Investigation of the axial heterogeneity of the retention factor of carbamazepine along an supercritical fluid chromatography column. I - Linear conditions.

The retention behavior of carbamazepine eluted in SFC was studied by changing the back pressure between 100 and 200bar and the volumetric flow rate between 0.5 and 5mL/min. Carbamazepine was eluted under linear conditions from a silica column with 90:10% (V/V) mixtures of supercritical carbon dioxide and methanol at 310K. The retention factor of carbamazepine decreases by 20% when the pressure drop along the column increases from 1 to 28bar and the outlet pressure increases from 100 to 200bar. The variation of the retention factor at low pressure drops (below 15bar) is negligible (a few percent) but it is significant (exceeding 20%) for pressure drops larger than 15bar. Under these latter conditions, the variation of the equilibrium constant along the column is significant at high pressure drops. Because the adsorption parameters depend on the local pressure, the extension of dynamic methods from LC to SFC should be made cautiously for the determination of adsorption isotherms. Our results suggest that the flow rate applied during the course of dynamic method measurements should be kept small. The determination of the adsorption parameters from measurements carried out at high flow rates might not be as accurate as it is when the column is axially uniform.