Step 3: Electron ionization (formerly known as electron impact ionization)
Upon exiting the GC column, the analytes are introduced to a hostile environment—the mass spectrometer's ion source. It is important to note that the mass spectrometer is under a 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. This means that different labs with different instruments can share spectral libraries. 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 be ionized into known ions, GC/MS can positively identify many compounds.
