How does an ESI ion source work?

The eluent is sprayed with nitrogen at atmospheric pressure in the presence of a strong electrostatic field. This creates charged droplets. An enveloping heated stream of nitrogen vaporizes the solvent molecules. Eventually, a small droplet carries so many charges that it bursts. This process continues faster and faster until practically only analyte ions remain. Depending on the polarity of the voltage applied to the ion source electrodes, protonated quasi-molecular ions [M + H]+ or negatively charged de-protonated quasi-molecular ions [M - H]- are formed. Which to prefer depends on type of molecule.

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