Amino acid dating offers an alternative to radioactive carbon dating for ascertaining the age of biological samples. 14C dating has a practical limit of 10 half-lives of 14C, and amino acid dating offers reliable age estimates for samples up to 400 000 years old.
The method relies on the extent of racemixization of chiral amino acids found in ancient biological samples. In living organisms, amino acids have a uniform L- configuration and are optically pure, this state requiring a complex biochemical maintenance process or the acids revert to an equilibrium of L- and D- forms.
After an organism dies, this maintenance process clearly stops, and a gradual thermal equilibrium process begins. The extent to which the acids have become racemic provides a reliable way to date the time elapsed since death.
The rate of racemixization varies between amino acids. The half-life of aspartic acid at 25 degrees centigrade and pH7 is 3 000 years, for alanine, it is 12 000. Racemixization rates also depend enormously on temperature, for example at zero degrees centigrade the half-life of aspartic acid increases to 430 000 years, so it is important to know the temperature at which the material was stored. Fortunately, the below-ground temperature at a given depth and climate tends to be fairly uniform and can be estimated with reasonable accuracy, but non-reliance on temperature estimates is a major point in favour of 14C dating. Amino acid dating, however, requires much smaller samples and can cover a much wider time span, although for samples less than 50 000 years old a combination of methods is often used for increased accuracy.