(401au) Adsorption Rate Constant and Equilibrium Constant in Chiral Separation for Trans-Stilbene Oxide, Linalool and Ibuprofen By Supercritical Fluid Chromatography

The enantiomers of chiral molecules have many different biological activities in R and S forms, and their separation is essential in the pharmaceutical, nutraceutical, and food industries [1]. Supercritical fluid chromatography (SFC) has been attracting attention for separating enantiomers with scCO2 by adding the small amount of an organic solvent. The mobile phase properties can be tuned by adding organic species as a modifier [2] as well as by changing temperature and pressure. However, while the elution time of an analyte in SFC is significantly affected by the analytical conditions such as temperature, pressure, modifier species and the composition, the effects of analytical conditions on retention factors have not been clarified well. It is difficult to find the optimum analytical conditions. A predictive method or a predictive correlation of retention factors as a function of analytical conditions is required. In this study, SFC separation was performed for the three kinds of enantiomers, i.e., trans-stilbene oxide (tSO), linalool, and ibuprofen, and the chromatograms were analyzed using the moment method [3].

As a result, the adsorption equilibrium constant K_A, defined as the rate constant ratio of desorption k_d to adsorption k_a were almost constant, independent of the flow rate of the mobile phase. On the other hand, the adsorption rate constant k_a can be approximated by a linear curve passing through the origin and it can be said that there is a tendency to depend strongly on the flow velocity with correlation coefficients R² = 0.99 or higher. As the flow velocity increases, since the number of analyte molecules adsorbing and desorbing passing through the silica gel surface increased, this resulted in increasing k_a and k_d. Thus, K_A did not depend on the flow rate and were almost constant for each component.

[1] Min, L. et al. J. Sep. Sci., 36, 3093 (2013).

[2] Horikawa, Y. Chromatogr., 32, 153 (2011).

[3] Wakao, N., Kaguei, S., Heat & Mass Transfer in Packed Beds, Gordon & Breach,

New York (1982).