148 CHROMATOGRAPHY 



to develop. It is usually wise to start slowly and then increase the amount 

 of acetone or ethyl ether in larger jumps if it seems warranted. It is 

 extremely difficult to predict the behavior of the development, apparently 

 because of variations in the amount of water dissolved in the solvents or 

 in the amount of moisture bound to the sugar particles. 



Paper Chromatography: In paper chromatography, the solid material is 

 ordinary filter paper. Various combinations of solvents can be used to 

 separate different kinds of mixtures. 



If we should wish to obtain a quick estimate of just which pigments 

 were contained in a mixed solution from leaves, a paper chromatogram 

 would allow a more rapid determination than would the preparation 

 of a column. The chief disadvantage of the paper technique is that only 

 small amounts of materials can be separated. The following description 

 includes several variations of the use of filter paper for chromatography 

 of plant pigments. 



(1) A strip of filter paper about 2 cm wide is cut to fit into a large 

 test tube. A hook in a cork stopper suspends the paper in the tube so 

 that the bottom of the paper strip almost touches the bottom of the tube 

 (see Fig. 11-2). A small amount of the solution of mixed pigments is 

 placed along a pencil line near the bottom of the paper and then dried. 

 A mixture of about 5 per cent acetone and 95 per cent petroleum ether 

 is placed in the bottom of the test tube. The paper and cork stopper are 

 fitted into the tube with the bottom of the paper dipping into the liquid 

 and the spot of pigment mixture about a centimeter above the surface of 

 the liquid. As the hquid rises in the filter paper by capillary action, vari- 

 ous pigments are carried along the paper strip. Some pigments move more 

 rapidly than others, and after a few minutes several distinct bands of 

 color become visible. 



(2) Larger amounts of the mixture can be separated, or several differ- 

 ent mixtures can be tested simultaneously, by using a large beaker or 

 battery jar instead of a test tube. A square of filter paper is prepared 

 by placing a line of the mixture near one edge or by placing spots of 

 several mixtures near the edge. The paper is then rolled and fastened 

 into a cylinder with the spots of mixtures at one end. This cylinder will 

 stand upright in the l)eaker or battery jar (Fig. 11-3). If solvent is placed 

 in the bottom of the glass container, it will rise in the paper just as in the 

 previous example. The same kind of separation of pigments occurs. 



(3) Sometimes the paper chromatogram will develop better if the 

 liquid is allowed to descend over the paper instead of rising by capil- 



