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 u sed 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 i n the helium carrier gas when humid samples are being analysed by SIFT-MS u sing H2O+ precursor ions. This requires an understanding of the kinetics of hydrated hydronium ion formation, the involvement of mass discrimination i n the analytical quadrupole mass spectrometer and the decreased diffusive l oss of the heavier hydrates along the flow tube. Thus, we show that the hum idity of breath and liquid headspace samples, typically at the few percent level, can be directly obtained on-line to the SIFT-MS instrument along wit h 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 me asurements to trace gas analysis using SIFT-MS. Copyright (C) 2001 John Wil ey & Sons, Ltd.