Flat-twisted tubing: Novel column design for spiral high-speed counter-current chromatography

The original spiral tube assembly for high-speed counter-current chromatography (HSCCC) is further improved by a new tube configuration called “flat-twisted tubing” which was made by extruding the tube (1.6 mm I.D.) through a narrow slot followed by twisting along its axis forming about 1 cm twisted screw pitch. This modification interrupts the laminar flow of the mobile phase through the tube and continuously mixes the two phases through the column. The performance of this spiral tube assembly was tested by three types of two-phase solvent systems with different polarities each with a set of suitable test samples such as DNP-amino acids, dipeptides and proteins at the optimal elution modes. In general all these test samples yielded higher resolution with the lower mobile phase than the upper mobile phase. In the most hydrophobic two-phase solvent system composed of hexane–ethyl acetate–methanol–0.1 M hydrochloric acid (1:1:1:1, v/v/v/v), DNP–amino acids were separated with Rs-a (peak resolution based on the same column capacity adjusted for comparison) at 4.40 and 73% of stationary phase retention at a flow rate of 0.5 ml/min with the lower mobile phase. In the polar solvent system composed of 1-butanol–acetic acid–water (4:1:5, v/v/v), dipeptide samples were resolved with Rs-a at 4.06, compared to 2.79 with the cross-pressed tube assembly at 45% stationary phase retention, each at a flow rate of 1 ml/min. Finally in the aqueous–aqueous polymer phase systems composed of polyethylene glycol 1000 – dibasic potassium phosphate each 12.5% (w/w) in water, protein samples were resolved with Rs-a at 2.53 compared to 1.10 with the cross-pressed tube assembly at 52% of stationary phase retention, each at a flow rate of 1 ml/min. These results indicate that the present system substantially improves the partition efficiency with a satisfactory level of stationary phase retention by the lower mobile phase.