Adsorption equilibrium for Z-ligustilide on C18-bonded silica from supercritical carbon dioxide

Adsorption equilibrium is of great importance for the preparative supercritical fluid chromatography (pre-SFC) in defining supercritical adsorption behavior and the industrial amplification. This paper presents adsorption isotherms of Z-ligustilide from supercritical carbon dioxide (SC-CO₂) on C18-bonded silica. Adsorption behavior was studied at 305.15 K, 313.15 K and 323.15 K with SC-CO₂ density varying from 0.687 g·cm^-3 to 0.863 g·cm^-3 with the elution by characteristic points (ECP) method. The adsorption amount of Z-ligustilide from SC-CO₂ on C18-bonded silica decreased with the increasing density of the mobile phase as well as the increasing temperature. Adsorption equilibrium data were fitted by Langmuir and Freundlich isotherm models, and the Langmuir isotherm model performed better for describing the whole adsorption process on the column. The monolayer saturation adsorption capacity of Z-ligustilide is in the range of 3.0×10^-4 mg·cm^-3 to 5.5×10^-4 mg·cm^-3 with an average value of 4.0×10^-4 mg·cm^-3.