Electron Impact (EI)
Definition and meaning of Electron Impact (EI) in chemistry.
Electron impact (EI) is an ionization technique in mass spectrometry in which a beam of high-energy electrons bombards vaporized analyte molecules, ejecting an electron to form a radical cation.
In more detail
The electron beam, typically accelerated to 70 electron volts, transfers far more energy than is needed for simple ionization, so the resulting molecular ion (M+•) is often left with excess internal energy and fragments extensively. This produces a reproducible fragmentation pattern that acts as a structural fingerprint, which is why EI is classified as a "hard" ionization method (contrasted with "soft" methods like electrospray or chemical ionization that largely preserve the intact molecular ion). Because the sample must be vaporized before ionization, EI is restricted to volatile, thermally stable compounds and is most commonly paired with gas chromatography (GC-MS).
Key facts
| Field | Analytical Chemistry |
|---|---|
| Standard electron energy | 70 eV |
| Ion produced | Radical cation, M+• |
| Common pairing | Gas chromatography (GC-MS) |
In GC-MS analysis of toluene (C7H8), a 70 eV electron beam ionizes the vapor to give a molecular ion peak at m/z 92, which loses a hydrogen atom to form the stable tropylium cation, producing a prominent fragment peak at m/z 91.
Frequently asked questions
Why is 70 eV used as the standard electron energy?
At 70 eV the ionization efficiency and fragmentation pattern are highly reproducible between instruments, which is why decades of EI mass spectra collected at this energy are cataloged in searchable libraries such as the NIST database for compound identification.
What is a major limitation of EI?
The sample must be volatile and thermally stable to be vaporized before ionization, and the extensive fragmentation can sometimes weaken or eliminate the molecular ion peak, making it hard to determine molecular weight directly.