Associating Molecular Physicochemical Properties with Ionization Efficiency for Compounds in Aprotic, Polar Solvent Using Field-free and Field-enabled cVSSI Techniques

The relationship between separate molecular physicochemical properties and ionization efficiency has been investigated for the new ionization technique capillary vibrating sharp-edge spray ionization (cVSSI). Intensity values have been recorded for both positively- and negatively-charged ions arising from various compounds in the aprotic, polar acetonitrile (ACN) solvent environment. These have been recorded for field-free cVSSI as well as field-enabled cVSSI and compared to results obtained from standard ESI. In general, the strongest correlating factors include the logarithm of the octanol/water partition coefficient (log P) and the compound proton affinity (PA) in both positive and negative ion mode. This is contrasted with results for the polar, protic solvents water and methanol where the log of the base dissociation constant (pKb) often produced the strongest correlation. The results suggest that, in the absence of abundant protonating reagent, pre-formed ions do not govern the ionization process for samples in the ACN solvent systems. Another notable result is the increased ion signal levels observed for the majority of the ions in positive ion mode upon their production by a field-free source; that is, remarkably, the application of a DC voltage to the solution serves to decrease the overall ion signal level. Overall, it appears that, regardless of whether or not ions are produced by the charged residue model or the ion evaporation model, gas-phase proton transfer reaction is the major process by which they are produced.