blood samples in forensic processing. The storage performance of breath samples in Mylar bags with a volume greater than 1 L has been shown to maintain the mixture integrity for ethanol but not for some other species.     

Kinetic study of the gas-phase reactions of OH and NO3 radicals and O3 with selected vinyl ethers

Kinetic studies on the gas-phase reactions of OH and NO3 radicals and ozone with ethyl vinyl ether (EVE), propyl vinyl ether (PVE) and butyl vinyl ether (BVE) have been performed in a 405 L borosilicate glass chamber at 298 +/- 3 K in synthetic air using in situ FTIR spectroscopy to monitor the reactants. Using a relative kinetic method rate coefficients (in units of cm3 molecule(-1) s(-1)) of (7.79 +/- 1.71) x 10(-11), (9.73 +/- 1.94) x 10(-11) and (1.13 +/- 0.31) x 10(-10) have been obtained for the reaction of OH with EVE, PVE and BVE, respectively, (1.40 +/- 0.35) x 10(-12), (1.85 +/- 0.53) x 10(-12) and (2.10 +/- 0.54) x 10(-12) for the reaction of NO3 with EVE, PVE and BVE, respectively, and (2.06 +/- 0.42) x 10(-16), (2.34 +/- 0.48) x 10(-16) and (2.59 +/- 0.52) x 10(-16) for the ozonolysis of EVE, PVE and BVE, respectively. Tropospheric lifetimes of EVE, PVE and BVE with respect to the reactions with reactive tropospheric species (OH, NO3 and O3) have been estimated for typical OH and NO3 radical and ozone concentrations.     

Temperature-dependent kinetics studies of the reactions of C2H5O2 and n-C3H7O2 radicals with NO

The rate coefficients for the gas-phase reactions of C₂H₅O₂ and n-C₃H₇O₂ radicals with NO have been measured over the temperature range of (201–403) K using chemical ionization mass spectrometric detection of the peroxy radical. The alkyl peroxy radicals were generated by reacting alkyl radicals with O₂, where the alkyl radicals were produced through the pyrolysis of a larger alkyl nitrite. In some cases C₂H₅ radicals were generated through the dissociation of iodoethane in a low-power radio frequency discharge. The discharge source was also tested for the i-C₃H₇O₂ + NO reaction, yielding k_{298 K} = (9.1 ± 1.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, in excellent agreement with our previous determination. The temperature dependent rate coefficients were found to be k(T) = (2.6 ± 0.4) × 10⁻¹² exp{(380 ± 70)/T} cm³ molecule⁻¹ s⁻¹ and k(T) = (2.9 ± 0.5) × 10⁻¹² exp{(350 ± 60)/T} cm³ molecule⁻¹ s⁻¹ for the reactions of C₂H₅O₂ and n-C₃H₇O₂ radicals with NO, respectively. The rate coefficients at 298 K derived from these Arrhenius expressions are k = (9.3 ± 1.6) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ for C₂H₅O₂ radicals and k = (9.4 ± 1.6) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ for n-C₃H₇O₂ radicals. © 1996 John Wiley & Sons, Inc.     

Gas-phase reaction of NO3 radicals with isoprene: a kinetic and mechanistic study

The gas-phase reaction of NO₃ radicals with isoprene was investigated under flow conditions at 298 K in the pressure range 6.8<P(mbar)<100 using GC-MS/FID, direct MS, and long-path FT–IR spectroscopy as detection techniques. By means of a relative rate method, the rate constant for the primary attack of NO₃ radicals toward isoprene was determined to be (6.86±2.60)×10⁻¹³ cm³ molecule⁻¹ s⁻¹. The formation of the possible oxiranes, 2-methyl-2-vinyl-oxirane and 2-(1-methyl-vinyl)-oxirane, was observed in dependence on total pressure. In the presence of O₂ in the carrier gas, the product distribution was found to be strongly dependent on the reaction pathways of formed peroxy radicals. If the peroxy radicals mainly reacted in a self-reaction, the formation of organic nitrates was detected and 4-nitroxy-3-methyl-but-2-enal was identified as a main product. On the other hand, when NO was added to the gas mixture and the peroxy radicals were converted via RO₂+NO→RO+NO₂, the formation of methyl vinyl ketone as the main product as well as 3-methylfuran and meth-acrolein was observed. From the ratio of the product yields if NO was added to the gas mixture it was concluded that the attack of NO₃ radicals predominantly takes place in the 1-position. A reaction mechanism is proposed and the application of these results to the troposphere are discussed. © 1997 John Wiley & Sons, Inc.     

Gas-phase reactivity of the O=P(OCH3)2 + phosphonium ion towards alpha,beta-unsaturated esters in a quadrupole ion trap

Ion-molecule reactions between the O=P(OCH3)2 + phosphonium ions and eight alpha,beta-unsaturated esters (methyl acrylate, ethyl acrylate, methyl crotonate, ethyl crotonate, methyl 3,3-dimethylacrylate, ethyl 3,3-dimethylacrylate, methyl methacrylate and ethyl methacrylate) were performed in a quadrupole ion trap mass spectrometer. The O=P(OCH3)2 + phosphonium ions, formed by electron ionization from neutral trimethyl phosphite, were found to react with alpha,beta-unsaturated esters to give an adduct [RR'C=CR'COOR', O=P(OCH3)2]+, which lose spontaneously a molecule of trimethyl phosphate (R'=CH3) or dimethyl ethyl phosphate (R'=C2H5). An ion corresponding to a protonated trialkyl phosphate is also observed when substituent R'=H. To confirm the experimental results, and to elucidate the mechanism for the formation of the ionic species, a theoretical study using the density functional theory (DFT) approach was carried out. The potential energy surface obtained from B3LYP/6-31G(d,p) calculations for the reaction between O=P(OCH3)2 + and methyl acrylate is described.     

A photoelectron photoion coincidence study of the vinyl bromide and tribromoethane ion dissociation dynamics: heats of formation of C2H3+, C2H3Br, C2H3Br+, C2H3Br2+, and C2H3Br3

The threshold photoelectron photoion coincidence (TPEPICO) technique has been used to measure accurate dissociative photoionization onsets of vinyl bromide and 1,1,2-tribromoethane. The reactions investigated and their 0 K onsets are C2H3Br + hnu --> C2H3+ + Br (11.902 +/- 0.008 eV); C2H3Br3 + hnu --> C2H3Br2+ + Br (10.608 +/- 0.008 eV); and (C2H3Br3 + hnu --> C2H3Br+ + 2Br (12.301 +/- 0.035 eV). The vinyl ion heat of formation (Delta(f)H degrees 298K = 1116.1 +/- 3.0 kJ/mol) has been calculated using W1 theory and used as an anchor along with the measured dissociation energies to determine the heats of formation, Delta(f)H degrees 298K, in kJ/mol, of the following bromine-containing species: C2H3Br (74.1 +/- 3.1), C2H3Br+ (1021.9 +/- 3.1), C2H3Br2+ (967.1 +/- 4.0), and C2H3Br3 (53.5 +/- 4.3). These results represent accurate and consistent experimental determinations of heats of formation for these bromine-containing species, which serve to correct the discrepancies in the literature for C2H3Br and C2H3Br+ and provide the first experimental determination for the enthalpies of formation of C2H3Br2+ and C2H3Br3.