Micropreparative purposes

As shown in [15] this concept may allow for the design of a separation system in such a way that two enantiomers certainly possessing the electric charge of the same sign will migrate toward opposite electrodes, which means that the enantioseparation factor becomes infinitely large (Fig. 2) [15], This technique certainly may be applied for micropreparative purposes also as well as for separations of achiral analytes not only in a binary mixture but also in multicomponent mixtures. 

Lanz et al. [89] reported the enantiomeric separation of methadone by CD-based capillary and recycling isotachophoreses. The latter technique was also applied for micropreparative purposes using two commercially available units, the model RR3 and MiniPhor. Both systems allowed to achieve a partial separation of / -(-)-methadone and 5-(+)-methadone being significantly enriched at the front and the back side. 

Thin-layer chromatography (TLC) is mainly applied in micropreparative taxoids separation [2-4]. Silica gel 6OF254 preparative plates are usually applied for this purpose. The problem of taxoids separation involves not only their similar chemical structure (e.g., paclitaxel versus cephalomannine) but also, due to different coextracted compounds usually encountered in crude yew extracts (polar compounds such as phenolics and nonpolar ones such as chlorophylls and biflavones), the separation is very difficult. The common band of paclitaxel and cephalomannine was satisfactorily resolved from an extraneous fraction in isocratic elution with ethyl acetate as a polar modifier [4] and n-heptane-dichloromethane as the solvent mixture and it was of suitable purity for high-performance liquid chromatography (HPLC) quantitative determination. 

As a micropreparative method, the high capacity of the thin layer it is easily possible to separate several milligrams of a mixture within an hour on a single 20 x 20 cm plate and to isolate radioactive compounds in amotmts which are adequate for most purposes. 

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