Resolution factors, molecularly imprinted

The CSPs prepared by the molecular imprint technique have also been used for chiral resolution by CEC [98-100]. Lin et al. [91] synthesized L-aromatic amino acid-imprinted polymers using azobisnitriles with either photoinitiators or thermal initiators at temperatures ranging from 4°C to 60° C. Methacrylic acid (MAA) was used as the functional monomer and ethylene glycol dimethacrylate (EDMA) was used as the cross-linker. The resulting polymers were ground and sieved to a particle size less than 10 pm, filled into the capillary columns, and used for enantiomeric separations of some amino acids at different temperatures. The relationships of separation factor and column temperatures were demonstrated to be linear between the logarithm of the separation factors and the inverse of the absolute temperature (Fig. 24). The authors also compared the obtained chiral resolution with the chiral resolution achieved by HPLC and reported the best resolution on CEC. The chromatograms of the chiral resolution of dl-... 

Chiral CEC will be discussed in detail later in the book but is included here to exemplify the application of the high efficiencies obtained with electro-driven techniques which makes them attractive for chiral analysis where selectivity factors are sometimes small. CE has made use of chiral additives in the electrolyte whilst LC tends to utilise chiral stationary phases. Both options have been explored for chiral CEC [27,28,77]. The small amount of packing material necessary for capillaries allows the use of chiral stationary phases that would be prohibitively expensive for standard LC. Cyclodextrins, proteins, antibiotics and molecular imprinting have all been used to form chiral stationary phases [78-80]. After some less than encouraging peak efficiencies obtained using the chiral CEC approach, much improved chiral resolutions have been achieved using CEC compared to LC or CE [81-83]. 

CEC was demonstrated in the analysis of /i-adrenergic antagonists using capillaries modified with propranolol-imprinted polymer. In situ molecular imprinting was performed via photo-polymerisation at — 20°C within a capillary that was premodified with 3-methacryloxypropyltrimethoxysilane. The time for UV irradiation was carefully determined so as to obtain a polymer coating of appropriate thickness. Enantioseparation of the racemate of propranolol was successfully demonstrated with a separation factor of 1.12 and a resolution factor of 1.26. 

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