Natural compounds occur as various isomeric or closely related structures in biological matrices. These compounds are difficult to separate from the complex mixtures, and hence, the need for effective and innovative separation techniques arises. Recycle HPLC allows the recycling of sample, in part or full, and increases the separation efficiency of the process while keeping the peak dispersion to a minimum. Recycling in an HPLC system has been used in the isolation and purification of different types of natural products including enantiomers, diastereomers, epimers, positional isomers, and structurally related or unrelated compounds having similar retention characteristics. The present paper overviews the development of instrumentation and setup of recycle HPLC and its applications in the separation of natural products. 1. Introduction The ever increasing needs for the isolation and purification of natural products have resulted in the constant improvements and advancements in separation techniques and instrumentation. The efficiency of a separation may be increased in a number of ways. A few include (1) increasing the column length; (2) using two columns in series, or (3) recycling the sample (in part or full) through the column. The first option, that is, to increase the column length, may not be feasible on an industrial scale as the columns used are of fixed dimensions. Sample recycling in an HPLC system came into picture in the middle of the twentieth century. Two key studies highlighted the instrumentation and applications of recycle HPLC [1, 2]. The aim of recycling is to increase the separation efficiency of the process while keeping the peak dispersion to a minimum. This aim is achieved by incorporating a recycle valve in the HPLC (preparative or semipreparative scale) system to recirculate the unresolved peaks into the column. Another added advantage of this recycling is that no fresh solvent is required in the recycling period. Currently, several recycling setups, based on mathematical models, have been devised, and the operating policies of these setups have been experimentally verified to be capable of improving product yields at laboratory, pilot, and industrial scales [1, 2]. These techniques have been utilized in the isolation and purification of a plethora of natural and synthetic molecules. As many as 23 recycles in the chromatographic system have been reported for the separation of structurally related compounds [3]. If a comparison is made between the conventional separations of isomeric/structurally related compounds and separation
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