A dating technique used in archaeology based on the relative amounts of the optical isomers of an amino acid in a sample. In most organisms, the l-isomer of the amino acid is the one produced by metabolism. When the organism dies, this isomer slowly converts into the d-form, and eventually an equilibrium is reached in which the two forms are present in equal amounts. Measuring the proportions of the l- and d-forms in a sample can, in principle, give an estimate of the time since death. Not all amino acids racemize at the same rate, and the rate of the process depends on other factors such as moisture and temperature. Most work has been done using leucine or aspartic acid.

A particular application in forensic science involves measuring the d/l ratio of aspartic acid in the dentine of teeth. Once a tooth has fully formed, the dentine is isolated by the enamel and then racemization takes place in the living subject at a fairly constant temperature and moisture level. Measuring the ratio gives a fairly good estimate of the age of the subject (rather than the time since death).