Proton transfer reaction rate coefficients between H3O+ and some sulphur compounds

Volatile sulphur compounds (VSCs) are key compounds in many fields of basic and applied science and technology, such as environmental sciences, food science, geochemistry, petrochemistry, agriculture, biology and medicine. Proton transfer reaction mass spectrometry (PTR-MS) allows for on-line monitoring of volatile organic compounds (VOCs) and, in particular, of VSCs with ultra low detection limits and a fast response time. In principle, with PTR-MS, absolute quantification of VOC concentrations without calibration is possible, provided the branching ratios are known. However, for this, the reaction rate coefficients between VOCs and the hydronium ion have also to be known. Several well-established theories may be used to determine ion-neutral molecule reaction rate coefficients. In the case of H3O+–VOC reactions proceeding in a PTR-MS drift tube, a key factor to be considered is the centre-of-mass energy, which is generally much higher than the thermal energy, due to the additional translational (drift) energy of the ion. Nevertheless, it is common practice to employ collision theories that do not show an explicit dependence on the centre-of-mass energy. First we review basic aspects of ion-neutral reactions in the PTR-MS drift tube and various methods to calculate reaction rate coefficients. Next, we calculate, on the basis of quantum chemical methods and different theoretical approaches for ion-molecule collisions, reaction rate coefficients between selected sulphur compounds and H3O+. Finally, we discuss proper methods for the calculations of ion-neutral molecule reaction rate coefficients in the context of PTR-MS and the corresponding experimental parameters involved.