The liquid containing the analyte(s) of interest is dispersed by electrospray into a fine aerosol. Because the ion formation involves extensive solvent evaporation, the solvents for electrospray ionisation are typically mixtures of water, methanol or acetonitrile. To aid ionisation and help minimise droplet size compounds that increase ionisation of the analyte (e.g. formic acid) are added to the analyte preparation. The resulting aerosol is sampled into the mass spectrometer through a capillary, which is normally heated to aid further solvent evaporation from the charged droplets. The solvent evaporates from the charged droplet until it becomes unstable upon reaching its Rayleigh limit. There are two major theories that explain the final production of gas-phase ions:
The Ion Evaporation Model suggests that as the droplet reaches a certain radius the field strength at the surface of the droplet becomes large enough to assist the field desorption of solvated ions.
The Charged Residue Model suggests that electrospray droplets undergo evaporation and fission cycles, eventually leading progeny droplets that contain on average one analyte ion or less. The gas-phase ions form after the remaining solvent molecules evaporate, leaving the analyte with the charges that the droplet carried.The ions observed may be quasimolecular ions created by the addition of a proton and denoted [M+H]+, or of another cation such as sodium ion, [M+Na]+, or in negative mode loss of a proton, [M-H]-. Multiple-charged ions such as [M+nH]n+ are often observed. For large macromolecules, there can be many charge states, resulting in a charge state envelope.