Proton-transfer rate constants for the determination of organic indoor air pollutants by online mass spectrometry

Proton transfer reaction mass spectrometry (PTR-MS) has become an indispensable analytical tool for indoor related sciences. With high-resolution techniques not only is the online monitoring of the selected ions in the gas phase possible, but also, with some limitations, the identification of substance mixtures without chromatographic separation. The quantification is carried out with the help of kinetic laws, which require knowledge of the conditions in the reaction chamber, the reduced ion moblilities and the reaction rate constant k(PT) under these conditions. Ion-dipole collision theory can be used to calculate k(PT). One approach is an extension of Langevin's equation and is known as average dipole orientation (ADO). In a further development, the analytical solution of ADO was replaced by trajectory analysis, which resulted in capture theory. The calculations according to ADO and capture theory require precise knowledge of the dipole moment and the polarizability of the respective target molecule. However, for many relevant indoor related substances, these data are insufficiently known or not known at all. Consequently, the dipole moment mu(D) and polarizability alpha of 114 organic compounds that are frequently found in indoor air had to be determined using advanced quantum mechanical methods. This required the development of an automated workflow that performs conformer analysis before computing mu(D) and alpha using density functional theory (DFT). Then the reaction rate constants with the H3O+ ion are calculated according to the ADO theory (k(ADO)), capture theory (k(cap)) and advanced capture theory (k*(cap)) for different conditions in the reaction chamber. The kinetic parameters are evaluated with regard to their plausibility and critically discussed for their applicability in PTR-MS measurements.

Automated summary

Proton transfer reaction mass spectrometry (PTR-MS) is an important tool for indoor related sciences, allowing online monitoring and identification of substance mixtures in the gas phase. Kinetic laws and collision theory are used to quantify and calculate reaction rate constants under different conditions in the reaction chamber. The dipole moment and polarizability of organic compounds frequently found in indoor air were determined using advanced quantum mechanical methods, and their applicability in PTR-MS measurements was critically discussed.