Elektronenimpactionisatie

Elektronenimpactionisatie, or electron impact ionization (EI), is a hard ionization technique used in mass spectrometry to convert neutral molecules into gas-phase ions by collision with high-energy electrons. It is commonly employed in conjunction with gas chromatography (GC–MS).

Mechanism: The ionization occurs in an electron ionization source where a filament emits electrons; electrons are accelerated to about 70 eV and directed at the neutral analyte molecules. The collision ejects one electron from the molecule, generating a molecular radical cation (M+•). The large excess energy (relative to the molecule's ionization energy) commonly induces extensive fragmentation, resulting in a spectrum of fragment ions.

Spectral characteristics: The EI spectrum typically contains a relatively intense molecular ion peak, though for many compounds it is weak or absent, and a series of diagnostic fragment peaks. Fragment patterns are highly reproducible, enabling library matching and compound identification. The method is particularly well suited for volatile and thermally stable small to medium-sized molecules.

Instrumentation and parameters: The EI source uses a hot filament to create electrons; after acceleration, the electron energy is typically fixed at 70 eV to standardize fragmentation patterns. The ion source operates under high vacuum; GC inlet interfaces the liquid or solid into the gas phase.

Comparison and limitations: EI is considered a hard ionization method, in contrast to softer methods such as electrospray ionization (ESI) or chemical ionization (CI), which produce little fragmentation. While EI provides rich structural information, it is less suitable for large, labile, or nonvolatile molecules. Other techniques exist for non-volatile analytes.