Effect of Functional Group Structure on Selectivity

To ascertain the effect of functional group structure on selectivity, the various resins should have a very similar exchange capacity so that identical elution conditions could be used for each resin. To accomplish this, the relative reactivities of various amines with chloromethylated XAD-1 had to be determined. Then the degree of chloro-methylation of XAD-1 could be adjusted so that the aminated resins would have similar capacities. 

The retention times of 17 monovalent anions on resins with different functional groups but with almost identical exchange capacities (average 0.027 mequiv/g) were compared with the use of a solution of a monovalent anion (sodium benzoate) as the eluent [8]. Relative retention times were calculated by dividing the measured retention times by that of chloride. Data for resins with various trialkylammonium groups are presented in Table 3.1. 

TMA= trimethylamine, TEtA= triethylamine, TPA= tripropylamine, TBA= tributylamine, THA= tri-hexylamine, and TOA = trioctylamine. 

The data show that the relative retentions of the weak-acid anions are almost independent of the size of the alkyl groups. However, as the size of the R groups increase, large changes occur with the more polarizable anions such as nitrate, iodide, chlorate and BF4 ions. 

Most of the monovalent anions exhibit only small changes in their relative retention on resins containing one, two, or three hydroxyethyl groups, compared to the tri-methylamine resins (Table 3.1). However, when a stronger eluent was used (phthalate, 

---