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12 FLASH CHROMATOGRAPHY AND LOW PRESSURE CHROMATOGRAPHIC TECHNIQUES FOR SEPARATION OF PHYTOMOLECULES Bibliography Glasl, S., Gunbilig, D., Narantuya, S., Werner, I., and Jurenitsch, J., 2001, Combination of chromatographic and spectroscopic methods for the isolation and characterization of polar guaianolides from Achillea asiatica, Journal of Chromatography A, 936: 193-200 Still, C. W., Kahn, M., and Mitra, A., 1978, Rapid chromatographic technique for preparative separations with moderate resolution, Journal of Organic Chemistry, 43: 2923 208
EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS 13 Counter-current Chromatography Abstract S. K. Srivastava Counter-current Chromatography (CCC) is based on liquid-liquid partitioning and is an excellent alternative to circumvent the problems associated with solid phase adsorbents and to preserve the chemical integrity of mixtures subjected to fractionation. The uniqueness of this technique is that it does not use a stationary phase and separation is achieved between two immiscible liquid phases. One phase is used as a stationary phase while the other one is used as mobile phase. The separation takes place on the basis of partition of a sample between the two phases. In contrast to other chromatographic techniques, since there is no irreversible adsorption of sample on the stationary phase, recovery is up to 100%. Centrifugal Partition Chromatography (CPC) and High Speed Counter-current Chromatography (HSCCC) are the two variants of CCC being used today. Separation of compounds with a wide range of polarities is possible with the use of aqueous and non-aqueous solvent systems. Separation of crude plant extracts, semipurifi ed fractions and synthetic mixtures can be carried out, with sample loads ranging from milligrams to grams. 13.1 Introduction Natural products have been a key source for the discovery of new drugs. Isolation of the active components from a natural product has always been associated with complex separation problems due to the enormous chemical complexity of the extracts, but recent advances in separation sciences have facilitated the isolation of these active components from natural products. Crude extracts of natural products which show desirable biological activity are subjected to activity-guided fractionation until an active component is isolated and identifi ed. This exploratory process of fractionation typically involves suboptimal chromatographic conditions; hence, in order to avoid destruction of potentially labile components, utmost care must be taken throughout the entire process of isolation. At present, most of the chromatographic separations of natural products are being carried out on solid supports. However, SiO 2 , Al 2 O 3 and reverse phase adsorbents are not chemically inert. Separation of a natural product on alumina or silica gel sometimes results in recovery of only 70%–90%. Sometimes severe losses of valuable materials result because of irreversible adsorption on a solid support. In addition, isolation of artifacts has also been reported due to chemical reactions of the substrates with solid phase adsorbents. 209
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CONTRIBUTORS Chapter 6 Aqueous Alco
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7 DISTILLATION TECHNOLOGY FOR ESSEN
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