electron ionization
/ ee-LEK-tron EYE-on-ih-ZAY-shun /
Imagine throwing a fast-moving cue ball through a rack of billiard balls: the impact is so energetic that the rack doesn't just scatter — it knocks pieces loose. Electron ionization does this to gas-phase molecules: a beam of fast electrons slams into them, knocking out an electron and leaving the molecule so jolted that it often breaks apart.
Electron ionization (EI) is a hard ionization method in which vaporized sample molecules are bombarded by a beam of high-energy electrons (conventionally 70 electron-volts), removing one electron to form a radical cation and depositing enough extra energy to cause extensive fragmentation. The pattern of fragments produced is highly reproducible at standard conditions.
It matters because that reproducibility built the great searchable spectral libraries: an EI spectrum taken anywhere can be matched against a reference, making EI the classic engine of gas chromatography-mass spectrometry for identifying small, volatile molecules. The caveat is that EI often fragments the molecule so heavily that the molecular ion is weak or missing, so the molecular mass can be hard to read directly.
A GC-MS using electron ionization identifies a residual solvent by matching its rich pattern of fragment peaks to a 70-electron-volt reference spectrum in the instrument's library, scoring a near-perfect match.
Electron ionization fragments molecules reproducibly, enabling library matching.
Electron ionization is sometimes loosely called 'electron impact', but the molecule and electron do not truly collide like balls — the fast electron's passing field jostles the molecule. EI works best for small, volatile, thermally stable compounds; fragile biomolecules need soft methods like ESI or MALDI.