ION-EXCHANGE CHROMATOGRAPHY 213 



materials of different structure and with less satisfactory results than are 

 currently expected. The ion-exchange separations of the nucleic acid con- 

 stituents and related products had the advantage of the newer materials 

 from the start. Thus all that has so far been published in this field is 

 comparable, since all utihzed the same or very similar materials. For this 

 reason, the discussion of ion-exchange resins which follows will be limited 

 to those few which are currently in use.* 



2. Ion-Exchange Equilibria- 



a. Hydrophilic Character 



We will consider only one type of matrix, the poljnmers of vinylbenzene 

 cross-linked with divinylbenzene to give three-dimensional polystyrene 

 beads of spherical shape. This matrix supports sulfonic acid (strong-acid 

 cation exchanger, such as Dowex-50 or Amberlite-120) or carboxylic acid 

 (weak-acid cation exchanger, such as Amberlite-IRC-50) or quaternary 

 ammonium (strong-base anion exchanger, such as Dowex-1, Dowex-2, and 

 Amberlite-IRA-400) residues attached to each one of the aromatic rings 

 ("nuclear" substitution).* The hydrophilic character of these substituents 

 gives the beads, in water, the properties of gels. They swell in water up to 

 limits determined by the degree of cross-linking and show typical Donnan 

 equilibrium effects due to the nondifTusible nature of the highly ionized 

 substituents. 



h. The Distribidion Coefficient- ^ 



The ionized substituents (e.g., resin SOs" H+) participate in ionic reac- 

 tions with diffusible ions (e.g., Na+) in the surrounding aqueous medium 

 according to the simple exchange reaction 



1 

 (resin SO^) H+ + Na+^oin. <=^ (resin SOD Na+ + H+.oin. 



2 



With respect to the diffusible ion (Na+soin.), sorption consists in favoring 

 reaction 1 (e.g., by low H+soin. concentration), desorption or elution in 

 favoring reaction 2 (e.g., high acidity). Under any given set of conditions, 

 an equilibrium can be reached which may be expressed in terms of the 

 mass law but which is usually designated by a "distribution coefficient" 



, _ moles C per g. exchanger 



moles C per ml. solution 



' A complete listing of commercially available ion exchangers, together with their 

 manufacturers, will be found in Samuelson," pp. 262-3. The same volume contains 

 a thorough discussion of all specifications as well as of the principles and applica- 

 tions of ion exchange. See also E. R. Tompkins.^ 



« E. R. Tompkins, Anal. Chem. 22, 1352 (1950). 



