Loading studies, which involve the injection of a range of sample loads to evaluate the isotherms of adsorption of the solutes,
demonstrated major differences among the solvents. The adsorption behavior of the α-methyl-α-phenylsuccinimide enantiomers
depended considerably on the nature of the mobile phase. In some cases (chloroform and acetonitrile/2-propanol), the separation
collapsed rapidly on loading with the second peak merging with the first at relatively low load. In the case of MTBE, the
second component peak moved to longer retention time at moderate load and only began to merge with the first eluting component
at high loadings. The high loading capacity in this system led to high productivity for the separations, both in HPLC and
SMB mode. The productivity of the separations (in kilograms of enantiomer produced per kilogram of CSP per day) is shown in
Table III for each of the mobile phases.
Separation in ethyl acetate also had a high loading capacity, which, combined with the high solubility in this solvent, resulted
in the highest production rate for both HPLC and SMB. Thus, solubility is only part of the answer for preparative separations.
The effect of the solvent on the loading capacity, presumably through its interactions with the stationary phase, is of great
importance in preparative separations. This result has been observed only by virtue of the solvent stability of the immobilized
polysaccharide-based CSPs.
Conclusion
The new generation of CSPs, based on immobilized polysaccharides, has proven to be versatile in development of enantioselective
separation methods, requiring relatively few stationary and mobile phases for success. The range of selectivity of the CSPs
is extended by using solvents hitherto forbidden for the older generation of coated polysaccharide stationary phases not only
for HPLC separations but also for SFC methods. Furthermore, the nature of the solvents in preparative separations may have
a profound influence on both the solubility of the samples and on the loading capacity of the preparative columns. The extended
range of solvents now accessible with the availability of immobilized CSPs extends the potential of preparative enantioselective
chromatography to new and larger-scale separations.
Geoffrey B. Cox is vice-president of technology at Chiral Technologies, 800 N. Five Points Rd., West Chester, PA 19380, tel. 610.594.2100,
fax 610.594.2325, gcox@chiraltech.com
.
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