Separations with imprinted polymers

As mentioned in Chap. 7, molecular imprinting of polymers15 can also be used to prepare hosts for the removal of the desired optical isomer from racemates. Foam flotation with imprinted polymers has been used in chiral separations.16 Flotation is often used to separate minerals from their ores. The polymers can be reused repeatedly. This may turn out to be the lowest-cost method of resolution. 

Chromatography with Imprinted Polymers. Chromatographic separations of the two enantiomers of phenyl-a-mannoside on a stationary silica phase with a polymer layer imprinted with 1 were performed 5T). In spite of the thin layer of polymer Che peaks were still rather broad. This cannot, therefore, be attributed to diffusion resistance in the polymer matrix, since these very thin layers should not show substantial diffusion hindrance. 

In a different approach, Lin et al. have used particles derived from a ground MI-bulk polymer and mixed with a polyacryl amide gel for chiral separation. Using a polymer imprinted with L-phenylalanine, D-phenylalanine could be separated from the template with a separation factor of 1.45 [35]. Although the combination of MIP with capillary electrochromatography is still not widely used, the ability to separate enantiomers in nanoliter samples promises interesting developments for the future. 

In a further application of MI-SPE, theophylline could be separated from the structurally related caffeine by combining the specific extraction with pulsed elution, resulting in sharp baseline-separated peaks, which on the other hand was not possible when a theophylline imprinted polymer was used as stationary phase for HPLC. A detection limit of 120 ng mb1 was obtained, corresponding to a mass detection limit of only 2.4 ng [45]. This combination of techniques was also used for the determination of nicotine in tobacco. Nicotine is the main alkaloid in tobacco and is the focus of intensive HPLC or GC analyses due to its health risk to active and passive consumers. However, HPLC- and GC-techniques are time-consuming as well as expensive, due to the necessary pre-purification steps required because the sample matrices typically contain many other organic compounds besides nicotine. However, a simple pre-concentration step based on MI-SPE did allow faster determination of nicotine in tobacco samples. Mullett et al. obtained a detection limit of 1.8 jig ml 1 and a mass detection limit of 8.45 ng [95]. All these examples demonstrate the high potential of MI-SPE to become a broadly applicable sample pre-purification tool. 

Comparable results were presented when the number of Mi-polymers was extended to other templates of similar steroid libraries. The use of a polymer imprinted with 11-deoxycortisol in a separation of 11-a-hydroxyprogesterone, progesterone and 11-deoxycortisol resulted in a chromatogram where the 11-de-oxycortisol eluted last (Fig. 17, bottom). When an 11-a-hydroxyprogesterone imprinted polymer was used, on the other hand, the 11 -a-hydroxyprogesterone was found to elute last (Fig. 17, top) [80]. 

One direct approach to the separation of chiral compounds is called molecular imprint polymers (MIPs) that involves the formation of a three-dimensional cavity with the shape and electronic features that are complementary to the imprinted or target molecule. 

Pichon V, Haupt K (2006) Affinity separations on molecularly imprinted polymers with special emphasis on solid-phase extraction. J Liq Chromatogr Related Technol 29(7—8) 989—1023... 

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-... 

One of the clearest pieces of evidence for the reversibility of some covalent interactions came from the work of Gunther Wulff at the University of Dusseldorf, Germany [172], He used imprinted polymers of o-aminomethyl phenylboronic acids as chromatographic stationary phases for the separation of saccharides. Older studies [173] also point to the reversible nature of the boronic acid-saccharide interaction. The pioneering studies of fluorescent transduction of this phenomenon by Czarnik and Yoon [174] (Ohio State University), Aoyama et al. [175] (Kyushu University, Japan), and Shinkai et al. [176] (Kyushu University, Japan) have been reviewed previously [9], Our concern in this review is particularly with the systems that clearly involve PET. Czarnik and Yoon s 93 [177] which interacts with catechol derivatives to produce 94 also belongs here. It... 

Another technique reported for polyaromatic hydrocarbons separation uses ODS-filled (with frits) or entrapped (without frits) columns with 80% acetonitrile-20% 0.1 M acetate buffer (v/v) at pH 3 (30 kV per 17-25 cm - long capillary, i.d. 100 pm) [10], The results are seen in Fig. 10.2. The technique of using entrapped sorbents can also be used for materials other than ODS (typically, imprinted polymers for chiral separations). 

Molecularly imprinted polymers with a variety of shapes have also been prepared by polymerizing monoliths in molds. This in situ preparation of MIPs was utilized for filling of capillaries [20], columns [21], and membranes [22, 23]. Each specific particle geometry however needs optimization of the respective polymerization conditions while maintaining the correct conditions for successful imprinting. It would be advantageous to separate these two processes, e.g., to prepare a molecularly imprinted material in one step, which then can be processed in a mold process in a separate step to result the desired shape. 

Polymer libraries are covered according to their numerous applications, each described through a specific example. The reported examples include libraries of copolymers as liquid/solid supports with different compositions, libraries of biodegradable materials for clinical applications, libraries of stationary phases for GC/LC separations, libraries of polymeric reagents or catalysts, libraries of artificial polymeric receptors or molecularly imprinted polymers, and libraries of polymeric biosensors. The opportunities that could arise in the near future from novel applications of polymer libraries are also briefly discussed. 

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