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.     

Quantification of methane in humid air and exhaled breath using selected ion flow tube mass spectrometry

In selected ion flow tube mass spectrometry, SIFT‐MS, analyses of humid air and breath, it is essential to consider and account for the influence of water vapour in the media, which can be profound for the analysis of some compounds, including H₂CO, H₂S and notably CO₂. To date, the analysis of methane has not been considered, since it is known to be unreactive with H₃O⁺ and NO⁺, the most important precursor ions for SIFT‐MS analyses, and it reacts only slowly with the other available precursor ion, O. However, we have now experimentally investigated methane analysis and report that it can be quantified in both air and exhaled breath by exploiting the slow O/CH₄ reaction that produces CH₃O ions. We show that the ion chemistry is significantly influenced by the presence of water vapour in the sample, which must be quantified if accurate analyses are to be performed. Thus, we have carried out a study of the loss rate of the CH₃O analytical ion as a function of sample humidity and deduced an appropriate kinetics library entry that provides an accurate analysis of methane in air and breath by SIFT‐MS. However, the associated limit of detection is rather high, at 0.2 parts‐per‐million, ppm. We then measured the methane levels, together with acetone levels, in the exhaled breath of 75 volunteers, all within a period of 3 h, which shows the remarkable sample throughput rate possible with SIFT‐MS. The mean methane level in ambient air is seen to be 2 ppm with little spread and that in exhaled breath is 6 ppm, ranging from near‐ambient levels to 30 ppm, with no significant variation with age and gender. Methane can now be included in the wide ranging analyses of exhaled breath that are currently being carried out using SIFT‐MS. Copyright © 2010 John Wiley & Sons, Ltd.     

The quantification of carbon dioxide in humid air and exhaled breath by selected ion flow tube mass spectrometry

The reactions of carbon dioxide, CO₂, with the precursor ions used for selected ion flow tube mass spectrometry, SIFT‐MS, analyses, viz. H₃O⁺, NO⁺ and O, are so slow that the presence of CO₂ in exhaled breath has, until recently, not had to be accounted for in SIFT‐MS analyses of breath. This has, however, to be accounted for in the analysis of acetaldehyde in breath, because an overlap occurs of the monohydrate of protonated acetaldehyde and the weakly bound adduct ion, H₃O⁺CO₂, formed by the slow association reaction of the precursor ion H₃O⁺ with CO₂ molecules. The understanding of the kinetics of formation and the loss rates of the relevant ions gained from experimentation using the new generation of more sensitive SIFT‐MS instruments now allows accurate quantification of CO₂ in breath using the level of the H₃O⁺CO₂ adduct ion. However, this is complicated by the rapid reaction of H₃O⁺CO₂ with water vapour molecules, H₂O, that are in abundance in exhaled breath. Thus, a study has been carried out of the formation of this adduct ion by the slow three‐body association reaction of H₃O⁺ with CO₂ and its rapid loss in the two‐body reaction with H₂O molecules. It is seen that the signal level of the H₃O⁺CO₂ adduct ion is sensitively dependent on the humidity (H₂O concentration) of the sample to be analysed and a functional form of this dependence has been obtained. This has resulted in an appropriate extension of the SIFT‐MS software and kinetics library that allows accurate measurement of CO₂ levels in air samples, ranging from very low percentage levels (0.03% typical of tropospheric air) to the 6% level that is about the upper limit in exhaled breath. Thus, the level of CO₂ can be traced through single time exhalation cycles along with that of water vapour, also close to the 6% level, and of trace gas metabolites that are present at only a few parts‐per‐billion. This has added a further dimension to the analysis of major and trace compounds in breath using SIFT‐MS. Copyright © 2009 John Wiley & Sons, Ltd.     

The analysis of 1-propanol and 2-propanol in humid air samples using selected ion flow tube mass spectrometry

Following the observation that propanol is present in the breath samples of cystic fibrosis (CF) patients infected by Pseudomonas aeruginosa (PA), a study of the reactions of H(3)O(+), NO(+) and O(2) (+.) with 1-propanol and 2-propanol has been conducted using selected ion flow tube mass spectrometry (SIFT-MS). In this study the number and the distribution of the product ions from NO(+) reactions with the two propanol isomers under humid air conditions were able to differentiate between the two isomers. The reaction mechanisms and the structures of the product ions for these reactions, especially those with H(3)O(+) and NO(+), have been proposed. As an example, 2-propanol was shown to be present in a breath sample from one CF patient infected with PA, and also in a PA isolate from another CF patient grown on Pseudomonas-selective media. The results of this study allow an analytical procedure to be advanced for the analysis of the two propanol isomers, which can also be utilised in other applications.     

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.     

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 acetaldehyde released by lung cancer cells in vitro using selected ion flow tube mass spectrometry

The production of volatile compounds from cancer cell lines in vitro has been investigated using selected ion flow tube mass spectrometry (SIFT-MS). This technique enables on-line quantitative analyses of the headspace above cell/medium cultures. This paper reports the discovery that acetaldehyde is released by the lung cancer cell lines SK-MES and CALU-1. The concentration of acetaldehyde in the headspace of the medium/cell culture was measured after 16 h incubation at 37 degrees C and found to be proportional to the number of cancer cells in the medium (typically 10(8)). From these data, the acetaldehyde production rates of the SK-MES cells and the CALU-1 cells in vitro are determined to be 1 x 10(6) and 1.5-3 x 10(6) molecules/cell/min, respectively. The potential value of this new technique in cell biology and in industrial cell biotechnology is discussed.     

