Relative influence of helium and nitrogen carrier gases on analyte ion branching ratios in SIFT-MS

Nitrogen carrier gas is now being used more frequently for SIFT-MS analyses than helium for the reasons of cost and supply. Yet the extensive kinetics database required has largely been compiled using data obtained in helium carrier gas. This paper asks the question: can the helium-based kinetics library be used with confidence for analyses in nitrogen carrier gas? To investigate this, the rate coefficients and product ion distributions for the reactions of H3O+, NO+ and O2+● with three monoterpenes, β-pinene, camphene and (R)-(+)-limonene, and the specific reactions (a) H3O+ with 2-propanol, (b) O2+● with acetone, (c) NO+ with acetaldehyde and (d) NO+ with 2,3-butanedione have been explored in both helium and nitrogen carrier gases using a Profile 3 SIFT-MS instrument. These reactions were chosen because several primary reaction mechanisms are involved, including proton transfer (a), charge transfer (b), parallel hydride ion transfer and adduct ion formation (c) and parallel charge transfer and adduct ion formation (d). The detailed results show that for the diverse monoterpene reactions that have multiple product ions and for the pure bimolecular reactions (a) and (b), the reaction kinetics in both helium and nitrogen carrier gases are essentially identical. However, reactions (c) and (d) in which adduct ions are partially formed exhibit a slow carrier gas pressure dependence in helium, but a much greater carrier gas pressure dependence in nitrogen, and different product ion distributions. The conclusion is drawn that for pure bimolecular reactions, e.g. (a) and (b), the helium-obtained kinetics data can be used with confidence for trace gas analysis by SIFT-MS in nitrogen carrier gas, whereas kinetics data for ion-molecule reactions that involve adduct ion formation must be obtained by measurements under the specific pressure (and temperature) of the nitrogen carrier gas at which gas analyses are to be performed.