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.     

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.     

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%.     

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.     

Gas-phase reactions and rearrangements of alkyl esters with H3O+, NO+, and O2*+: a selected ion flow tube study

Selected ion flow tube mass spectrometry (SIFT-MS) has been employed to study the ion-molecule reactions of 17 alkyl esters reacting with the common SIFT-MS reagent ions, H3O+, H3O+.nH2O (n = 1, 2, 3), NO+, and O2+. The majority of reactions were observed to proceed at or near collision rate, with the exception of H3O+.3H2O, which was found to be slow for 8 of 17 alkyl esters. Unexpected product ions in the form of the parent carboxylic acid cation were observed to arise from the H3O+ and NO+ reactions of some alkyl esters. The observed reactions have been probed by the ab initio CBS-4M and G2(MP2,SVP) methods. The postulated reaction pathway involves a 1,5 H atom migration from a beta-carbon onto the carbonyl oxygen.     

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.     

Analysis of ketones by selected ion flow tube mass spectrometry

A selected ion flow tube mass spectrometry (SIFT-MS) study of the reactions of H3O+, NO+ and O2+* ions with the ketones (M) 2-heptanone, 2-octanone, 2-nonanone, 2-undecanone and 2-aminoacetophenone has been conducted in preparation for studies of volatile emissions from bacteria. The H3O+ reactions all proceed rapidly via exothermic proton transfer, producing only MH+ ions that form their monohydrates when water vapour is present in the helium carrier gas. The O2+* reactions proceed rapidly via dissociative charge transfer producing parent cations M+* and some fragment ions. The NO+ reactions form the NO+M adduct ions at rates which are dependent on the pressure of the helium carrier gas. Combining the present NO+ kinetic data with those available from previous SIFT studies, the phenomenon of charge transfer complexing is clearly demonstrated. This results in adduct formation in these NO+/ketone reactions at or near the collisional rate. SIFT-MS spectra are presented to illustrate the simplicity of SIFT-MS analysis of ketones using both H3O+ and NO+ precursor ions.     

Selected ion flow tube mass spectrometry of exhaled breath condensate headspace

Collection of exhaled breath condensate (EBC) is a relatively simple noninvasive method of breath analysis; however, no data have been reported that would relate concentration of volatile compounds in EBC to their gaseous concentrations in exhaled air. The aim of the study was to investigate which volatile compounds are present in EBC and how their concentrations relate to results of direct breath analysis. Thus, samples of EBC were collected in a standard way from several subjects and absolute levels of several common volatile breath metabolites (ammonia, acetone, ethanol, methanol, propanol, isoprene, hydrogen cyanide, formaldehyde and acetaldehyde) were then determined in their headspace using selected ion flow tube mass spectrometry (SIFT-MS). Results are compared with those from on-line breath analyses carried out immediately before collecting the EBC samples. It has been demonstrated that SIFT-MS can be used to quantify the concentrations of volatiles in EBC samples and that, for methanol, ammonia, ethanol and acetone, the EBC concentrations correlate with the direct breath levels. However, the EBC concentrations of isoprene, formaldehyde, acetaldehyde, hydrogen cyanide and propanol do not correlate with direct breath measurements. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Analysis of petrol and diesel vapour and vehicle engine exhaust gases using selected ion flow tube mass spectrometry

We have used selected ion flow tube mass spectrometry (SIFT-MS) to analyse the vapours emitted by petrol and diesel fuels and the exhaust gases from petrol (spark ignition) and diesel (compression ignition) engine vehicles fitted with catalytic converters. Only those components of these media that have significant vapour pressures at ambient temperatures were analysed and thus particulates were obviously not detected. These media have been analysed using the full scope of SIFT-MS, i.e., with the three available precursor ions H3O+, NO+ and O2+. The combination of the H3O+ and NO+ analyses is seen to be essential to distinguish between different product ions at the same mass-to-charge ratio (m/z) especially in identifying aldehydes in the exhaust gases. The O2+ precursor ions are used to detect and quantify the large amount of nitric oxide present in the exhaust gases from both engine types. The petrol and diesel vapours consist almost exclusively of aliphatic alkanes, alkenes and alkynes (and dienes) and aromatic hydrocarbons. Some of these compounds appear in the exhaust gases together with several aldehydes, viz. formaldehyde, acetaldehyde, pentanal, pentenal (acrolein), butenal, and also methanol and ethanol. Acetone, nitric oxide and ammonia are also present, acetone and nitric oxide being much more abundant in the diesel exhaust gas than in the petrol exhaust gas. These data were obtained from samples collected into pre-evacuated stainless steel vessels. Trapping of the volatile compounds from the gas samples is not required and analysis was completed a few minutes later. All the above compounds are detected simultaneously, which demonstrates the value of SIFT-MS in this area of research.     

