EI (Electron Ionisation)
Definition
Hard ionisation at 70 eV. Produces extensive fragmentation, ideal for library search. Standard ion source in GC-MS.
- Ionisation energy
- 70 eV
- Ionisation type
- Hard ionisation
- Effect
- Extensive, reproducible fragmentation
- Common instrument
- GC-MS
Common questions
Why does EI use 70 eV rather than a lower energy?+
70 eV is a standardised setting adopted across the field because it produces consistent, reproducible fragmentation patterns across different instruments, which is what allows a spectrum collected in one laboratory to be matched against a shared reference library like NIST.
What is the tradeoff of using such a hard ionisation method?+
The extensive fragmentation that makes library matching reliable can also destroy the molecular ion, so a compound's exact mass or molecular weight is sometimes not directly visible in the spectrum and must be inferred from fragment patterns or confirmed with a softer ionisation technique.
Related terms
- Base Peak
- The most intense peak in a mass spectrum, normalised to 100 percent relative abundance. All other peaks in the spectrum are reported...
- ESI (Electrospray Ionisation)
- Soft ionisation at atmospheric pressure for polar and thermally fragile analytes in solution. Standard source in LC-MS for drugs and biomolecules.
- Fragmentation
- Breakdown of the molecular ion into smaller charged and neutral pieces. The fragmentation pattern is the chemical fingerprint that identifies the compound.
- M/z (Mass-to-Charge Ratio)
- The x-axis of every mass spectrum. Ions are separated by their mass divided by the number of charges they carry. For singly-charged...
- MALDI
- Matrix-Assisted Laser Desorption / Ionisation. A soft method that uses a UV-absorbing matrix to ionise large biomolecules. Almost always paired with a...
- Molecular Ion (M+)
- The intact ionised molecule with no fragmentation, formed by removing a single electron from the parent. Its m/z gives the molecular weight...
- Nitrogen Rule
- An organic molecule with an odd-numbered molecular weight contains an odd number of nitrogen atoms. A staple trick for narrowing down a...
- TOF (Time-of-Flight)
- Mass analyser that separates ions by their flight time down a field-free drift tube. Theoretically unlimited mass range, very high speed.