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EI — Electron Ionization

A hard ionization that fragments molecules · paired with GC-MS

EI (Electron Ionization) is a hard ionization technique: it breaks molecules into fragments. Gas-phase molecules are struck by high-energy electrons, and the resulting fragmentation pattern acts like a fingerprint for identifying each compound. EI is the classic source for GC-MS with small, volatile molecules.

How it works

Send a molecule and watch it get struck by the electron beam and shatter into fragments.

molecule in → 70 eV electron hit → shatters → fragments → analyzer
neutral molecule 70 eV electron fragment ion → analyzer
  1. Molecule in: a gas-phase sample molecule enters the ion source (under vacuum).
  2. Electron beam: a hot filament emits a 70 eV electron beam across the source toward an electron trap.
  3. Impact: an electron knocks an electron off the molecule, forming a molecular ion (M+•).
  4. Fragmentation: 70 eV is far more than the molecule can hold, so it shatters into fragment ions of many sizes — the fingerprint pattern.

Hard vs soft

Unlike ESI and MALDI, which keep the molecule intact, EI deliberately fragments it. Those fragments are highly reproducible, so EI spectra can be matched against large reference libraries to identify unknown compounds.

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