Page 188 - Chiral Separation Techniques

166 6 Enantiomer Separations using Designed Imprinted Chiral Phases

Fig. 6-7. Asymmetry factor (A ) of the L-enantiomer versus sample load (A) and versus flow rate (B) on
s
–1
L-PA-imprinted polymers. Flow rate: 1.0 ml min . Mobile phase: MeCN/[potassium phosphate 0.05 M,
pH 7] (7/3, v/v).

6.6 Factors to Consider in the Synthesis of MICSPs

In spite of the fact that molecular imprinting allows materials to be prepared with
high affinity and selectivity for a given target molecule, a number of limitations of
the materials prevent their use in real applications. The main limitations are:

1 Binding site heterogeneity
2 Extensive nonspecific binding
3 Slow mass transfer
4 Bleeding of template
5 Low sample load capacity
6 Unpractical manufacturing procedure
7 Poor recognition in aqueous systems
8 Swelling–shrinkage: may prevent solvent changes
9 Lack of recognition of a number of important compound classes
10 Preparative amounts of template required
It is clear that improvements aiming at increasing the yield of high-energy bind-
ing sites or modifying the site distribution in other ways will have a large impact on
the performance of the materials (affecting limitations 1, 2, 4 and 5). The strategies
adopted to achieve this have been focusing either on prepolymerization measures,
aimed at stabilization of the monomer template assemblies prior to polymerization,
or postpolymerization measures aimed at modifying the distribution of binding sites
by either chemical or physical means. The most important of these factors will now
be discussed, together with techniques allowing their optimization.