Chiral phases

Chiral stationary phases are the most common first approach for enantiomer separation. To be able to separate racemic mixture of stereoisomers , the chiral phase has to form a diastereomeric complex with one of the isomers, or has to have any other type of stereospecific interactions. Detailed mechanism of chiral recognition is not well understood yet.

A large number of different bonded stationary phases are commercially available. Basically we can distinguish four major types of chiral bonded phases:

1. Donor-acceptor (Pirkle) type

Those bonded ligands should have three possible points of interactions with analyte molecules. These interactions can include hydrogen bonding, dipole-dipole interactions, or charge transfer. Obviously, these ligands have to be a stereoisomeric. Due to the type of interactions these adsorbents can be used with normal phase eluents.

2. Chiral cavity phases

Cavity type phases such as cyclodextrins bound to the surface of silica through a spacer are act like a cone shape cavity open at both ends with a relatively hydrophobic interior. Chiral separation will occur if one of the analyte stereoisomers fits exactly into the cone and its functional groups can interact with the hydroxyl groups at the wide openings of the cavity.

These type of chiral adsorbents are used in the reversed-phase HPLC mode.

3. Helical polymer phases

Some helical polymers such as cellulose esters whose chirality comes from helicity also can be used for enantiomer separations. It is thought that  the mechanism of separation on these columns involves a combination of attractive interactions and inclusion of the analyte in a chiral cavity.

4. Ligand exchange columns

Silica surface with bonded chiral moiety, such as proline usually treated with copper salts. This treatment leads to the formation of the reversible complex with copper. Chiral analytes participate in the ligand exchange process. Depending on the type of enantiomer (R or S) formed, the complex may be more or less stable which leads to the chiral separation.