A novel concept for phase equilibria measurements of chiral systems using polarimetry and Raman spectroscopy

A. Butka1, C. Pauls1, K. Leonhard1 and A. Bardow1

1Chair of Technical Thermodynamics, RWTH Aachen University, Germany

Keywords: Raman spectroscopy, polarimetry, chiral separation
property: distribution coefficient
material: chiral substances

Within this work, a novel optical measurement technique for the investigation of liquid-liquid equilibria of chiral systems is developed.

Most chiral chemicals are synthesized as racemates, i.e., with equal ratio of both enantiomers. Since the two enantiomers usually differ in pharmacological effects or levels of activity, their separation is of vital interest and gained considerable attention in several industries[1]. For chiral separations, techniques such as liquid-liquid extraction[2] and pertraction[3] are regarded as promising approaches. Information on the liquid-liquid equilibria of the enantiomeric systems is very important to understand and optimize these separation processes.

The presented measurement technique is based on a combination of polarimetry and Raman spectroscopy. The Raman spectra yield the concentrations in the two layered phases from which the distribution coefficients of the components can be determined. However, for two enantiomers the Raman signals are practically indistinguishable. So an additional enantiospecific quantity is needed allowing the determination of the distribution coefficients of the enantiomeric species. Therefore in the suggested setup the optical activity is measured by means of polarimetry.

This presented set-up allows the in-situ analysis of both chiral and achiral compounds simultaneously. To prevent fluorescence an excitation wavelength of approx. 765 nm is used. In addition it is possible to acquire kinetic data using an optical multi-channel analyzer as detection unit.

This novel measurement technique is used to investigate the equilibria occurring in pertraction processes. The principle of this separation technique is the complex formation between one enantiomer and a selective chiral molecule (carrier). First measurement results using a suitable enantiomer-carrier combination are presented.

  1. T.J. Ward, Anal. Chem. 74, pp. 2863-2872 (2002)

  2. B. Schuur, B.J.V. Verkuijl, A.J. Minnaard et al., Organic & Biomolecular Chemistry 9, pp. 36-51( 2011)

  3. L. Gou, S. Robl, K. Leonhard, H. Lorenz, M. Sordo, A. Butka, S. Kesselheim, M. Wolff, A. Seidel-Morgenstern, K. Schaber, Chirality 23, pp. 118-127 (2011)

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