General Properties of Chiral Stationary Phases

The surface of the adsorbent in a chiral stationary phase (CSP) often contains two different types of adsorption sites [103, 110, 111], type-I and type-II sites. In most chiral columns only type-II site is chiral selective, but the saturation capacity for most columns is very low resulting in that when the sample size is increased, the selective retention mechanism is rapidly overloaded and the chiral separation disappears. The type-II saturation capacity is also called the true chiral saturation capacity. 

The type-I sites have, in protein based CSPs, identical behavior toward the two enantiomers, and cannot distinguish between them. Many columns contain mostly type-I sites. On type-I sites all possible molecular interactions, between the analyte molecules and atoms or groups of atoms belonging to the adsorbent surface, take place. These interactions can originate from the nonchiral parts of the protein and/or from the adsorbent (silica) matrix. The energies of each interaction on type-I sites are small. The other type of adsorption sites have, in protein based CSPs, much higher adsorption energy and are enantioselective (chiral). These sites, type-II sites, are responsible for the enantiomeric separations. On most CSPs the type-II sites are relatively few. 

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