Different reactivities of H 3 O + (H 2 O) n with unsaturated and saturated aldehydes; ligand switching reactions govern the quantitative analytical sensitivity of SESI-MS.

The trends in SESI-MS sensitivities are rationally explained by differences in the rates of the ligand switching reactions, which are justified by theoretically calculated equilibrium rate constants derived from thermochemical DFT calculations of Gibb's free energy changes. Humidity present in SESI gas thus favours the reverse reactions of the saturated aldehyde analyte ions, effectively suppressing their signals in contrast to their unsaturated counterparts.