Enantiomeric excess of a mixture is the absolute difference between the percent composition of the major enantiomer and that of the minor enantiomer. The resulting value is numerically equivalent to the mixture's optical purity.
A hypothetical example is in a mixture with one substance of 75 percent, and the other substance is 25 percent. The difference, or enantiomeric excess, is 50 percent. Enantiomeric excess, abbreviated "ee" in formulas, is also calculated by dividing the observed optical rotation of a substance by the optical rotation of the pure enantiomer, then multiplying by 100 to obtain a percentage. Enantiomers are a type of stereoisomers, compounds with the same molecular formula and same structure, that are mirror images of each other. Optical purity is a concept of organic chemistry that measures a substance's optical activity, which is a solution's ability to rotate plane-polarized light.