Electron ionization (EI) was the first ionization technique developed for mass spectrometry (MS) and has since been the most commonly used hard ionization technique . The robust ionization energy (normally at 70 eV) ionizes and fragments analyte molecules and produces highly reproducible mass spectra with certain mass-tocharge-ratio (m/z) distributions which are searchable in spectral libraries (e.g., National Institute of Standard and Technologies [NIST]) . Although fragmentation complicates analysis, electron ionization mass spectrometry (EIMS) still provides valuable structural and compositional information on unknown compounds , , . Readers not in the