Evaluation of laser-based polarimetry for the determination of enantiomeric excess (ee) at the extremes of the ee scale
by Linder, S. W.; Yanik, G. W.; Bobbitt, D. R.
The development of new approaches for assessing enantiomeric purity is an area of critical need in the pharmaceutical and biotechnological industries. Recent work has shown that two enantiomers, although virtually identical physically, may exhibit widely divergent physiological properties. In extreme situations, the contaminant enantiomer may itself be physiologically active leading to significant health consequences. Laser-based polarimetric detection, in combination with a chiral-selective separation mode, has been shown to be capable of assessing enantiomeric purity in situations where the two enantiomers exist as either a racentic or near racemic mixture. The laser-based polarimetric detector responds directly to the intrinsic optical activity of the eluting analyte. The sign of the response can provide unique information with respect to which enantiomer, of the enantiomeric pair is eluting. The detector has been shown to be sensitive and linear under analytical conditions. However, this combined methodology has not been rigorously evaluated under conditions of low enantiomeric excess (ee). Such situations would be encountered, for example, when one is attempting to characterize a pharmaceutical preparation marketed as enantiomerically pure. In this work, laser-based polarimetric detection, in combination with a chiral-selective separation mode will be evaluated under conditions at the extreme of the ee scale. Under such conditions, this approach will be shown to be capable of making measurements below 0.2%ee, even in the presence of a 1000-fold excess of the major enantiomer. UV/Vis detection, evaluated under identical conditions, could not accurately predict the ee even when baseline resolution of the enantiomers was possible. This was attributed to the presence of absorbing impurities that can co-elute with the minor enantiomer. Since the polarimetric detection system responds only to optically active analytes, these impurities do not affect the quality of the enantiomeric measurement.