Density dependence of retention factors of trans-stilbene oxide for chiral separation by supercritical fluid chromatography

Retention factors for two enantiomers of trans-stilbene oxide, k₁ and k₂, were measured with a chiral AD-H column using two syringe pumps to feed CO₂ and methanol as a co-solvent at various temperatures, pressures and co-solvent mole fractions to determine the effects of these operating conditions on the retention factors. The retention factors k₁ and k₂ are for the (R,R)- and (S,S)-forms, respectively. When the isothermal compressibilities of a mixture of CO₂ and MeOH were lower than 0.01, far from the critical locus of the CO₂ and methanol mixture, both retention factors were well expressed with the solvent density and temperature with an average absolute relative deviation of 1-2%. In the vicinity of the critical locus, however, where the isothermal compressibilities were much larger than 0.01, the relationship between retention factor and density was complicated. Both retention factors were proportional to the isothermal compressibility, irrespective of methanol mole fraction at each temperature.