Why Ascending Method of Paper Chromatography is Preferred?
The ascending method is preferred because of the simplicity of the set up. In actual method the substance to be separated is applied as a spot in a suitable solvent with the help of a capillary tube and hot blower. The paper is then suspended vertically inside a sealed chamber to make its lower end dip into a solvent. Due to capillary action, the solvent ascends along the filter paper that forms the mobile phase, while the water held immobile by the cellulose fibres of the paper forms the stationary phase.
After the solvent front has run almost to the upper end of the filter paper, the latter is removed from the chamber and dried. The paper is then suitably stained to locate separate functions as coloured spots. Sometimes, to enhance the resolution of the spots and to verify the authenticity of the material, the chromatogram is subjected to two dimensional paper chromatography. After it is run in single dimension in one solvent system, it is dried and rotated to 900 and rechromatographed using a second altogether different solvent system. Finally the filter paper is dried and stained. The spots are better resolved after chromatography in the second dimension. In descending paper chromatography, the filter paper is suspended vertically down from the solvent chamber at the top so that the liquid and the solutes migrate down the paper.