Determination of enantiomeric excess using the ultraviolet-circular dichroism and the high-performance liquid chromatography-circular dichroism methods

Abstract

Two useful numerical methods using ultraviolet (UV) and circular dichroism (CD) spectroscopies are proposed to determine enantiomeric excess (e.e). An algorithm is also proposed to generate self-consistent pure R and S enantiomer reference spectra. After all pure reference spectra are generated, a simulated annealing algorithm is applied to minimize the mismatch between the experimental spectra and the spectra after a least-squares fit. Optimal factors for R and S enantiomers are then used to determine e.e. The ultraviolet-circular dichroism (UV-CD) method uses the combined UV and CD spectra in a composite form, while the high-performance liquid chromatography (HPLC)-CD method only employs the CD spectra with the total concentrations of R plus S enantiomers provided by HPLC using a non-chiral stationary phase. Both methods were successfully tested on mixtures with known composition and then applied to real experimental data (unknown compositions). Compared with the UV-CD method, the results show that excellent results are more readily obtained using the HPLC-CD method. With the systems studied, the latter usually provided outstanding estimations of e.e with low error percentages.