Elution-extrusion countercurrent chromatography. Use of the liquid nature of the stationary phase to extend the hydrophobicity window

Countercurrent chromatography (CCC) is a liquid chromatography technique with a liquid stationary phase. Taking advantage of the liquid nature of the stationary phase, it is possible to perform unique operations not possible in classical liquid chromatography with a solid stationary phase. It is easy to avoid any solute-irreversible absorption in the CCC column. If the retention volumes of solutes become too high, the dual mode will be used. The roles of the phases are reversed. The stationary phase becomes the mobile phase, and the CCC column is started again. The solutes elute rapidly in what was previously the stationary phase. The theoretical basis of the dualmode method is recalled. The dual-mode method is a discontinuous method. The separation should be stopped when the phase switch is performed. The elution-extrusion procedure is another way to avoid any irreversible adsorption of solutes in the column. The method uses the fact that the liquid volumes occupied by the solutes highly retained inside the column can be orders of magnitude lower than the mobile-phase volume that would be needed to elute them. The elution-extrusion method also has two steps: the first step is a regular CCC chromatogram. Next, the stationary phase containing the partially separated hydrophobic solutes is extruded out of the column in a continuous way using the liquid stationary phase. The theory of the process is developed and compared to the dual-mode theory. Alkylbenzene homologues are experimentally used as model compounds with the heptane/methanol/water biphasic liquid system to establish the theoretical treatment and compare the performance of two types, hydrodynamic and hydrostatic, of CCC columns. It is shown that the method can dramatically boost the separation power of the CCC technique. An apparent efficiency higher than 20 000 plates was obtained for extruded octylbenzene and a 160-mL hydrodynamic CCC column with less than 500 plates when conventionally used.