A quadrupole mass analyzer is used for the determining of the mass of ionized compounds. Quadrupole mass spectrometers used in liquid chromatography are most often equipped with electrospray (ESI) or atmospheric pressure chemical ionization (APCI) ion sources. A separation technique such as 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.