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The page below is a sample from the LabCE course Drug Testing Methods in the Clinical Toxicology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Electron Ionization (EI)

Electron ionization (formerly know as electron impact ionization)
Upon exiting the GC column, the analytes are introduced to a very hostile environment--the ion source of the mass spectrometer. It is very important to note that the mass spectrometer is under very high vacuum produced by a turbomolecular pump or diffusion pump aided by a backing pump known as the rough pump. This minimizes background signal. In the ion source, the analytes are bombarded by an electron beam created by a wire filament with current flowing through it at 70 eV. 70 eV is more than enough energy to ionize molecules. The impact of electrons to the molecule creates positive radical ions by knocking off an outer shell electron. This radical cation is referred to as the molecular ion. Radical cations are very unstable and tend to fragment into charged and neutral fragments.
This type of ionization is known as electron impact ionization. Because of the standard electron energy of 70 eV used to ionize the molecules the mass spectra is very reproducible from one instrument to another. Because of this high degree of reproducibility, spectral libraries have been developed. It is the most common ionization technique employed in the analysis of drugs in the clinical laboratory.