WHAT IT IS
The electron impact ion source uses high-energy electrons to bombard neutral molecules, causing them to lose an electron and form positively charged ions. This ionization process is characterized by the extensive fragmentation of the target molecules, resulting in a unique mass spectrum that serves as a "fingerprint" for compound identification. It is one of the most widely used ionization techniques in mass spectrometry.
HOW IT WORK
Sample Introduction – The sample is vaporized and introduced into the ion source as a gas. Solid or liquid samples require prior volatilization.
Electron Beam Generation – A filament, usually made of tungsten or rhenium, emits electrons when heated. These electrons are accelerated through a potential difference.
Molecular Ionization – The electron beam interacts with the neutral sample molecules, knocking out an electron and forming positively charged molecular ions:M e−→M 2e−
Fragmentation – The excess energy imparted during ionization causes the molecular ions to fragment into smaller ions and neutral fragments.
Ion Focusing and Analysis – The resulting ions are focused into a beam and directed into the mass analyzer, where they are separated based on their mass-to-charge ratio (m/z).
KEY FEATURES OF ELECTRON IMPACT IONIZATION
Hard Ionization: EI is considered a "hard" ionization technique due to the high energy involved, which results in extensive molecular fragmentation.
Fragmentation Patterns: The fragmentation produces a reproducible pattern unique to each compound, aiding in structure elucidation and identification.
High Sensitivity: The method is highly sensitive for volatile and thermally stable compounds.
Limited Molecular Ion Presence: Molecular ions (the intact ionized molecule) may be weak or absent in the mass spectrum due to extensive fragmentation.
ADVANTAGES
Reproducible Spectra: The standardized 70 eV ionization energy ensures consistent fragmentation patterns across instruments and laboratories.
Extensive Library Support: Thousands of compounds have EI mass spectra stored in databases, facilitating rapid identification.
High Sensitivity for Volatile Compounds: Effective for analyzing low concentrations of thermally stable, volatile molecules.
Structural Information: The fragmentation provides valuable clues about the molecular structure of the analyte.
CHALLENGES AND LIMITATIONS
Not Suitable for Non-Volatile or Thermally Unstable Compounds: Samples must be gaseous or capable of being vaporized without decomposition.
Excessive Fragmentation: Extensive fragmentation can obscure the molecular ion peak, complicating the determination of the molecular weight.
Limited Application to Large Biomolecules: EI is unsuitable for large, non-volatile compounds such as proteins or polymers.
Dependency on Volatility: Requires analytes to be thermally stable enough to enter the gas phase without degradation.