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

The reactions of the reagent ions used for trace gas analysis in selected ion flow tube mass spectrometry (SIFT-MS), R + , viz. H 3 O + , NO + and O 2 + , with the major gases in air and breath samples, M, viz. N 2 , O 2 , CO 2 and H 2 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 volatile organic compound (VOC) analyte ions. Methods:The ternary association rate coefficients mediated by helium (He) carrier gas atoms, k 3a , have been determined for all combinations of R + and M, which formCO 2 ). 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, k 3b , to be estimated.Results: Values of k 3a were found to range from 1 Â 10 À31 cm 6 s À1 to 5 Â 10 À29 cm 6 s À1 , 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 H 2 O) react with H 2 O molecules by ligand switching forming the readily recognised monohydrates of the initial reagent cations R + H 2 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 + H 2 O 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. | INTRODUCTIONSelected ion flow tube mass spectrometry, SIFT-MS, combines a fast flow tube chemical ionisation reactor with quantitative mass spectrometry. 1,2 Since its inception, 3 it has been used primarily for real-time analysis of trace gases in ambient air, 4,5 exhaled breath, 6,7 This paper is dedicated to Professor John J. Monaghan. As the editor of RCM he handled some 30 of our submissions, was always very professional, yet courteous, understanding the challenges of authors and reviewers alike. When choosing a suitable journal for publication, these qualities were for us a true incentive to choose RCM.