Ternary association reactions of H3O+, NO+ and O2+• with N2, O2, CO2 and H2O; implications for selected ion flow tube mass spectrometry analyses of air and breath

Abstract

Rationale: The reactions of the reagent ions used for trace gas analysis in selected ion flow tube mass spectrometry (SIFT-MS), R+, viz. H3O+, NO+ and O2+, with the major gases in air and breath samples, M, viz. N2, O2, CO2 and H2O, are investigated. These reactions are seen to form weakly-bound adduct ions, R+M, by ternary association reactions that must not be mistaken for genuine volatile organic compound (VOC) analyte ions. Methods: The ternary association rate coefficients mediated by helium (He) carrier gas atoms, k3a, have been determined for all combinations of R+ and M, which form R+M adduct ions ranging in m/z from 47 (H3O+N2) to 76 (O2+•CO2). This was achieved by adding variable amounts of M (up to 0.5 mbar pressure) into the He carrier gas (pressure of 1.33 mbar) in a SIFT-MS flow tube at 300 K. Parabolic curvature was observed on some of the semi-logarithmic decay curves that allowed the rate coefficients mediated by M molecules, k3b, to be estimated. Results: Values of k3a were found to range from 1 × 10−31 cm6 s−1 to 5 × 10−29 cm6 s−1, which form mass spectral R+M “ghost peaks” of significant strength when analysing VOCs at parts-per-billion concentrations. It was seen that the R+M adduct ions (except when M is H2O) react with H2O molecules by ligand switching forming the readily recognised monohydrates of the initial reagent cations R+H2O. Whilst this ligand switching diminishes the R+M adduct ghost peaks, it does not eliminate them entirely. Conclusions: The significance of these adduct ions for trace gas analysis by SIFT-MS in the low m/z region is alluded to, and some examples are given of m/z spectral overlaps of the R+M and R+H2O adduct cations with analyte cations of VOCs formed by analysis of complex media like exhaled breath, warning that ghost peaks will be enhanced using nitrogen carrier gas in SIFT-MS.