Electrospray Ionization - Ionization Mechanism

Ionization Mechanism

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 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. Large-flow electrosprays can benefit from additional nebulization by an inert gas such as nitrogen or carbon dioxide. The aerosol is sampled into the first vacuum stage of a mass spectrometer through a capillary, 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 and emits charged jets in a process known as Coulomb fission. 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) 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 Charge Residue Model (CRM) 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.

A large body of evidence, which is consider either direct or indirect that small ions are liberated into the gas phase through the ion evaporation mechanism, while larger ions form by charged residue mechanism

A third model invoking combined charged residue-field emission has been proposed.

The ions observed by mass spectrometry may be quasimolecular ions created by the addition of a hydrogen cation and denoted +, or of another cation such as sodium ion, +, or the removal of a hydrogen nucleus, −. Multiply charged ions such as 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.