Ultraviolet–visible absorption
The molar absorptivity coefficient can be calculated by measuring the absorption of a sample of a known concentration and known pathlength.  The absorptivity coefficient units are dependent on the pathlength and concentration units.  Molar absorptivity units are L mol-1 cm-1 when the concentration units are molarity.  The absorptivity is a measure of how strongly a molecule absorbs light at a particular wavelength.  Once this constant is determined, unknown concentrations can be determined from this relationship.