TABLE OF CONTENTS Sometimes pre-washing of the TLC/HPTLC layer is necessary to remove impurities usually originating from the binder. Normally this is only a problem if the detection reagent is sensitive to the impurities or fluorescence quenching is being employed as the detection technique. The use of polar mobile phases causes a concentration of these impurities at the solvent front. As the resolution of components of the sample would be expected to be poor near the solvent front, the concentration of these impurities is not usually a problem. Non-polar mobile phases do not cause any migration of the impurities and hence there is little background interference when scanning. The best way to pre-wash a plate is by blank development in a TLC tank. Either the mobile phase for the separation can be used, or methanol, or mixtures of methanol and chloroform. Acids and bases are best avoided. The plate should be marked to show the direction of blank development. When the plate is then later used for chromatography of the sample, the same direction of development can and should be employed.
