388 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



studies with radioisotopes along the hnes of analytical separation, iden- 

 tification, and isolation of labeled compounds in pure form. 



GENERAL METHODS 



Ion-exchange separation methods usually fall into either of two general 

 classifications: ion exchange, in which exchangeable ions are separated 

 from nonexchangeable ions or from nonelectrolytes ; and ion-exchange 

 chro7natography , in which exchangeable ions are separated from each other 

 on the basis of differences in affinity for the exchanger. 



The general principles and terminology may be illustrated by schematic 

 diagrams showing the deionization of an aqueous mixture of potassium 

 phosphate and ethyl alcohol [after (2)]. As indicated in Fig. 9-1, if the 

 mixture is passed over a cation exchanger, the potassium ions will be 

 removed by the exchanger to give a solution containing only the alcohol 

 plus phosphate. If the exchanger is then treated with acid, the cation 

 solution that results contains the potassium ions from the original sample 

 but none of the alcohol molecules or phosphate ions (Fig. 9-2). Similarly, 

 by the use of an anion exchanger, it is possible to produce a solution con- 

 taining the alcohol and potassium with no phosphate (Fig. 9-3), and also, 

 by elution, a solution containing the phosphate but with no alcohol or 

 potassium (Fig. 9-4). Finally, if the cation and anion exchanger are used 

 in sequence, it is possible to obtain the deionized solution, that is, the 

 alcohol free of both potassium and phosphate (Fig. 9-5). 



Cation Exchangers. Cation or base exchangers can be regarded as 

 polyvalent anions with attached positively charged exchangeable ions. 

 Many of the organic cation exchangers utilize sulfonic acid as the func- 

 tional group. These materials can be used over practically the entire pH 

 range. The weakly acid type of exchanger may contain carboxylic or 

 phenolic groups that can be employed only above pH 4 and pH 8, respec- 

 tively. Table 9-1 presents a listing of some of the commonly used cation 

 exchangers that are commercially available. 



Anion Exchangers. Naturally occurring anion exchangers are not 

 generally available. Anion exchangers are polyvalent cations with neg- 

 atively charged exchangeable ions, or polymeric insoluble bases or salts 

 thereof. The active groups are usually amino or quaternary ammonium 

 groups. Table 9-2 presents a listing of commonly used anion exchangers 

 that are commercially available. 



GENERAL TECHNIQUES 



For general discussions of technique, the reader is referred to the texts 

 of Samuelson (2) and Kunin and Myers (1). Detailed procedures for 



