Electron Ionisation (EI)
Definition
The standard ionisation mode in GC-MS, using a 70 eV electron beam in high vacuum to ionise and fragment vapour-phase analytes. Produces reproducible fragmentation patterns searchable against the NIST mass spectral library.
- Energy
- 70 eV electron beam
- Environment
- High vacuum
- Applies to
- Vapour-phase analytes in GC-MS
- Output use
- Searchable against NIST library
Common questions
Why must the sample be in the vapour phase for EI to work?+
The electron beam ionises analytes as they pass through the ion source in gas form after separation on the GC column, so any analyte that cannot be volatilised without decomposing is unsuitable for EI-GC-MS and needs a different ionisation or separation technique such as LC-MS.
How does the reproducibility of EI fragmentation help explosives casework specifically?+
Because degradation products and additives in explosives formulations produce consistent fragment patterns under EI, an analyst can match a recovered residue's spectrum against reference library entries for known explosive compounds with high confidence even from a small recovered sample.
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