In stereochemistry, enantiomeric excess (ee) is a measurement of purity used for chiral substances.
If one knows the moles of each enantiomer produced then: Enantiomeric excess is used as one of the indicators of the success of an asymmetric synthesis.
This has led to informal use the two terms as interchangeable, especially because optical purity was the traditional way of measuring enantiomeric excess.
However, other methods such as chiral column chromatography and NMR spectroscopy can now be used for measuring the amount of each enantiomer individually.
For example, The term enantiomeric excess was introduced in 1971 by Morrison and Mosher in their publication Asymmetric Organic Reactions.
It has been suggested that the concept of ee should be replaced by that of er which stands for enantiomeric ratio or er (S:R) [6] or q (S/R) because determination of optical purity has been replaced by other techniques which directly measure R and S and because it simplifies mathematical treatments such as the calculation of equilibrium constants and relative reaction rates.