KB001935: What is ElectroSpray Ionization (ESI)? - General LC/MS 

This is an atmospheric pressure ionization technique. Ions are formed in solution (droplets) and then the droplets are evaporated with a drying gas (Nebulised) in the presence of a strong electrostatic field . This will disassociate molecules , increase the charge concentration eventually the repulsive force between ions with like charges exceeds the cohesive forces and ions are ejected in to the gas phase.