Because enantiomers can have different properties when they interact with other chiral systems, such as biological ones, it is important to have a measure of the “optical purity”.
The enantiomeric excess (ee) is a measure of composition of a mixture of enantiomers. It can be calculated using formula:
ee = ((R-S)/(R+S)) with R and S, respectively amounts of enantiomer (R) and (S).
It’s an important characteristic since it’s difficult to obtain specifically one enantiomer for the chemist. Indeed, contrary to reactions existing in the nature, this class of compounds is often obtained as a mixture of each form in a laboratory. So, an ee of 100% correspond to a perfectly pure enantiomer whereas ee = 0% describe a mixture with enantiomers of each configuration in an equal amount.
Enantiomeric ratio (er) is a proportion of enantiomer in a mixture. For the historical reasons more often is used measure of Enantiomer excess.