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.     

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.     

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.     

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%.     

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.     

Quantification of volatile compounds in the headspace of aqueous liquids using selected ion flow tube mass spectrometry

We describe a method by which the concentrations of volatile compounds in the headspace of their dilute aqueous solutions in sealed containers can be determined using on-line selected ion flow tube mass spectrometry (SIFT-MS). Thus, the changing number density of the molecules of the volatile compound in the carrier gas of the SIFT-MS instrument is described in terms of its changing flow rate as the pressure in the sealed container decreases during the sampling procedure. It is shown that the best analytical procedure is to determine the mean concentration of the trace gas in the liquid headspace over a given sampling time and relate this to the required concentration, which is the initial equilibrium concentration established before the pressure in the sealed container reduces significantly. To test the validity of this analytical approach, the headspace concentrations of acetaldehyde, ethanol and acetone above aqueous solutions of known concentrations have been determined. Hence, the Henry's Law constants for these compounds have been determined and found to agree with the published values. The confirmation of the quality of this sampling methodology combined with SIFT-MS for the analysis of volatile compounds in liquid headspace paves the way for the rapid analyses of biological liquids such as urine and serum for clinical diagnosis and physiological monitoring.     

Major volatile compounds in head-space above olive oil analysed by selected ion flow tube mass spectrometry

Selected ion flow tube mass spectrometry (SIFT-MS) is a technique that is well suited to the real-time analysis of head-space. SIFT-MS gives a non-discriminatory snapshot of the volatiles present and their amounts, and is considered to display less bias than chromatographic techniques as neither sample pre-treatment nor separation are necessary in most cases. The technique has been used for analysis of virgin olive oil head-space on more than 100 different oils. Twenty of these are reported. The results obtained using this technique differ from those normally reported from chromatographic analyses in that the dominant species in the head-space of all oils tested were methanol and ethanol. These volatiles were present in the head-space in the concentration ranges of 2.8-11.3 ppm (methanol) and 0.4-4.9 ppm (ethanol). (E)-2-Hexenal, normally reported as the dominant olive oil volatile, is found in significantly lower concentrations and is in the range of 0.02-1.6 ppm.     

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.     

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.     

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.     

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.     

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.     

An exploratory comparative study of volatile compounds in exhaled breath and emitted by skin using selected ion flow tube mass spectrometry

Selected ion flow tube mass spectrometry (SIFT-MS) has been used to carry out a pilot parallel study on five volunteers to determine changes occurring in several trace compounds present in exhaled breath and emitted from skin into a collection bag surrounding part of the arm, before and after ingesting 75 g of glucose in the fasting state. SIFT-MS enabled real-time quantification of ammonia, methanol, ethanol, propanol, formaldehyde, acetaldehyde, isoprene and acetone. Following glucose ingestion, blood glucose and trace compound levels were measured every 30 min for 2 h. All the above compounds, except formaldehyde, were detected at the expected levels in exhaled breath of all volunteers; all the above compounds, except isoprene, were detected in the collection bag. Ammonia, methanol and ethanol were present at lower levels in the bag than in the breath. The aldehydes were present at higher levels in the bag than in breath. The blood glucose increased to a peak about 1 h post-ingestion, but this change was not obviously correlated with temporal changes in any of the compounds in breath or emitted by skin, except for acetone. The decrease in breath acetone was closely mirrored by skin-emitted acetone in three volunteers. Breath and skin acetone also clearly change with blood glucose and further work may ultimately enable inferences to be drawn of the blood glucose concentration from skin or breath measurements in type 1 diabetes.     

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.