Since about 1975 it is possible to separate enantiomers chromatographically and to determine their amounts quantitatively, using so-called chiral columns in liquid or gas chromatography. Alternatively in certain NMR experiments both enantiomers can be observed and quantified side-by-side. By these methods the enantiomer ratio is immediately obtained. However, the ratio is not normally given, rather the quantity enantiomeric excess, ee, defined as follows:

ee(%) = 100·([R]-[S])/([R]+[S]) ,

This format of result has advantages:

• Numbers are between 0 and 100% as for o.p., and results obtained for an identical sample by the two methods numerically agree (in the best case).

Example:   [R] : [S]  =   70 : 30

• ee is numerically more robust against measurement errors than the enantiomer ratio. If, for example, an enantiomer ratio of 99.5:0.5 is measured, while the true value is 99:1, then the ratio would be given as 199:1 instead of 99:1, the ee as 99% instead of 98%.