DISCREPANCY BETWEEN THE THEORETICAL PLATE NUMBER (N) AND PEAK RESOLUTION (RS) FOR OPTIMIZING THE FLOW-RATE IN COUNTERCURRENT CHROMATOGRAPHY

This paper deals with a problem in optimizing the flow rate of the mobile phase in countercurrent chromatography (CCC). A set of data including theoretical plate number (N), resolution factor (Rs), % retention, etc. are obtained from the separation of three indole auxins in a two-phase solvent system of n-hexane / ethyl acetate / methanol / water (1:1:1:1) by applying the flow rates of the mobile phase at 0.5 - 8.0ml/min. The experimental results revealed a disagreement between the efficiencies expressed in parameters N and Rs at the high flow rates of the mobile phase in the CCC separation. Namely, the partition efficiency expressed in terms of Rs decreases with an increased flow rate of the mobile phase, while the N rises with increasing the flow rate. Since Rs represents the actual separation of the solute peaks, the results indicate that the N used in high performance liquid chromatography is not always a reliable measure in CCC separation. However, the application of a high flow rate yields better min/Rs (time in min required to yield Rs=1) than that of a low floe rate. On the other hand an extremely slow flow rate of the mobile phase results in an abrupt increase of the partition efficiency in terms of both N and Rs. Therefore, the present findings suggest new strategies to improve the separation efficiency in CCC.