ION-EXCHANGE CHROMATOGRAPHY 



215 



to "sharpen" those eluticn peaks which are diffused due to noiipolar forces 

 as well as to shift their relative positions (see Stein and Moore''). 



3. Ion-Exchange Chromatography 



a. Sorption and Elution 



Column chromatography in general requires two steps, the sorption of 

 the sample containing the components to be separated and an elution 

 sequence in which the various components are brought off the column sepa- 

 rately. The sorption step usually utilizes conditions of high affinity (high 



LIQUID H£4D »ND FLOW 

 R4TE DETERMINED BY 

 LENGTH OF STEM 



ION -EXCHANGE COLUMN 



Fig. 1. lon-exchango column and receiver (after Tompkins).^' ' 



distribution coefficient) between solutes and exchanger (e.g., high charge 

 on the solute, low ionic strength in the solution) to bring about retention of 

 the sample in the topmost layers of the column (Fig. 1). Elution, on the 

 other hand, utilizes conditions in which a larger fraction of the constituent 

 in question is released from the resin (lower distribution coefficient), thus 

 setting up a distribution between solvent and exchanger which permits a 

 reasonable degree of movement of the solute do\vn the column with the 

 solvent flow. The conditions for elution are the opposite of those for sorp- 

 tion; hence, elution can be accomplished by a reduction in charge of the 

 sorbed constituent (by pH adjustment or complex formation or the reverse), 

 or by an increased concentration of competing ion (ionic strength adjust- 

 ment), or by increased temperature (decreased nonpolar attraction), or by 

 some combination of these. 



9 E. R. Tompkins, J. Chem. PJduc. 26, 32, 92 (1949). 



