The liquid containing the analytes of interest (typically 10−6 - 10−4 M needed ) is
dispersed by electrospray, into a fine aerosol. Because the ion formation involves extensive solvent evaporation (also termed desolvation), the typical solvents for electrospray ionization are prepared by mixing water with
volatile organic compounds (e.g. methanol
acetonitrile). To decrease the initial droplet size, compounds that increase the conductivity (e.g. acetic acid) are customarily added to the solution. These species also act to provide a source of protons to facilitate the ionization process. Large-flow electrosprays can benefit from
nebulization of a heated inert gas such as
nitrogen or carbon dioxide in addition to the high temperature of the ESI source. The aerosol is sampled into the first vacuum stage of a mass spectrometer through a capillary carrying a potential difference of approximately 3000V, which can be heated to aid further solvent evaporation from the charged droplets. The solvent evaporates from a charged droplet until it becomes unstable upon reaching its Rayleigh limit. At this point, the droplet deforms as the electrostatic repulsion of like charges, in an ever-decreasing droplet size, becomes more powerful than the surface tension holding the droplet together. At this point the droplet undergoes Coulomb fission, whereby the original droplet 'explodes' creating many smaller, more stable droplets. The new droplets undergo desolvation and subsequently further Coulomb fissions. During the fission, the droplet loses a small percentage of its mass (1.0–2.3%) along with a relatively large percentage of its charge (10–18%). There are two major theories that explain the final production of gas-phase ions: the ion evaporation model (IEM) and the charge residue model (CRM). The IEM 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 CRM suggests that electrospray droplets undergo evaporation and fission cycles, eventually leading progeny droplets that contain on average one
analyte ion or less. while larger ions (from folded proteins for instance) form by charged residue mechanism. A third model invoking combined charged residue-field emission has been proposed. Another model called chain ejection model (CEM) is proposed for disordered polymers (unfolded proteins). The ions observed by mass spectrometry may be quasimolecular ions created by the addition of a
hydrogen cation and denoted [
M + H]+, or of another
cation such as
sodium ion, [
M + Na]+, or the removal of a hydrogen nucleus, [
M − H]−. Multiply charged ions such as [
M +
nH]
n+ are often observed. For large
macromolecules, there can be many charge states, resulting in a characteristic charge state envelope. All these are even-electron ion species:
electrons (alone) are not added or removed, unlike in some other ionization sources. The analytes are sometimes involved in
electrochemical processes, leading to shifts of the corresponding peaks in the
mass spectrum. This effect is demonstrated in the direct ionization of noble metals such as copper, silver and gold using electrospray. The efficiency of generating the gas phase ions for small molecules in ESI varies depending on the compound structure, the solvent used and instrumental parameters. The differences in ionization efficiency reach more than 1 million times. == Variants ==