When we perform an asymmetric reaction or extract a chiral molecule from some organism (the chiral pool) we need to measure its enantiopurity. There are a number of approaches to this including Polarimetry, Chiral Chromatography and NMR techniques. Chiral chromatography is currently the most used method based on its reliability and accuracy.
Chiral chromatography is a method of separating chiral molecules . Once separated the amount of each can easily be determined. Chiral columns for GC (gas chromatograph) or HPLC (High Pressure Liquid Chromatography) are available. A chiral column has other chiral molecules bound to its stationary phase making it a chiral environment.
Recall that enantiomers only behave differently in a chiral environment. Chiral stationary phase is a chiral environment. Each enantiomer of a mixture will interact with the chiral surface differently. Together the chiral surface and chiral molecule being separated form a diastereomeric complex. Since diastereomers have different properties they are separated. Chiral chromatography is not required for separating diasteromers.
Below is a reversed phase chiral HPLC chromatogram of a racemic mixture of warfarin (anti-coagulant used in the prevention of thrombosis and thromboembolism,). The warfarin mixture is separated into its R & S enantiomers at 4.65 and 5.22 minute retention times.
Enantiomeric excess is a measure of chiral purity. It is defined as follows;
%ee = (R-S)/(R+S)*100
R and S are the amount (%, or Concentration or mass fractions) of the R and S enantiomers. You would get these values from Polarimetry or chromatography.