Measuring Chiral Purity

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

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

Enantiomeric excess is a measure of chiral purity.  It is defined as follows;

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.