Understanding the Concept:
Electron Ionization (EI) is a hard ionization technique used in mass spectrometry. In an EI source, a current passing through a tungsten or rhenium filament emits high-energy thermal electrons, which are accelerated toward an anode across an applied voltage potential grid.
Step 1: Determine the lower bound energy boundary
To knock an electron out of a neutral gas-phase molecule (\(\text{M}\)) to form a radical cation (\(\text{M}^{+\bullet}\)), the incoming electron must possess energy equal to or greater than the molecule's first ionization potential:
\[
\text{M} + e^-_{\text{fast}} \longrightarrow \text{M}^{+\bullet} + 2e^-_{\text{slow}}
\]
For most volatile organic molecules, this ionization potential threshold falls between 7 eV and 15 eV. Therefore, an energy source lower than 10 eV would fail to ionize the majority of analyte molecules.
Step 2: Analyze the standard operating energy selection
If the electron energy is increased above the ionization threshold, the efficiency of ionization rises significantly. The transition probability reaches a broad plateau between 70 eV and 100 eV. At these higher energies, the wavelength of the fast-moving electrons matches typical molecular bond lengths, maximizing the kinetic energy transfer during collisions. This excess internal energy also drives reproducible fragmentation pathways, creating a structural "fingerprint" spectrum.
To ensure consistent, reproducible fragmentation matching across standard reference libraries (such as the NIST library), mass spectrometers almost universally operate at a standard setting of 70 eV, which fits directly within the 10 eV to 100 eV range.