Influence of organic solvent on the behaviour of camphor and α-pinene enantiomers in reversed-phase liquid chromatography systems with α-cyclodextrin as chiral additive

Reversed-phase liquid chromatography has been applied in order to gain insight into the α-cyclodextrin (α-CD)–solute complexation process, which occurs in the aqueous mobile phases containing a secondary achiral modifier. The model compounds tested were (±)-camphor and (±)-α-pinene. Methanol, ethanol, and 1 or 2-propanol were used as secondary modifiers. Retention factors and enantioseparation factors have been determined on a RP 18 stationary phase as a function of the α-CD concentration, secondary modifier content, and temperature changes. The shortest retention and the best separation of studied compounds were achieved for aqueous–methanol eluents. Apparent stability constants in various binary aqueous–organic solvent mixtures have been evaluated for α-CD complexes of camphor enantiomers. Using the competition concept, values for the stability constants in pure water have been calculated. It has been found that: (1) the quotient of the stability constants for both enantiomers, denoted as absolute enantioselectivity E, always remains constant at a fixed value (E≅1.9), which may indicate that the complex composition does not change, (2) only the first step in the complexation process is altered by changing the solvent, which does not seem to affect the separation of the enantiomers, (3) the remarkable enantioselectivity that is observed results from the second step in the complexation process, (4) enthalpy changes are much more favourable for camphor–α-cyclodextrin complex formation than for the transfer of camphor to the stationary phase, which means that complexation dominates over adsorption and retention is shorter at lower temperatures, (5) the difference in free energy changes of complexation (ΔΔG) between the enantiomers of camphor is about 1.5 kJ/mol at 20°C.