ionization
/ EYE-on-ih-ZAY-shun /
Think of a crowd of people walking quietly through a doorway — nobody notices them because they carry no badge. Now hand each person a glowing badge: suddenly a sensor at the exit can count them, sort them, and steer them with a magnet. Ionization is the act of giving molecules that electric 'badge', a charge, so that a mass spectrometer can grab hold of them.
More precisely, ionization is the process of converting a neutral atom or molecule into an ion by adding or removing one or more electrons (or by attaching or losing a charged particle such as a proton). The charged species can then be pushed and pulled by electric and magnetic fields, which is what lets a mass analyzer sort it by mass-to-charge ratio.
It matters because everything downstream in mass spectrometry depends on it: only ions are detected. The choice of ionization method shapes the whole experiment — a gentle method may leave the molecule whole, while a harsh one may shatter it into fragments. The caveat is that ionization efficiency varies wildly between substances, so the size of a peak reflects both how much is present and how easily that compound ionizes.
When a protein is sprayed through electrospray ionization, it picks up several extra protons; each added proton is one act of ionization, turning the silent neutral protein into a charged ion the instrument can finally see.
Ionization gives a molecule the charge that lets the instrument detect it.
Ionization in mass spectrometry is broader than just losing an electron: a molecule can also become an ion by gaining a proton, losing a proton, or attaching to a charged adduct such as sodium. The net result is the same — it now carries charge.