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Several techniques for the fractionation of complex plant extracts exist. Researchers investigating a plant extract's activity often find it necessary to fractionate such mixtures, in order to identify the one or more components in the mixture that express the biological activity they are after. All too often, the complexity of the mixture hides the activity (or activities) of one or more of the constituents.
By cutting up the extracts into more manageable fractions, it becomes easier to appreciate the true value of the compounds in the mixture.
However, on the other hand, in some cases it may also be that an extract's activity derives from a specific combination of constituents. One always has to keep in mind the possibility of such a so-called synergism, which - for instance - has been described for the antidepressant activity of St. John's Wort Hypericum perforatum.
A 'classic' method for the separation of compounds from a - not too - complex mixture is preparative HPLC. A column filled with stationary phase, similar to that used for analytical runs (only much larger) is used. The advantage is that results obtained from the analytical (small scale) runs can be extrapolated to runs on the large column. A good separation can thus be achieved. However, it is still fairly easy to overload the column, resulting in deteriorated resolution, or even a completely failed run. Besides, there always is the risk of irreversible adsorption of components in the sample to the column. This can, however, be prevented, although only partially, by the use of a precolumn.
Centrifugal Partition Chromatography
This is a very powerful technique for the fractionation of complex mixtures. It comprises two immiscible liquids, that are used to partition the sample. By using one of the liquids as stationary phase and the other as a mobile phase, a continuous partitioning is achieved, resulting in elution of the compounds in order of their ratio of solubility in the 2 phases. But one can also use a principle of ion-pairing in the separation, or use the pKa-values of the components as determinants.
The big advantages of CPC are the high sample load and the ability to retrieve all the start material in the unlikely event that something goes wrong. Besides, no expensive columns are needed and one can 'tune' the separation by adjusting the ratio and composition of the two used liquids.