Ternary association reactions of H3 O+ , NO+ and O2+• with N2 , O2 , CO2 and H2 O; implications for SIFT-MS analyses of air and breath.

The reactions of the reagent ions used for trace gas analysis in SIFT-MS, R+ , viz. H3 O+ , NO+ and O2+ , with the major gases in air and breath samples, M, viz. N2 , O2 , CO2 , and H2 O, 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 VOC analyte ions.The ternary association rate coefficients mediated by 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 (H3 O+ 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.k3a were found to range from 1x10-31 cm6 s-1 to 5x10-29 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 H2 O) react with H2 O molecules by ligand switching forming the readily recognized monohydrates of the initial reagent ions R+ H2 O. Whilst this ligand switching diminishes the R+ M adduct ghost peaks, it does not eliminate them entirely.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+ H2 O adduct ion with analyte ions 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.