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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.

David R. Smith,P. Španěl

Published 2021 in Rapid Communications in Mass Spectrometry

ABSTRACT

RATIONALE 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. METHODS 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. RESULTS 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. 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+ 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.

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