Apparatus and technique for chromatographic separations

19 APPARATUS AND TECHNIQUE FOR CHROMATOGRAPHIC SEPARATIONS In paper and thin layer chromatography we may use ascending or descending technique. In the former the solvent moves upwards, while in the latter downwards. 

The solvent should be put into the tank about an hour before the thin layer sheet is introduced. The plates are then run either for a fixed time (when the final position of the solvent front must be marked), or up to a fixed distance (12-35 cm), pre-marked with a pencil. The sample (2-5 pi) is taken with a micro syringe (e.g. Hamilton type) or micro pipette (Shandon) and dropped to a point on the lower end of the thin layer. The position of the point must be chosen in such a way that it will not be covered by the liquid solvent when the thin layer plate is placed in it. The location of the starting point should be clearly indicated by a pencil mark. If a wider thin layer plate is used, more chromatograms can be run simultaneously. In this case the individual samples should be placed at least 1 5 cm apart along the starting line. 

The chromatograms are then allowed to run at a constant temperature, and then finally taken out of the tank and dried. A hair drier can often be used to speed up the process of drying. 

The dried chromatograms can then be examined with an ultraviolet lamp or developed by spraying with suitable reagents or both. 

With paper chromatography a descending technique may be more convenient (though an ascending method can of course also be used). 

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