Although the first enantiomeric separations were carried out during the early past century, it was not until 1973 that the first generally useful chiral stationary phase (CSP), cellulose triacetate, was developed. This phase was used primarily for preparative separations until the work of Professor Pirkle in the United States and Professor Okamoto in Japan made analytical CSPs readily available. Although the Pirkle phases provided good separations, the polysaccharide-based CSPs developed by Okamoto had very wide applicability.
It was subsequently found that the majority of enantiomeric separations could be achieved using just four different derivatives—two based on cellulose and two based on amylose. These polysaccharide phases were prepared by first derivatizing native amylose or cellulose and then coating these chiral polymers on a silica-gel base. This preparation method led to phase instability to certain organic solvents in which the polymers are partially or fully soluble.
Many methods have been developed to allow the immobilization or chemical bonding of the polysaccharide-based polymers to the silica support (1). Methods include cross-linking the polymer by use of diisocyanates (2), unsaturated groups that are subsequently polymerized (3, 4), silyl derivatives (5), direct bonding of the polysaccharide to silica by enzymatic or chemical routes before derivatization (6), and cross-linking of the chiral polymer by photochemical or free-radical reactions (7). All of these methods produce somewhat different products with different chromatographic properties.Immobilization of the chiral polymer to the silica support has several benefits. The CSP is not destroyed on contact with a solvent that swells or dissolves (even partially) the polymer. The sensitivity of the first-generation CSPs to even small concentrations of such solvents often required the use of dedicated equipment to eliminate all chances of accidental introduction of a "forbidden" solvent.
Immobilization of the chiral polymer thus brings a new generation of polysaccharide-based CSPs into line with the stability of other high-performance liquid chromatography (HPLC) phases (such as C18 and CN), which means a significant extension of the column lifetime and the elimination of sudden column failure because of an accident. Others benefits of the immobilization process are that otherwise difficult separations can be achieved by using solvents that previously could not be used with the coated phases and that new selectivity can be developed by using previously unusable chiral selectors.
Selectivity and separations