Write Notes on Electrospray Ionization.

Electrospray Ionization:

Electrospray Ionization (ESI) uses electrical energy to assist the transfer of ions from solution into gaseous phase before they are subjected to mass spectrometric analysis.

During standard ESI, the sample is dissolved in a polar, volatile solvent which is prepared by mixing water with volatile organic compounds like methanol.

ESI involves three steps.

1) Dispersal of a fine spray of charged droplets.

2) Followed by solvent evaporation , and

3) Ion ejection from the highly charged droplets.

The sample solution is pumped through a narrow tube (capillary) which is maintained at high voltage of 2-6 Kv . As a consequence of this strong electric field, the sample emerging from the tip is dispersed into an aerosol of highly charged droplets. Application of nebulising gas (e.g. Nitrogen) enhances the sample flow rate.

Electrospray Ionization

The charged droplets generated from the electrospray tip, pass down a pressure gradient and potential gradient towards the analyzer region of the mass spectrometer. 

The charged droplets are continuously reduced in size by solvent evaporation , assisted by flowing of drying gas. It leads to an increase of surface charge density and decrease of the droplet radius.

Finally the electric field within the charged droplets reach a critical point at which it is kinetically and energetically possible for the ions at the surface of the droplets to be ejected into the gaseous phase.

Application: ESI is mainly used to study protein folding.