Selectors and Stationary Phases

Chromatographic enantiomer separation can be performed in two different ways. The so-called direct method is based, as described above (Section 2.6.2.4), on the formation of a diastereomeric transient complex between the chiral selector and the analyte. The selector can be present in various forms in the column, either linked directly to the silica particle, or as a coating on the carrier particles. However, it can also be dissolved in the mobile phase as an additive. In contrast, the indirect 

According to their chemical structures, CSPs can be divided into three different groups. A multitude of chiral stationary phases is derived from (modified) natural or synthetic polymers, e.g., the polysaccharides, proteins or polyacrylamides. A second type of selectors is based on large chiral ring systems, such as cyclo-dextrins, macTocycKc antibiotics, and crown ethers. The last group comprises moleailes of small and medium size, such as amino acids and their derivatives, alkaloids, and fuUy synthetic selectors. 

Chiral stationary phases can exist in different forms [10] (see Fig. 8). Some selectors can be used as particulate phase materials, such as polymeric cellulose triacetate. Polymeric cellulose and amylose derivatives are often coated onto silica carrier particles so that only 20% of the CSP consists of the chiral selector. This combination of stationary phase and chiral polymer combines good chromatographic properties (due to the homogeneous particle size distribution) with a high density of chiral adsorption sites in the polysaccharide derivatives. Another approach is selected for the so-called brush-type CSPs. In these, the chiral selector is covalently bound to the surface of the silica particles. These phases show high chemical inertness and allow the use of a multitude of different mobile phases. 

Alkane/alcohols Alkane / tetrahydrofuran Alkane/dioxan Chlorinated solvents 

Alkane/alcohols Alkane/chlorinated solvents Acetonitrile/ethyl acetate 

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