Page 307 - Chiral Separation Techniques

Chiral Separation Techniques: A Practical Approach, Second, completely revised and updated edition
Edited by G. Subramanian
Copyright © 2001 Wiley-VCH Verlag GmbH
ISBNs: 3-527-29875-4 (Hardcover); 3-527-60036-1 (Electronic)
12 Sub- and Supercritical Fluid Chroma-

tography for Enantiomer Separations *

Karen W. Phinney

12.1 Introduction

Growing recognition of the role of chirality in biological activity and subsequent
regulatory guidelines for chiral drug development have spurred tremendous growth
in chiral separations technology. Applications of this technology range from phar-
macokinetic studies during drug development to assessment of asymmetric synthe-
sis strategies for the expanding list of drugs marketed in single-isomer form. Chro-
matographic methods have proven to be the most reliable and versatile analytical
techniques for measurement of stereochemical composition, although capillary elec-
trophoresis (CE) has become a viable alternative for certain applications [1, 2]. Chi-
ral chromatographic methods are not limited to analytical-scale applications. Prepar-
ative scale chromatography is often the most efficient approach to resolve sufficient
quantities of enantiomers for drug discovery activities [3, 4] .
Development and commercialization of chiral stationary phases (CSPs) for liquid
chromatography (LC) revolutionized the field of enantiomer separations. Suddenly,
the analyst could choose from a variety of immobilized chiral selectors to achieve
direct enantiomer resolution and significantly reduce the need for chiral derivatizing
agents [5]. However, a number of limitations of this approach also became apparent.
First, prediction of CSP selectivity remains elusive, and method development often
requires a trial-and-error approach. Second, successful chiral resolution often occurs
within a narrow range of mobile phase compositions, and parameters that yield suc-
cessful enantiomeric separation may not be suitable for resolving the enantiomers
from other sample components. Finally, chromatographic efficiency of CSPs tends
to be inferior to the efficiency of nonchiral stationary phases for LC, and broad chro-
matographic peaks can hinder quantification of desired analytes [1].
Supercritical fluid chromatography (SFC) provides a means of minimizing the
limitations of CSPs developed for LC while retaining the impressive chiral selectiv-
ity that has been achieved through the evolution of CSPs during the past two decades
[6, 7]. The use of supercritical fluids as eluents for chromatographic separations was

* Contribution of the National Institute of Standards and Technology. Not subject to copyright.