150 



CHROMATOGRAPHY 



is prepared by placing a ring of the mixture to be separated around the 

 center of the paper. After drying, the paper is placed over the dish, with 

 the point of the cone piercing the center of the paper disk. An identical 



glass dish is placed over the paper to serve 

 as a cover. The paper cone acts as a w^ick 

 to deliver solvent to the center of the 

 paper disk. From this point the solvent 

 moves outward into the paper in all direc- 

 tions. The finished chromatogram will 

 be a circular disk of paper with concen- 

 tric rings of the various separated pig- 

 ments. 



(5) Paper chromatographic separations 

 can even be performed in two dimensions. 

 We place a spot of the mixture of ma- 

 terials at one corner of a square of paper. 

 Then, using one solvent mixture, we 

 allow the various materials to move up- 

 paper was rolled into a cylinder ^^j-J ^Jong one edge of the paper. Now 

 for this chromatogram. The ^^ ^^^^^^ ^^^ p^p^^ ^^ ^1^^^ ^^le separated 



spots are at the bottom and run the sep- 

 aration again, this time using a different 

 solvent mixture (see Fig. 11-4). The fin- 

 ished two-dimensional chromatogram will 

 have spots distributed in various locations 

 all over the paper because of the differ- 

 ent rates with which the individual pigments move in different solvent 

 combinations. 



Fig. 11-3. A square of filter 



edge of the paper could be 

 fastened together with staples, 

 adhesive strips, or with thread. 

 Spots of a mixture were placed 

 at the circles marked M, and the 

 solvent has risen to point S. 



Physical principles involved in chromatography 



There is no general agreement on the physical principles which cause 

 the separation of materials. The chances are reasonably good that the 

 disagreement results from the fact that different principles are involved 

 in the different kinds of chromatography. The following physical prin- 

 ciples certainly are involved at one time or another, although it may not 

 be easy to tell which is operating in any particular case. 



Differential Adsorption: Adsorptioa is a phenomenon which occurs at 

 the surfaces or interfaces between two different kinds of material, as 



