ESI — Electrospray Ionization

ESI (Electrospray Ionization) turns molecules dissolved in solution into gas-phase ions. It is a soft ionization technique that does not break the molecule, making it ideal for large molecules such as peptides and proteins, and it is the source most commonly paired with LC-MS.

How it works — four stages

Adjust and watch the animation below — ions (green) flow from the spray needle all the way to the analyzer inlet.

1. Spray: solution is pushed through a high-voltage needle (kV); the liquid surface is drawn into a cone (Taylor cone) and sprayed into a fine mist.
2. Charged droplets: the small droplets carry charge on their surface.
3. Shrink: solvent evaporates, the droplets shrink, and the surface charge becomes more concentrated.
4. Coulomb fission: when charge repulsion exceeds surface tension (the Rayleigh limit), the droplet bursts and releases free ions into the analyzer.

Why ESI is “soft”

Ions form as droplets gradually evaporate and burst, rather than being hit directly with high energy. The molecule therefore stays intact (a molecular ion) — unlike hard ionization such as EI, which shatters the molecule into fragments.