Mass spectrometry identifies a chemical by measuring characteristic mass fragments produced as the chemical is ionized. Sample molecules are ionized with an electron beam, and the resulting molecular ion and fragment ions then pass through a mass analyzer where their masses are measured. A separation technique such as gas chromatography is typically added to the front end of the MS to separate mixtures into individual components. Mass spectrometry is generally considered the benchmark for identification of unknown organic chemicals because it is highly sensitive and selective, and mass spectra are easily searchable against vast reference databases.