Monitoring chloramines and bromamines in a humid environment using selected ion flow tube mass spectrometry

The selectivity and sensitivity of selected ion flow tube mass spectrometry (SIFT‐MS) for individual breath analysis of haloamines has been improved by heating the flow tube in a commercial instrument to around 106°C. Data is presented showing the marked reduction in the number density of water clusters of product ions of common breath metabolites that are isobaric with the product ions from monochloramine and monobromamine that are used to monitor the haloamine concentrations. These results have direct relevance to the real‐time monitoring of chloramines in drinking water, swimming pools and food processing plants. However, once the isobaric overlaps from water cluster ions are reduced at the higher temperatures, there is no conclusive evidence showing the presence of haloamines on single breath exhalations in the mid parts per trillion range from examination of the breaths of volunteers. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of water vapour on selected ion flow tube mass spectrometric analyses of trace gases in humid air and breath

Selected ion flow tube mass spectrometry (SIFT-MS) detects and quantifies in real time the trace gases, M, in air/breath samples introduced directly into a flow tube. Inevitably, relatively large partial pressures of water vapour are introduced with the sample and the water molecules become involved in the ion chemistry on which this analytical technique depends. When H(3)O(+) ions are used as the precursors for chemical ionisation and SIFT mass spectrometric analyses of M, they generally result in the formation of MH(+) ions. Also, when water vapour is present the H(3)O(+) ions are partially converted to hydrated hydronium ions, H(3)O(+).(H(2)O)(1,2,3). The latter may act as precursor ions and produce new product ions like MH(+).(H(2)O)(1,2,3) via ligand switching and association reactions. This ion chemistry and the product ions that result from it must be accounted for in accurate analyses by SIFT-MS. In this paper we describe the results of a detailed SIFT study of the reactions involved in the quantification of acetone, ethyl acetate, diethyl ether, methanol, ethanol, ammonia and methyl cyanide by SIFT-MS in the presence of water vapour. This study was undertaken to provide the essential data that allows more accurate analyses of moist air and breath by SIFT-MS to be achieved. It is shown using our standard analysis procedure that the error of SIFT-MS quantification caused by the presence of water vapour is typically 15%. An improved analysis procedure is then presented that is shown to reduce this error to typically 2%. Additionally, some fundamental data have been obtained on the association reactions of protonated organic molecules, MH(+) ions, with water molecules forming MH(+).H(2)O monohydrate ions. For some types of M, reaction sequences occur that lead to the formation of dihydrate and trihydrate ions.     

Quantification of hydrogen cyanide in humid air by selected ion flow tube mass spectrometry

Following our recent observation that Pseudomonas bacteria in vitro emit hydrogen cyanide, we have found it necessary to investigate the ion chemistry of this compound and to extend the kinetics database for selected ion flow tube mass spectrometry (SIFT-MS) to allow the accurate quantification of HCN in moist air samples, including exhaled breath. Because of the proximity of the proton affinities of HCN and H2O molecules, the presence of water vapour can significantly distort HCN analysis in the presence of water vapour and a more sophisticated analytical procedure has to be developed. Thus, the reactions of H3O+(H2O)0,1,2,3 ions with HCN molecules have been studied in the presence of varying concentrations of water vapour, reactions on which SIFT-MS analysis of HCN relies. The results of these experiments have allowed an analytical procedure to be developed which has extended the kinetics database of SIFT-MS, such that HCN can now be quantified in humid air and in exhaled breath.     

Quantification of hydrogen sulphide in humid air by selected ion flow tube mass spectrometry

We report the results of a study of the reactions of H(3)O(+), NO(+) and O(2)(+.) ions with H(2)S. This study was undertaken to provide a thorough understanding of the ion chemistry required for accurate quantification of H(2)S in humid air by selected ion flow tube mass spectrometry (SIFT-MS). It shows that slow reactions occur between H(3)S(+), the primary product ions of the H(3)O(+)/H(2)S reaction, and the abundant H(2)O molecules present in humid air and breath. These reactions disturb somewhat the quantification of H(2)S by this analytical method, but the kinetic data obtained in this study facilitate precise quantification of H(2)S in humid air. This study also shows that NO(+) does not react with H(2)S, and that O(2)(+.) does react rapidly with H(2)S, but the product H(2)S(+.) ions react rapidly with H(2)O. Thus, NO(+) and O(2)(+.) cannot be used as precursor ion for analysis of H(2)S in moist air by SIFT-MS. A sample SIFT mass spectrum is shown from which H(2)S and several other volatile compounds have been quantified in a sample of cow rumen gas.     