Quantification of methylamine in the headspace of ethanol of agricultural origin by selected ion flow tube mass spectrometry

Selected ion flow tube mass spectrometry, SIFT-MS, has been used to investigate if absolute levels of trace compounds in the headspace of ethanol/water vapour mixture can be quantified. This case study was directed towards the analysis of methylamine in distilled ethanol of agricultural origin because of its relevance to quality control legislation in the distillery industry. This has required a detailed study of the ion chemistry occurring – initiated by H₃O⁺ precursor ions – when ethanol/water vapour mixtures are introduced into the H₃O⁺/helium carrier gas swarm and has resulted in the construction of a full scheme of the complex ionic reactions that occur. It has been found that under the SIFT-MS flow reactor conditions (He pressure 130 Pa and temperature 299 K) the terminating ions of the several parallel and sequential reactions that occur are the proton bound ethanol clusters ions, C₂H₅OH₂⁺(C₂H₅OH)_n, with n = 1,2,3, proton bound trimer (n = 2) being the dominant species. These ethanol cluster ions can be used as precursor (reagent) ions for the chemical ionisation of the methylamine present in the ethanol/water vapour, which produces two characteristic product ions CH₃NH₂H⁺(C₂H₅OH)_1,2 that are used for the methylamine analysis. The ratio of the product ion count rate to the precursor ion count rate is used in an analogous way to the routinely used for SIFT-MS analyses to quantify the methylamine concentration. The results of calibration experiments show that using SIFT-MS it is possible to quantify methylamine in liquid ethanol/water mixtures at levels of 0.1 mg/L or greater.     

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.     

Quantification of character-impacting compounds in Ocimum basilicum and 'Pesto alla Genovese' with selected ion flow tube mass spectrometry

Basil (Ocimum basilicum) is an important flavourant plant which constitutes the major ingredient of the pasta sauce 'Pesto alla Genovese'. The characteristic smell of basil stems mainly from a handful of terpenoids (methyl cinnamate, eucalyptol, linalool and estragole), the concentration of which varies according to basil cultivars. The simple and rapid analysis of the terpenoid constituents of basil would be useful as a means to optimise harvesting times and to act as a quality control process for basil-containing foodstuffs. Classical analytical techniques such as gas chromatography/mass spectrometry (GC/MS) are, however, slow, technically demanding and therefore less suitable for routine analysis. A new chemical ionisation technique which allows real-time quantification of traces gases, Selected Ion Flow Tube Mass Spectrometry (SIFT-MS), was therefore utilised to determine its usefulness for the assay of terpenoid concentrations in basil and pesto sauce headspace. Trace gas analysis was performed using the NO(+) precursor ion which minimised interference from other compounds. Character-impacting compound concentration was measured in basil headspace with good reproducibility and statistically significant differences were observed between cultivars. Quantification of linalool in pesto sauce headspace proved more difficult due to the presence of interfering compounds. This was resolved by careful selection of reaction product ions which allowed us to detect differences between various commercial brands of pesto. We conclude that SIFT-MS may be a valid tool for the fast and reproducible analysis of flavourant terpenoids in basil and basil-derived foodstuffs.     

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.     

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.     

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.     

On-line measurement of the absolute humidity of air, breath and liquid headspace samples by selected ion flow tube mass spectrometry

We describe how selected ion flow tube mass spectrometry (SIFT-MS) can be used to determine the absolute humidity of air, breath and liquid headspace samples. This involves the determination of the relative count rates of the H3O+ ions and those H3O+.(H2O)(1,2,3) hydrate ions that inevitably form in the helium carrier gas when humid samples are being analysed by SIFT-MS using H3O+ precursor ions. This requires an understanding of the kinetics of hydrated hydronium ion formation, the involvement of mass discrimination in the analytical quadrupole mass spectrometer and the decreased diffusive loss of the heavier hydrates along the flow tube. Thus, we show that the humidity of breath and liquid headspace samples, typically at the few percent level, can be directly obtained on-line to the SIFT-MS instrument along with the concentrations of trace gases, which are present at much lower levels. We emphasise the value of parallel humidity measurements in ensuring good real-time sampling of breath and liquid headspace and the value of such measurements to trace gas analysis using SIFT-MS.     

The use of selected ion flow tube mass spectrometry to detect and quantify polyamines in headspace gas and oral air

Polyamines are a class of aliphatic compounds which include putrescine, cadaverine, spermine and spermidine. They are involved in a variety of cellular processes and have been implicated in a number of different pathophysiological mechanisms. Polyamines are volatile compounds having a distinctive odour normally perceived as being unpleasant. The measurement of their abundance has, however, been restricted to compounds present in the aqueous phase. Using selected ion flow tube mass spectrometry (SIFT‐MS) we have shown that the polyamines react with the ions H₃O⁺, NO⁺ and O to form distinctive product ions allowing their levels to be quantified in the vapour phase. The low volatility of spermine did not allow extensive analysis of this compound by SIFT‐MS while the adherent properties of cadaverine and putrescine required the use of PTFE transfer lines and couplers. Our data suggested the presence of cadaverine and putrescine in both oral air and the headspace of putrefying bovine muscle, while product ions corresponding to putrescine and spermidine were found in the headspace of human semen. SIFT‐MS therefore appears to be a practical means of measuring vapour‐phase polyamine levels, having applications in biology, medicine and dentistry, and food science. Copyright © 2009 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.