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.
The 3,5-dimethylphenylcarbamates of amylose and cellulose are the most useful polysaccharide-based CSPs. They are commercialized
as "Chiralpak IA" and "Chiralpak IB," respectively (Daicel Chemical Industries, Osaka, Japan) (see Figure 1). Other polysaccharide-based
chiral selectors include the 3,5-dichlorophenylcarbamate of cellulose, which was first introduced by Okamoto as a coated CSP.
Although it had limited utility because of its solubility in hexane-based mobile phases, it demonstrated sufficiently interesting
selectivity when used with pure alcohol mobile phases to be of interest as an immobilized phase. Daicel introduced a CSP of
similar structure, immobilized by a proprietary technology, as "Chiralpak IC" (see Figure 1).
Figure 1: Structures of the immobilized chiral stationary phases.
Selectivity and separations