Repeatability of the measurement of exhaled volatile metabolites using selected ion flow tube mass spectrometry

Selected ion flow tube mass spectrometry, SIFT-MS, has been used to determine the repeatability of the analysis of volatile metabolites within the breath of healthy volunteers, with emphasis on the influence of sampling methodology. Baseline instrument specific coefficients of variability for examined metabolites were as follows: acetone (1%), ammonia (1%), isoprene (2%), propanol (6%), ethanol (7%), acetic acid (7%), and hydrogen cyanide (19%). Metabolite concentration and related product ion count rate were identified as strong determinants of measurement variation. With the exception of ammonia, an orally released metabolite, variability in repeated on-line breath analysis tended to be lower for metabolites of systemic origin. Standardization of sampling technique improved the repeatability of the analysis of selected metabolites. Off-line (bag) alveolar breath sampling, as opposed to mixed (whole) breath sampling, likewise improved the repeatability of the analysis of all metabolites investigated, with the exception of acetic acid. We conclude that SIFT-MS analysis of common volatile metabolites within the breath of healthy volunteers is both reliable and repeatable. For selected metabolites, the finding that repeatability is improved through modification of sampling methodology may have implications in terms of future recommended practices.     

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.     

On-line analysis of diesel engine exhaust gases by selected ion flow tube mass spectrometry

Selected ion flow tube mass spectrometry (SIFT-MS) has been used to analyse on-line and in real time the exhaust gas emissions from a Caterpillar 3304 diesel engine under different conditions of load (idle and 50% of rated load) and speed (910, 1500 and 2200 rpm) using three types of fuel: an ultra-low-sulphur diesel, a rapeseed methyl ester and gas oil. SIFT-MS analyses of the alkanes, alkenes and aromatic hydrocarbons in the headspace of these fuels were also performed, but the headspace of the rapeseed methyl ester consists mainly of methanol and a compound with the molecular formula C4H8O. The exhaust gases were analysed for NO and NO2 using O2+* reagent ions and for HNO2 using H3O+ reagent ions. The following aldehydes and ketones in the exhaust gases were quantified by using the combination of H3O+ and NO+ reagent ions: formaldehyde, acetaldehyde, propenal, propanal, acetone, butanal, pentanal, butanone and pentanone. Formaldehyde, acetaldehyde and pentenal, all known respiratory irritants associated with sensitisation to asthma of workers exposed to diesel exhaust, are variously present within the range 100-2000 ppb. Hydrocarbons in the exhaust gases accessible to SIFT-MS analyses were also quantified as total concentrations of the various isomers of C3H4, C3H6, C4H6, C5H8, C5H10, C6H8, C6H10, C7H14, C6H6, C7H8, C8H10 and C9H12.     

Quantitative selected ion flow tube mass spectrometry: The influence of ionic diffusion and mass discrimination

Selected ion flow tube mass spectrometry, (SIFT-MS), involves the partial conversion of mass-selected precursor ions to product ions in their reactions with the trace gases in an air sample that is introduced into helium carrier gas in a flow tube. The precursor and product ions are then detected and counted by a downstream quadrupole mass spectrometer. Quantification of particular trace gases is thus achieved from the ratio of the total count rate of the product ions to that for the precursor ions. However, it is important to appreciate that in this ion chemistry the light precursor ions (usually H3O+ ions) are invariably converted to heavier product ions. Hence, the product ions diffuse to the flow tube walls more slowly and thus they are more efficiently transported to the downstream mass spectrometer sampling orifice. This phenomenon we refer to as diffusion enhancement. Further, it is a well-known fact that discrimination can occur against ions of large mass-to-charge ratio, (m/z), in quadrupole mass spectrometers. If not accounted for, diffusion enhancement usually results in erroneously high trace gas concentrations and mass discrimination results in erroneously low concentrations. In this experimental investigation, we show how both these counteracting effects can be accounted for to increase the accuracy of SIFT-MS quantification. This is achieved by relating the currents of ions of various m/z that arrive at the downstream mass spectrometer sampling orifice disc to their count rates at the ion detector after mass analysis. Thus, both diffusion enhancement and mass discrimination are parameterized as a function of m/z and these are combined to provide an overall discrimination factor for the particular analytical instrument.