WHAT IT IS
Electron energy is the kinetic energy (measured in electron volts, eV) of electrons used to ionize analyte molecules in the ion source. This is most relevant in Electron Ionization (EI), the most common ionization technique in GC-MS.
The electron energy affects how molecules are fragmented and directly influences the mass spectra, sensitivity, and structural information obtained from the analysis. The standard and most widely used electron energy in EI is 70 eV.
HOW IT WORKS
Inside the ion source of the mass spectrometer, a filament emits a beam of electrons at a set energy level (e.g., 70 eV).
As analyte molecules exit the GC column and enter the ion source, they collide with the high-energy electrons.
This collision causes the molecules to lose an electron, forming positive molecular ions (M⁺•), and often fragmenting into smaller ions.
The resulting mixture of ions is sent into the mass analyzer, where they are separated by their mass-to-charge ratio (m/z) and detected.
IMPACT ON PERFORMANCE
Spectral Consistency: 70 eV provides reproducible fragmentation patterns, which match standard mass spectral libraries (e.g., NIST). This ensures reliable compound identification.
Fragmentation Control: Higher electron energy causes more fragmentation, which can help identify compound structures. Lower energy produces less fragmentation, preserving the molecular ion peak.
Ionization Efficiency: The electron energy impacts how many ions are formed. Too low = weak signals; too high = excessive fragmentation or signal loss.
Molecular Ion Detection: In cases where identifying the molecular ion is critical (e.g., for unknowns or structural confirmation), lowering the electron energy (e.g., to 20–40 eV) can be helpful.