Investigations on the field dependency of proton and charge transfer kinetics in atmospheric pressure corona discharge sources

Abstract

Proton and charge transfer are two commonly observed reaction mechanisms in the chemical ionisation of neutral analytes in mass spectrometry and ion mobility spectrometry. While those reactions show a strong influence on effective temperature, influenced by the applied electric field strengths, a detailed understanding of the underlying mechanisms under such conditions is still lacking. Using a high kinetic energy ion mobility spectrometer, we examine the reaction dynamics of proton transfer from H₃O⁺(H₂O)_n and charge transfer from NO⁺(H₂O)_m to the model analytes benzene, toluene, and p-xylene experimentally and compare this to quantum chemical modelling of the ionisation processes over a wide range of effective temperatures. Our findings underline that the reduced field strength (E/N) significantly affects the ion-neutral reactions due to its influence on effective temperature, leading to significant changes in reaction rate coefficients. Particularly when ionisation proceeds via H₃O⁺ and NO⁺, the reaction rate coefficients approach the association rate in most cases. However, hydration of the reactant ions can slow down the reaction since the dynamics of intermediate reaction complexes need to be considered, as these can introduce additional internal barriers. Especially when subjected to high E/N, those reaction complexes can either dissociate back to the reactants or towards the products, with the branching ratio determined by the kinetics of both reaction paths. Particularly when the ionisation energies or proton affinities of the neutral precursor of the reactant ion and the neutral analyte are similar, this product branching can introduce deviations between the experimentally observed reaction rates and the association rates.