What is %ee? Chiral HPLC?

What is %ee? ee is the abbreviation of enantiomeric excess, a measurement of purity of chiral substances. As an example, there is a sample that contains enantiomers: X and Y. The optical rotation for X is 60 degrees. However, when X exists with Y, the optical rotation becomes 30 degrees. So, how much is the %ee? From this case, we also can conclude that X is available in a higher amount than Y because X that rotates while Y is turned off. If the X optical rotation becomes 30 degrees, means that the remaining 30 degrees were canceled by Y optical rotation. Then to calculate the %ee of X, let's see the formula below: 

The calculation above for determining the %ee is obtained from optical rotation measurement. Besides that, %ee can be also calculated from the measurement using chiral HPLC. We just need to compare the area of the enantiomers. For example:

When using HPLC, if we use column with conventional reversed phase, we only can separate the diastereomers. If we want to separate enantiomers, we require chiral stationary phase. To know the difference between diastereomers and enantiomers, see this post. 

How can Chiral HPLC separate enantiomers? The HPLC uses a single-enantiomer stationary phase. Therefore, the separation depends on the affinity of the analyte to the single-enantiomer stationary phase. So, the enantiomers will exit the column at different time.  

The chiral stationary phase usually be prepared by attaching the chiral compound to the surface of achiral compound such as silica gel. 

An example of chiral column is CHIRALPAK and CHIRAL CEL, these are a well known chiral chromatography column produced by Daicel. Daicel had been developing these products since 1980s.