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The analyte, supposedly composed of two enantiomers (R and S), is chromatographed on a chiral stationary phase composed of a single enantiomer (e.g. R‐type). Between the analyte enantiomers and the stationary phase, reversible interactions of the R (analyte)/R (stationary phase) and S (analyte)/R (stationary phase) are created, with slightly different stabilities. This leads to distinct migration speeds for the two enantiomers. Finally, this leads to two peaks on the chromatogram for this single analyte (Figure 2.9), whose areas reflect the abundances of the R and S forms, from which the analyte’s optical purity is calculated.

The optical purity of the analyte refers to its enantiomeric excess (e.e.), calculated from the following relationship, where S_R and S_S refer to the areas of the two enantiomer peaks:

(2.2)

According to this formula, a pure chemical compound present as a racemic mixture will yield two peaks equal in size, each corresponding to an enantiomer. Its optical purity will thus be equal to 0.

Figure 2.9 Example of a separation with a chiral phase which contains grafted cyclodextrins. The use of a chiral column to separate a racemic mixture of compounds (alcohols 2 and 4). This chromatogram in isothermal mode enables the calculation of retention indexes for the separated compounds.