a) Can you separate the key steps leading to the formation of isolated gas phase ions in ESI?
Ion formation in ESI can be regarded as divided into three steps:
i) creation of an electrically charged spray,
ii) dramatic reduction of the droplets’ size, and finally
iii) liberation of fully desolvated ions.
b) Can you describe the formation of an electrospray more closely?
At the open end of the capillary, the electric field causes charge separation in the liquid and finally deformation of the meniscus into a cone. When a certain field strength is reached, the starts ejecting a fine jet of liquid from its apex towards the counter electrode. The jet carries a large excess of ions of one particular charge sign. Such a jet cannot remain stable for an elongated period, but breaks up into small droplets. Due to their charge, these droplets are driven away from each other by Coulombic repulsion. Overall, this process causes the generation of a fine spray.
c) The initially created droplets are several hundred times larger in diameter and carry a tremendously higher number of charges than the final droplets do. How does the shrinking of droplets proceed?
When a micrometer-sized droplet evaporates some solvent, the charge density on its surface is continuously increased. As soon as electrostatic repulsion exceeds the surface tension, disintegration of the droplet into smaller sub-units will occur. It has been demonstrated that the microdroplets eject a series of much smaller microdroplets from an elongated end. The smaller offspring droplets carry off only about 1-2 % of the mass, but 10-18 % of the charge of the parent droplet. The concept of this so-called droplet jet fission is not only based on theoretical considerations but can be proven by flash microphotographs.
d) Briefly explain the two theories to describe the final act of liberation of isolated gas phase ions.
The elder model of ion formation, the charged-residue model (CRM), assumes the complete desolvation of ions by successive loss of all solvent molecules from droplets that are sufficiently small to contain just one analyte molecule in the end of a cascade of Coulomb fissions.
A later theory, the ion evaporation model (IEM), describes the formation of desolvated ions as direct evaporation from the surface of highly charged microdroplets.