Ion exchange retention mode

This mode is seen for templates with protolytic functional groups, i.e. Bronsted-basic or acidic groups. In the case where, respectively, an acidic (i.e. MAA) or basic (i.e. VPY) functional monomer is used in the preparation of the corresponding MIPs, the polymeric phases often show good chromatographic performance and excellent selectivity in aqueous mobile phases, where the retention is driven by ion exchange [129]. 

Since the second term in both the numerator and the denominator are identical, an increase in a s will obviously lead to a decrease in a. In support of this explanation, the non-specific binding increases above pH pp = 6. This is clearly seen in the plot of k versus pHapp for BA on the L-PA MIP (Fig. 5.34) and in the parallel increase in a for the latter (Fig. 5.35). This can also be seen from the plot of the estimated separation factor of BA ( ba) versus pHapp (Fig. 5.34), where a is highest at low pH values (below pATa(BA)) and decreases over a large pH interval. Furthermore, the potentiometric titrations showed that the LPA MIP had a lower average pAfa than a non-imprinted blank polymer (see Chapter 2). This strongly suggests that the carboxylic acid groups of the selective sites are more acidic than those of the non-selective sites. 

As noted in the first proposed explanation (a) for the change in enantioselectiv-ity, an increase in pH may per se lead to a decrease in the enantioselectivity of the imprinted sites. One way to separate these effects is to measure the complete isotherms at several pH values and evaluate how the relative population of nonspecific and specific sites, as well as their affinity for the template, change with respect to the mobile phase pH [40,50]. 

A number of other studies using uncharged templates or acids imprinted using VPY as functional monomer show pH dependent selectivities that can be rationalised considering this type of ion exchange model [144]. 

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