Selectivity of MIPs

A problem related to the previous one is the separation of a group of similar compounds from a complex sample matrix. A group of chemically similar compounds, like a family of drugs or pesticides, may include widely dissimilar members with respect to the properties on which typical separations are based. For example, the hydrophobicity of a drug and its metabolites often differ substantially despite relatively small changes in the overall structure of the molecule. In such situations the inherent shape selectivity of MIPs may be quite useful to achieve a group separation. 

The first MISPE protocol reported in the literature [18] involved the use of an imprinted dispersion polymer in a column format, for the selective enrichment of pentamidine (1) in urine. In this case the high selectivity of the polymer allowed the analyte to be detected directly in the eluate without the need for any further chromatographic separation (entry A in Tables 15.1 and 15.2). In view of the high selectivity of MIPs, this is a viable approach that brings the benefits of shorter analysis times and simpler instrumentation. In most cases, however, MISPE has been used prior to a chromatographic separation step. The MIP has been applied in a batch-wise extraction [23] or in columns or cartridges [20-22,24-26,33]. 

The selectivities of MIPs are in many cases comparable to those of commercially available CSPs. For example, a separation factor (a) of 17.8 was found for the separation of the two enantiomers of a dipeptide on poly(methacrylic acid-co-EDMA) imprinted with one of the enantiomers (Fig. 17.5) [43]. 

A number of commonly occurring pesticides and herbicides was also examined and found to cause no interference, even at extremely high concentrations. Such sensors, which combine the selectivity of MIPs with the extremely sensitive fluorescence properties of lanthanide complexes, ensure that this will be a very fruitful area of research. 

Much of the selectivity of MIPs arises from the interactions between the templates and the functional monomers. The functional monomers are chosen so that their functionalities complement the functionalities of the template molecules. A wide variety of functional monomers, including acidic, basic, neutral and hydrophobic ones, have been tested in MIP synthesis (Table 2.1). Methacrylic acid is the most widely used monomer and has been applied in the synthesis of MIPs selective for a wide range of templates. 

Numerous research groups have attempted to make use of the selectivity of MIPs to prepare enzyme analogues with a catalytic activity [71,155-170]. One strategy was to prepare a MIP around a template molecule with a structure similar to that of the substrate. The functional groups that play a catalytic role in the imprinted site are judiciously placed, by interaction with the functional groups on the guest molecule. An example of this strategy, represented in Fig. 19(2) [171, 172] concerns the catalysis of the dehydrofluorination of an... 

An investigation on the effects of the electrostatic force and the Mulliken charge distribution on the selectivity of MIPs was carried out based on DFT for hydroxyzine- and cetirizine-imprinted polymers [Azimi et al., 2014]. The results showed a correlation between the selectivity coefficients and the theoretical charge distributions and also showed that charge distribution based model was able to predict the selectivity coefficients of MIP based potentiometric sensors. 

Table 4 Comparison of Selectivity of MIP Made Using the Template oc-Methylbenzylamine (10) vs. MIP Made Using P-Methyl Phenethylamine as Template (11)...

Table 9 Selectivity of MIPs Against L-Phenylalanine Made with Different Porogens (Adapted from Ref. 30)...

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