Abstract
The basic physics and ion chemistry involved in the application of selected ion flow tube mass spectrometry, SIFT-MS, to accurate quantification of concentrations of trace gases in air and breath is now well understood. Algorithms, documented in previously published literature and based on a detailed understanding of the reactions of reagent ions with analyte molecules in the presence of larger concentrations of water vapour, carbon dioxide, nitrogen and oxygen molecules, are implemented in the instrument software that allow absolute real time analyses at the parts-per-billion by volume, ppbv, level, analyses made more robust by accounting for differential diffusion of the reagent and product ions along the reactor flow tube and mass discrimination in the analytical quadrupole mass spectrometer. In the present article, other important details of this analytical method are discussed for the first time, including the choice of an optimum sample flow rate and the acquisition times of reagent and product ion counts. The method of calculating absolute concentrations is re-iterated, and the sample kinetics library entries are described, using subsets of available product ions, accounting for overlaps of product isobaric ions originating from different compounds, and accounting for reverse reactions of product ions with H2O. Strategies for support to data interpretation using isotopologues of carbon- and sulphur-bearing ions, and the value in analysis of parallel GC-MS/SIFT-MS measurements and a GC/SIFT-MS combination are also alluded to.
Keywords: Selected ion flow tube mass spectrometry, SIFT-MS, volatile organic compounds, real time quantification, absolute analysis.
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