electrospray ionization
/ ee-LEK-troh-spray EYE-on-ih-ZAY-shun /
Picture a perfume atomizer, but instead of plain mist it sprays a fine cloud of electrically charged droplets. As each droplet flies and dries, it shrinks until the crowded charges inside burst it into still smaller droplets, again and again, until at last bare charged molecules are floating free in the gas. That delicate cascade is electrospray ionization.
Technically, electrospray ionization (ESI) is a soft ionization method in which a liquid sample is pushed through a fine needle held at high voltage, producing a spray of charged droplets that progressively evaporate and divide until they release gas-phase ions of the dissolved molecules. Because the molecule is coaxed into the gas phase rather than blasted, large and fragile species survive intact.
It matters because ESI made it routine to weigh big, delicate biomolecules — proteins, peptides, DNA — that older harsh methods would have destroyed, and it pairs naturally with liquid chromatography since both work with liquids. The honest caveat is that ESI typically produces multiply charged ions and is sensitive to salts and matrix in the liquid, which can suppress the signal if the sample is not cleaned up.
When a protein solution flows out of an electrospray needle, the intact protein emerges carrying many protons; the spectrum shows a row of peaks from the same protein at different charges, which together pin down its true mass.
Electrospray gently lifts large molecules into the gas phase as multiply charged ions.
ESI is called 'soft' because it tends to leave the molecule whole, in contrast to 'hard' methods like electron ionization that deliberately fragment it. The multiply charged ions ESI produces are a feature, not a flaw — they let huge molecules fit a normal m/z range.