How does electron impact ionization work?

Electron impact ionization (EI) is a common method used in mass spectrometry to turn molecules into ions.

Simple explanation:

1. Sample enters:
   The sample (usually a gas or vapor) is introduced into the ionization chamber.
2. Electron beam hits the sample:
   A beam of high-energy electrons (usually 70 eV) is fired at the molecules.
3. Electrons knock off other electrons:
   When the fast electrons hit the molecules, they knock out one of the molecule’s own electrons.
4. Ion is formed:
   The molecule loses an electron and becomes a positively charged ion (M⁺).
   Example:
   M + e⁻ → M⁺ + 2e⁻
5. Fragmentation (breaking apart):
   The ion may break into smaller pieces (fragments), giving extra information about the structure of the molecule.
6. Ions go to the analyzer:
   These ions (both parent and fragments) are sent to the mass analyzer to measure their mass-to-charge ratio (m/z).

In short:

Electron impact ionization works by bombarding molecules with high-energy electrons, which knock out electrons from the molecules to form positive ions and sometimes fragments. These ions are then analyzed to identify the substance.