Ion formation mechanism of cortisone molecules and clusters in charged nanodroplets

Mass spectrometry measurements coupled with classical molecular dynamics (MD) simulations have been conducted in recent years to understand the final stage of ion formation in electrospray ionization (ESI). Here, to characterize the ion formation mechanism in the recently developed droplet-assisted ionization (DAI) source, MD simulations with various conditions (solute number, temperature, ions, composition) were performed to help explain DAI-based measurements. The specific binding ability of cortisone with preformed ions (ions of sodium, cesium and iodide) in evaporating nanodroplets makes the ion formation process characteristic of both the ion evaporation and charge residue models (IEM and CRM, respectively). Most preformed ions are ejected with dozens of solvent molecules to form gas-phase ions by IEM, while clusters of one or more cortisone molecules with one or more preformed ions remain in the evaporating droplet to form gas-phase ions by CRM. As the ratio of cortisone molecules to preformed ions increases, the number of preformed ions held in the droplet without ejection by the IEM increases. In other words, increasing the molecular solute to preformed ion ratio in the droplet increases the fraction of gas-phase ions formed by CRM relative to IEM. The increase in CRM relative to IEM is accompanied by an increase in the calculated activation energy barrier, which can explain the activation energy measurements by DAI, where droplets without preformed ions exhibit higher activation energies for gas-phase ion formation than droplets containing large numbers of preformed ions.