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 important to note that the mass spectrometer is under very high vacuum produced by at least two strong pumps. This removes air and other background molecules to reduce the background signal. In the ion source, the compounds in the sample are bombarded by an electron beam created by a wire filament with current flowing through it at a high voltage (70 eV). This ionizes the molecules.
This type of ionization is known as electron impact ionization. Because the standard electron energy of 70 eV used to ionize the molecules is very reproducible from one instrument to another, there is a high degree of reproducibility and spectral libraries have been developed. These libraries can be used to identify unknown compounds. Since a given compound will elute off a specific GC column at a known time, and will be ionized into known ions, GC/MS can positively identify many compound(s).
