Ionic diffusion and mass discrimination effects in the new generation of short flow tube SIFT-MS instruments

The major thrust of this paper is to describe how the current selected ion flow tube mass spectrometry (SIFT-MS) Profile 3 instruments can be configured to provide reliable quantification of the trace gases present in air and exhaled breath by accounting for the phenomena of differential diffusion of the analytical precursor and product ions in the flow tube reactor and mass discrimination in the ion sampling/analytical quadrupole mass spectrometer/detection system. If not accounted for these phenomena, especially the latter, can result in serious errors in quantification. Hence, it is described how H3O+ precursor ions are totally converted to a range of product ions within the mass-to-charge ratio, m/z, range from 18 to 201 and, thus, how the ion currents collected by the downstream ion sampling orifice disc and the count rates of these ions as determined by the analytical detection system at the various m/z are used to provide values of both the diffusion enhancement coefficient, De, and the mass discrimination factor, Mr, that are required to allow accurate trace gas analyses. It is indicated how all such SIFT-MS instruments can be properly configured using a similar but abbreviated procedure. Finally, it is shown how the properly configured Profile 3 instrument provides consistent and reliable analyses of acetone using H3O+, NO+ and O2+, 2-pentanone and 2-hexanone using H3O+ and NO+, and ammonia using H3O+ and O2+ precursor ions in (relatively dry) air samples and in (humid) single breath exhalations.