Studies of interstellar ion reactions using the SIFT technique: isotope fractionation

Impact of oral cleansing strategies on exhaled volatile organic compound levels

RATIONALE

The analysis of volatile organic compounds (VOCs) within exhaled breath potentially offers a non-invasive method for the detection and surveillance of human disease. Oral contamination of exhaled breath may influence the detection of systemic VOCs relevant to human disease. This study aims to assess the impact of oral cleansing strategies on exhaled VOC levels in order to standardise practice for breath sampling.

METHODS

Ten healthy volunteers consumed a nutrient challenge followed by four oral cleansing methods: (a) water, (b) saltwater, (c) toothbrushing, and (d) alcohol-free mouthwash. Direct breath sampling was performed using selected ion flow tube mass spectrometry after each intervention.

RESULTS

Proposed reactions suggest that volatile fatty acid and alcohol levels (butanoic, pentanoic acid, ethanol) declined with oral cleansing interventions, predominantly after an initial oral rinse with water. Concentrations of aldehydes and phenols (acetaldehyde, menthone, p-cresol) declined with oral water rinse; however, they increased after toothbrushing and mouthwash use, secondary to flavoured ingredients within these products. No significant reductions were observed with sulphur compounds.

CONCLUSIONS

Findings suggest that oral rinsing with water prior to breath sampling may reduce oral contamination of VOC levels, and further interventions for oral decontamination with flavoured products may compromise results. This intervention may serve as a simple and inexpensive method of standardisation within breath research.

SIFT-MS and FA-MS methods for ambient gas phase analysis: developments and applications in the UK

The origins of SIFT created to study interstellar chemistry and SIFT-MS developed for ambient gas and exhaled breath analysis and the UK centres in which these techniques are being exploited.

Ambient analysis of trace compounds in gaseous media by SIFT-MS

The topic of ambient gas analysis has been rapidly developed in the last few years with the evolution of the exciting new techniques such as DESI, DART and EESI. The essential feature of all is that analysis of trace gases can be accomplished either in the gas phase or those released from surfaces, crucially avoiding sample collection or modification. In this regard, selected ion flow tube mass spectrometry, SIFT-MS, also performs ambient analyses both accurately and rapidly. In this focused review we describe the underlying ion chemistry underpinning SIFT-MS through a discourse on the reactions of different classes of organic and inorganic molecules with H(3)O(+), NO(+) and O(2)(+)˙ studied using the SIFT technique. Rate coefficients and ion products of these reactions facilitate absolute SIFT-MS analyses and can also be useful for the interpretation of data obtained by the other ambient analysis methods mentioned above. The essential physics and flow dynamics of SIFT-MS are described that, together with the reaction kinetics, allow SIFT-MS to perform absolute ambient analyses of trace compounds in humid atmospheric air, exhaled breath and the headspace of aqueous liquids. Several areas of research that, through pilot experiments, are seen to benefit from ambient gas analysis using SIFT-MS are briefly reviewed. Special attention is given to exhaled breath and urine headspace analysis directed towards clinical diagnosis and therapeutic monitoring, and some other areas researched using SIFT-MS are summarised. Finally, extensions to current areas of application and indications of other directions in which SIFT-MS can be exploited for ambient analysis are alluded to.

Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis

Selected ion flow tube mass spectrometry (SIFT-MS) is a new analytical technique for the real-time quantification of several trace gases simultaneously in air and breath. It relies on chemical ionization of the trace gas molecules in air/breath samples introduced into helium carrier gas using H(3)O(+), NO(+), and O(2) (+.) precursor ions. Reactions between the precursor ions and trace gas molecules proceed for an accurately defined time, the precursor and product ions being detected and counted by a downstream mass spectrometer, thus effecting quantification. Absolute concentrations of trace gases in single breath exhalation can be determined by SIFT-MS down to ppb levels, obviating sample collection and calibration. Illustrative examples of SIFT-MS studies include (i) analysis of gases from combustion engines, animals and their waste, and food; (ii) breath and urinary headspace studies of metabolites, ethanol metabolism, elevated acetone during ovulation, and exogenous compounds; and (iii) urinary infection and the presence of tumors, the influence of dialysis on breath ammonia, acetone, and isoprene, and acetaldehyde released by cancer cells in vitro. Flowing afterglow mass spectrometry (FA-MS) is briefly described, which allows on-line quantification of deuterium in breath water vapor.

Recombination of D3+ ions in the afterglow of a He-Ar- D2 plasma

We report measurements of the rate coefficient alpha for recombination of D3+ with electrons in a He-Ar-D2 plasma. The observed rate coefficient is dependent on the deuterium number density indicating that third-body assisted recombination is efficient and significantly contributes to recombination. When the deuterium number density is decreased to 7x10(10) cm(-3), the rate coefficient also decreases to alpha approximately 6x10(-9) cm3 x s(-1). These data indicate that the binary dissociative recombination of D3+ is very slow with alphaDR<6x10(-9) cm3 x s(-1). The observation of a deionization process proceeding via formation of D5+ is also reported.

Selected ion flow tube: a technique for quantitative trace gas analysis of air and breath

The selected ion flow tube (SIFT) technique for trace gas analysis of air and breath is based on soft chemical ionisation of the trace gases to the exclusion of the major air and breath gases, in fast-flowing inert carrier gas, exploiting the ion-molecule reactions that occur between the trace gases and the pre selected precursor ions (H3O+, NO+ and O2+). The physics and ion chemistry involved in the SIFT technique are described, as are the kinetics of the ion-molecule reactions that are exploited to quantitatively analyse the trace gases. Fast on-line data-acquisition hardware and software have been developed to analyse the mass spectra obtained, from which partial pressures of the trace gases down to about 10 parts per billion can be measured. The time response of the instrument is 20 ms, allowing the profiles of the trace gas concentrations on breath to be obtained during a normal breathing cycle. Pilot results obtained with this SIFT technique include detection and quantification of the most abundant breath trace gases, analysis of cigarette smoke, detection of gases present on smokers' breath and accurate measurement of the partial pressures of NH3, NO and NO2 in air. The simultaneous analysis of several breath trace gases during a single exhalation is clearly demonstrated, and thus different elution times for isoprene and methanol along the respiratory tract are observed. This technique has great potential in many clinical and biological disciplines, and in health and safety monitoring.