Investigation on the role of pneumatic aspects in electrospray, desorption electrospray surface ionization and surface-activated chemical ionization

This review reports the results of some studies carried out by us on the role of pneumatic aspects in electrospray and desorption electrospray surface ionization, with the aim of proposing some relevant aspects of the mechanisms involved in these ionization methods. Electrospray ion sources, with the exception of the nano-electrospray source, operate with the concurrent action of a strong electric field and a supplementary coaxial gas flow. The electric field is responsible for electrospraying of the analyte solution but the use of a coaxial gas flow leads to a significant increase in the analyte signal and allows the use of higher solution flows. However, by employing capillary voltages much lower than those necessary to activate the electrospray phenomenon, analyte ions are still observed and this indicates that different mechanisms must be operative for ion production. Under these conditions, ion generation could take place from the neutral pneumatically sprayed droplet by field-induced droplet ionization. Also, in the case of desorption electrospray ionization, and without any voltage on the spraying capillary as well as on the surface of interest, ions of analytes present on the surface become detectable and this shows that desorption/ionization of analytes occurs by neutral droplets impinging the surface. Consequently, the pneumatic effect of the impinging droplets plays a relevant role and, for these reasons, the method has been called pneumatic-assisted desorption. Some analogies existing between pneumatic-assisted desorption and surface-activated chemical ionization, based on the insertion of a metallic surface inside an atmospheric pressure chemical ionization source operating without corona discharge, are discussed.