Flash chromatography on cartridges for the separation of plant extracts – Rules for the selection of chromatographic conditions, and comparison with MPLC

Flash chromatography on cartridges has become increasingly popular for the rapid purification of compounds, mainly of synthetic origin. In contrast, its application for natural product isolation is poorly documented, and easy-to-use procedures for optimization of the separation conditions are lacking. Using Sepacore® cartridges (Büchi Labortechnik), we have established empirical rules for the selection of chromatographic conditions with an emphasis on gradient mode. Reversed phase HPLC separations can be transposed by increasing the gradient time by a factor 2–4. For normal phase separations, solvent compositions resulting in Rf values of 0.15–0.2 on TLC for the most lipophilic and the most hydrophilic constituents, respectively, should be selected as gradient endpoints. We applied these rules to the separation of complex plant extracts, with Curcuma xanthorrhiza, Piper nigrum and Salvia milthiorrhiza as examples of medicinal and commercial importance. The performance of the cartridges was compared to that of classical MPLC (medium pressure liquid chromatography) glass columns. Sepacore cartridges enabled a good separation of compounds with a broad range of polarity, as typically found in plant extracts. The chromatographic resolution remained, however, lower than that achieved by MPLC on columns packed with material of smaller particle size. For poorly soluble extracts, solid introduction gave better results than liquid injection. Despite lower resolution as compared to MPLC, pre-packed cartridges are an attractive alternative for the purification of extracts and crude fractions due to their ease of use and speed of separation.