H3O+, NO+ and O2+[rad] reactions with saturated and unsaturated monoketones and diones; focus on hydration of product ions

Abstract

A selected ion flow tube, SIFT, study has been carried out for the reactions of the SIFT-MS reagent ions H3O+, NO+ and O2+ with 21 ketones, M, comprising 13 saturated monoketones of which 10 are linear and 3 are branched, 5 unsaturated monoketones and 3 diones. The collisional rate coefficients, kc, were calculated for the H3O+ reactions that all proceed via rapid proton transfer and the relative rate coefficients for the NO+ and O2+ reactions were obtained experimentally. The product ion distributions for all 63 reactions were obtained at two sample gas absolute humidity (0.5% and 5.5%), the higher humidity intended to simulate that of exhaled breath. A major objective was to understand how the product ions of the reactions formed hydrates, which is important for accurate quantification of metabolites in humid air samples by SIFT-MS and by secondary electrospray ionisation, SESI. The MH+ product ions of the H3O+ reactions readily hydrate at the higher humidity, the monohydrate being dominant for both the saturated and unsaturated ketones, whereas the dihydrate was dominant for the dione reactions. In the NO+ reactions, formation of the NO+M adduct ions was dominant for the saturated and unsaturated ketones, but charge transfer producing the M+ parent cation was very dominant in the dione reactions. The O2+ reactions all proceeded via dissociative charge transfer producing the parent cation M+ and closed shell and open shell cation fragments. Except for the dione parent cations, the M+ ions hydrated, as did the open shell fragment ions of the O2+ reactions, but the closed shell fragment ions did not hydrate.