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Electrospray Ionization (ESI)

The electrospray ion source works at atmospheric pressure and consists of a stainless steel or metalized silica capillary held at a high potential, typically about 3 kV, and placed a few millimeters from a counter electrode.  A solution of the sample flows through the capillary and exits into the ion source.  A voltage is applied between the capillary and sampling plate to generate an electrospray. 

This applied voltage causes charge separation at the capillary tip and defines the mode of spraying, as an excess of either positive or negative charges are created in the sprayed droplet.  Neutral analytes may be charged via redox, proton transfer, adduction or electrostatic processes.  According to the nature of the analyte of interest, positive or negative mode can be selected.

Positive Ion Electrospray

 
 
Negative Ion Electrospray
 
 
As the liquid eluting from the column, now containing multiple charges, begins to exit the capillary tip it becomes conical in shape.  Small charged droplets are released as a spray from the end of the cone, known as the Taylor cone, which forms to relieve coulombic repulsion at the sprayer tip once the Rayleigh limit of surface tension is reached. 
Taylor Cone
 
 

This spray of charged droplets emerges into the ion source where it meets a counter flow of warm nitrogen gas, sometimes referred to as the curtain gas, which vaporizes the solvent.  The stream of droplets emerging from the Taylor cone contains predominantly ions and this is what is known as the electrospray. 

As the droplets travel across the desolvation zone, hydrated ions migrate from within the bulk of the droplet to the surface, from where they can ultimately be transferred to the gas phase.  The process continues, accumulating higher concentrations of ions in lower volume offspring droplets, until they are liberated by one of two mechanisms into the gas phase in the atmospheric pressure interface.

Some of these ions then enter a sampling cone, placed off-axis or orthogonal to the solvent flow, and are introduced into the mass spectrometer. Typical LC-MS solvents are mixtures of water and either methanol or acetonitrile; pure water or nonpolar organic solvents are less effective. Traces of acetic or formic acid can be added to the sample solution to promote protonation. Other additives such as metal salts can be used to produce [M + Metal]+ ions or to promote negative ion formation.

Droplet Desolvation

 
 

 

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