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

Abstract

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.

From molecules in space to molecules in breath

“…Some of the trace compounds detected and quantified in breath by SIFT-MS are indicated in Figure 1. The association of particular trace metabolites with disease states are also indicated together with the concentration reference ranges for ostensibly healthy individuals resulting from longitudinal inter-individual and intra-individual studies [4,5]. Careful scrutiny of Figure 1 reveals that the trace biomarkers are present in concentrations ranging from sub-ppbv (parts-per-billion by volume) to a few ppmv (parts-per-million by volume), i.e.…”

“…Breath is exhaled at the sample entry port of the SIFT-MS; on inhalation, the ambient air is sampled and analysed, as is important in breath analysis research. Details of the SIFT-MS method, the physics and ion chemistry involved, a line diagram showing the structure of the instrumentation and its application to breath analysis, have been given in recent authoritative reviews [4,5].…”

Selected Ion Flow-Drift Tube Mass Spectrometry: Quantification of Volatile Compounds in Air and Breath

“…25 The concentration of M in the sample being analysed can then be determined from the known sample gas dilution in the helium carrier/buffer gas, as described for SIFT-MS in previous papers. 1,17,25 Uncertainty of the quantification can be estimated as ±20% by error analysis from the combined uncertainties of the gas flow rates and all parameters in equation (7). Validation of SIFDT-MS analyses for various analyte neutral compounds will require much experimentation along the same lines as carried out to establish SIFT-MS.…”

“…[3][4][5] In this regard, gas chromatography with mass spectrometry, GC-MS, has made a major contribution to gas analysis, but it is not a real time technique. Whilst nanomaterial-based sensors, 6 laser spectroscopic techniques 7 and ion mobility spectrometry, IMS, 8 are partly a solution for real time analysis, there is a need for further development of techniques based on chemical ionization during defined reaction times, exemplified by selected ion flow tube mass spectrometry, SIFT-MS, 1 and proton transfer reaction mass spectrometry, PTR-MS. 2,9,10 Both of these techniques are widely used for real time trace gas analysis in various areas and are able to accurately quantify trace compounds and biomarkers in air and exhaled breath. Yet there are clear intrinsic differences between these techniques which reflect their relative strengths and weaknesses as analytical tools that need to be appreciated in further development of analytical methods based on gas phase ion molecule reactions.…”

“…Some areas in which such analyses are important have been reviewed recently. 1,2 These gaseous media are usually humid, especially so in the case of exhaled breath, and so ideally the analytical method must allow quantification of the VOCs without de-humidification that can compromise the sample. Furthermore, size, weight and cost of instrumentation are important issues and versatility is very desirable for research applications when a great variety of VOCs need to be detected and analysed.…”