Atmospheric pressure photoionization mechanisms - 1. The case of acetonitrile

Experimental data show that acetonitrile, even though it has an ionization energy higher than the photon energy employed in atmospheric pressure photoionization (APPI) conditions, participates in the protonation of furocumarins. In order to clarify this unexpected behaviour, the processes activated by APPI on acetonitrile have been studied in detail. The formation of protonated molecules (C2H4N+ cations), and of a complex between acetonitrile and water (C2H6NO+ cations), is observed. To clarify the nature of this species, and labelling experiments and product ion spectra have been employed. The data suggest that photon irradiation leads to a first isomerization of acetonitrile molecules that leads to species that exhibit an IE<10 eV and that consequently are able to generate photoionization products. The C2H6NO+ species, already detected in stratosphere and produced in drift tube experiments, reasonably originates from the reaction of C2H4N+ ions with water molecules. Both of these species can be considered as protonating agents.