Secondary electrospray ionization proceeds via gas-phase chemical ionization

Our main goal was to gain further insights into the mechanism by which gas-phase analytes are ionized by interaction with plumes of electrospray solvents. We exposed target vapors to electrosprays of either water or deuterated water and mass analyzed them. Regardless of the solvent used, the analytes were detected in protonated form. In contrast, when the ionization chamber was humidified with deuterated water, the target vapors were detected in deuterated form. These observations suggest that either there is no interaction between analytes and electrospray charged droplets, or if there is any, a subsequent gas-phase ionmolecule reaction governs the process. Implications in practical examples such as breath analysis are discussed.