Electrospray Ionisation

Electrospray Ionisation (ESI)

Schematic showing mechanism for Charge Residue Model in ESI

The possibility of generating gas-phase ions by spraying a solution from the tip of an electrically charged capillary was reported by Dole and co-workers. This early work was hampered by the use of an ion-drift spectrometer for ion analysis. Building upon the ideas of Dole, Fenn and co-workers developed electrospray as an ionisation technique for mass spectrometry.

The development of electrospray ionisation (ESI) has had a major impact on the mass spectrometric analyses of a broad range of analytes, and in particular for the analysis of macromolecules. Recent emphasis has been on the analysis of compounds of biological significance.

In electrospray ionisation, an analyte solution is passed, at atmospheric pressure, through a capillary into a small-diameter tip held at high potential (a few kV). The effect of the electric field as the solution emerges from the tip is to generate a spray of highly charged droplets that pass down a potential (and pressure) gradient towards the analyser. In order to establish a stable spray, it is important to have full control over parameters such as:

* the conductivity of the capillary and the needle tip
* the surface tension of the analyte solution
* the flow rate of the analyte solution
* the composition of the analyte solution
* the potential applied to the capillary tip

In most commercial instruments the facility to use a sheath flow of nebulizer gas is incorporated. Its use is determined by the flow rate employed, the composition of the solvent and the potential applied to the capillary tip. In addition, some electrospray source designs allow a part of the capillary to be heated to facilitate droplet evaporation. Ultimately, fully desolvated ions result from evaporation of the solvent or field desorption of the droplets (see below). The desolvated ions are passed through one or two (usually conical) skimmer electrodes into increasingly higher-vacuum regions.

Theory of Electrospray Ionisation

The solution at the tip of the electrospray needle experiences the electric field associated with the maintenance of the tip at high potential. Assuming a positive potential, positive ions in the solution will accumulate at the surface, and are drawn out to form a ‘Taylor cone’. If the surface tension at the tip of the Taylor cone is exceeded by the applied electrostatic force, a droplet is formed.

There is still no consensus on the mechanism by which solute ions are formed from charged droplets. Dole and co-workers proposed that ions originate from small droplets containing one molecule of the analyte, the ‘charge residue model’ (CRM). Evaporation of the solvent from the initially formed droplet as it traverses a pressure gradient toward the analyser leads to a reduction in diameter, and an increase in surface field, until the Rayleigh limit is reached. A coulomb explosion occurs, as the magnitude of the charge is sufficient to overcome the surface tension holding the droplet together. The resulting instability disperses the droplet into a collection of smaller droplets, that continue to evaporate until they too reach the Rayleigh limit and disintegrate. A continuation of this process may be envisaged to result in the formation of an ion containing a single analyte molecule. The molecule retains some of its droplet’s charge to become a free ion as the last of the solvent vaporises.
A second mechanism of ion formation, the ion desorption model (IDM) is based on the work of Iribarne and Thomson. The ion desorption model assumes that before a droplet reaches the ultimate stage its surface electric field becomes sufficiently large to lift an analyte ion at the surface of the droplet over the energy barrier that prevents its escape. A diagram illustrating the CRM and IDM models of ion formation in electrospray is shown below:

References

All of the above text is an excerpt from:

Bottrill, A.R., Ph.D. Thesis, University of Warwick, 2000.
'High-energy Collision-induced Dissociation of Macromolecules using Tandem Double-focusing/Time-of-flight Mass Spectrometry.'

maintained by:
Dr. Mike Naldrett