It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized. Da-MDa orders of magnitude observed in proteins and their associated polypeptide fragments. ESI is a so-esi mass spectrometry pdf ‘soft ionization’ technique, since there is very little fragmentation.
Another important advantage of ESI is that solution-phase information can be retained into the gas-phase. The electrospray ionization technique was first reported by Masamichi Yamashita and John Fenn in 1984. The development of electrospray ionization for the analysis of biological macromolecules was rewarded with the attribution of the Nobel Prize in Chemistry to John Bennett Fenn in 2002. One of the original instruments used by Dr.
Fenn is on display at the Chemical Heritage Foundation in Philadelphia, Pennsylvania. In 1882, Lord Rayleigh theoretically estimated the maximum amount of charge a liquid droplet could carry before throwing out fine jets of liquid. This is now known as the Rayleigh limit. In 1914, John Zeleny published work on the behaviour of fluid droplets at the end of glass capillaries and presented evidence for different electrospray modes.
The first use of electrospray ionization with mass spectrometry was reported by Malcolm Dole in 1968. John Bennett Fenn was awarded the 2002 Nobel Prize in Chemistry for the development of electrospray ionization mass spectrometry in the late 1980s. 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. 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. The gas-phase ions form after the remaining solvent molecules evaporate, leaving the analyte with the charges that the droplet carried.
IEM, CRM and CEM schematic. A third model invoking combined charged residue-field emission has been proposed. For large macromolecules, there can be many charge states, resulting in a characteristic charge state envelope. 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 electrosprays operated at low flow rates generate much smaller initial droplets, which ensure improved ionization efficiency. In 1993 Gale and Richard D. Currently the name nanospray is also in use for electrosprays fed by pumps at low flow rates, not only for self-fed electrosprays.
Although there may not be a well-defined flow rate range for electrospray, microspray, and nano-electrospray, studied “changes in analyte partition during droplet fission prior to ion release” . In this paper, they compare results obtained by three other groups. Applications of this method include the analysis of fragile molecules and guest-host interactions that cannot be studied using regular electrospray ionization.