PROPERTIES OF THE CAROTENOIDS 057 



valuable pigment may be lost if one sections the column at a right angle to 

 the periphery. According to Zechmeister,^" in the preparation of the por- 

 tion to be eluted: "With complicated columns one indulges almost in a 

 form of sculpture!" 



The nature of the adsorption medium is of the greatest importance in 

 obtaining satisfactory results in chromatography. Theoretically, all sub- 

 stances in a finely divided form, or present as a powder, as well as in the 

 form of fibers, can adsorb, provided, of course, that they are not soluble in 

 the solvent employed. A satisfactory adsorbent must exert no chemical 

 action on the adsorbed material, and must be capable of releasing it to an 

 eluent if the procedure is one in which identification, purification, or prep- 

 aration of a pigment is desired. However, in some cases in which the 

 object is simply to remove a single component so that an examination of 

 the filtrate can be carried out, a reaction between the adsorbent and the 

 product to be removed may be desirable. Such a procedure is used in the 

 determination^'^ of vitamin A: the removal of all vitamin A and carot- 

 enoids is effected by passing their solution through a floridin column ; the 

 vitamin A reacts quantitatively with the floridin and cannot be eluted 

 from it. 



Another condition which may preclude the use of certain adsorbents is 

 their color. If they are dark, it may be impossible to establish the sequence 

 of layers. However, when it is desired to remove only one component, this 

 objection is no longer valid. Strongly acid or strongly basic adsorbents 

 are usually unsatisfactory. 



In order to serve effectively, an adsorbent must be able to bind the 

 adsorbed products with sufficient tenacity to hold them on the colunm, but 

 in such a manner that differentiation may occur in the strength of such 

 attraction for the various materials. It is also necessary that the adsorp- 

 tion affinity be of such a degree that elution can be accomplished when de- 

 sired. The strong adsorbents are referred to as "active." In certain cases, 

 it is possible to increase, activate, or deactivate adsorbents. Activation is 

 frequently brought about by washing with water or acid, followed by heat- 

 ing of the material. AVhen the solid phase acts too energetically, it binds 

 all substances indiscriminately, and it has no value as an absorbent in 

 chromatography. 



The optimum proportion of adsorbed material to adsorbent varies 

 greatly with different combinations; it may vaiy between 1:10 to 1:100,- 

 000. The size of the particle of adsorbent, also, may result in a marked 

 variation in its behavior. Finer particles ordinarily act extremely vigor- 

 ously as adsorbing agents, although Koschara"^ reports that coarser par- 

 ticles of alumina may be much more active toward certain pigments than 



5'9 J. Awaparu, F. H. Mattson, J. W. Mehl, and H. J. Dcucl, Jr.. Science. lOL 602-004 

 (1040). 



