The location of the fronts in thin-layer chromatography (TLC) with multicomponent mobile phases is discussed with reference to the possibilities of mobile phase transfer via analytical overpressured-layer chromatography (OPLC) to preparative medium-pressure liquid chromatography (MPLC). The advantage of this procedure is that the mobile phase of the TLC separation where the substance zones are distributed over the whole RF range can be transferred to preparative column chromatography without limitations.
Because OPLC may be used as equilibrated or nonequilibrated planar column systems and all mobile-phase fronts may be seen, it can be applied as a pilot method for preparative MPLC. Depending on the results of the analytical OPLC separation, four possibilities exist for mobile phase transfer to MPLC; these are discussed.
The generally useful method is to equilibrate the dry-filled column (silica of TLC quality with an average particle size of 15 pm) with a solvent in which the substances to be separated do not migrate and which was used for the prerun in analytical OPLC. The separation is then started with the optimized TLC mobile phase where the substances are distributed over the whole RF range. Transfer of the optimized TLC mobile phase via OPLC to MPLC is demonstrated by the separation of furocoumarin isomers from the roots of Heracleum sphondylium and the ginsenosides from Panax ginseng C. A. Meyer. The separation of anthraquinone aglycones from Rhamnus,franguZa is an example of the direct transfer of the mobile phase from TLC to MPLC.