Selected ion flow tube, SIFT, studies of the reactions of H3O+, NO+ and O2+ with some biologically active isobaric compounds in preparation for SIFT-MS analyses

A study of the reactions of H 3 O + , NO + and O 2 + ions with two groups of four isobaric, biologically active compounds with molecular weights, MW, of 86u and 88u has been carried out using the selected ion flow tube, SIFT, technique in preparation for the analyses of these compounds in the headspace of bacterial and cell cultures using SIFT-MS. These compounds are: MW 86u: 2,3 butanedione (diacetyl), C 4 H 6 O 2 ; allyl ethyl ether, C 5 H 10 O, cyclopropane carboxylic acid, C 4 H 6 O 2 : γ-butyrolactone, C 4 H 6 O 2 . MW 88u: 3-hydroxybutanone (acetoin), C 4 H 8 O 2 ; n-butyric acid, C 4 H 8 O 2 ; ethyl acetate, C 4 H 8 O 2 ; pyruvic acid, C 3 H 4 O 3 . All 24 reactions proceed at the gas kinetic rate, the dominant primary product ions of the H 3 O + reactions being the protonated reactant molecules, MH + with a common mass-to-charge ratio, m / z , in each group of isobaric compounds; those of the NO + reactions being the adduct ions, NO + M, also with a common m / z value; for the O 2 + ion reactions, fragmentation ions of the nascent parent ion M + are produced. SIFT-MS analysis of each compound individually is straightforward, but when more than one isobaric compound is present, SIFT-MS analyses become more challenging. However, hydration of some, but not all, of the primary MH + and NO + M primary ions occurs, variously generating MH + (H 2 O) 1,2 and NO + MH 2 O ions, and the occurrence/non-occurrence of these hydrates can assist in distinguishing between some of the isobaric compounds. From the kinetic data obtained in this study, the required SIFT-MS kinetics library entries for each compound can be constructed that allow their quantification. As an example, SIFT-MS spectra are presented relating to the analysis of the headspace of incubated yoghurt, which show the presence of several volatile organic compounds, including acetoin and diacetyl.