Monolithic imprinted polymer capillary columns

Another approach to monolithic imprinted polymer capillary columns has been reported. The polymerisation was performed at 60°C and thus thermally initiated [70,71]. The result is a dense polymer inside the whole capillary. The resulting MIP capillary then has to be connected to an electrolyte-filled open capillary via a teflon tube and a detection window is prepared on the open capillary to facilitate detection (as described above. Fig. 16.2). Also, ammonium acetate (1-2 mM) is added as a conducting agent to the pre-polymerisation mixture. This is done to allow exchange of the solvent of polymerisation for electrolyte, which can be achieved electrophoretically by stepwise increase of the electric field until a stable base-line is obtained. 

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. 

Thus far, the most successful approach to M IP-based CEC utilises capillary columns filled with a monolithic, super-porous imprinted polymer [39-41]. The morphology of a certain MIP monolith is depicted in Fig. 16.3. Using this system enantiomer separations with baseline resolution have been carried out in less than 2 minutes. The M IP-filled capillaries are obtained by an in situ photo-initiated polymerisation process (Fig. 16.4]. The capillary is filled with a pre-polymerisation mixture of imprint molecule, functional and cross-linking monomers (MAA and TRIM, respectively), radical initiator (2,2 -azobisisobutyronitrile) and solvent (toulene). Both ends of the capillary are sealed and the polymerisation is performed... 

Fig. 16.4. Preparation of a capillary column with a super-porous MIP monolith. A mixture of imprint molecule, functional and cross-linking monomer, radical initiator and a solvent is prepared and introduced into the capillary. Both ends of the capillary are sealed and the polymerisation reaction is initiated by placing the capillary under a UV source. After completion of the polymerisation reaction, the solvent used for polymerisation is exchanged for electrolyte. The flushing also extracts imprint molecules and remaining traces of unreacted monomer and radical initiator out of the polymer. The capillary column is then ready for CEC. Reprinted from [57] Copyright (1998), with permission from Elsevier Science.

However, it is still unclear how the efficiencies and resolutions of MIP monoliths compare to other methods of capillary fabrication using molecularly imprinted polymers. Future studies that directly compare the chromatographic results obtained using the same imprinting system under different preparation techniques (i.e., packed columns, coated thin films, immobilized particles, and monoliths) have... 

Figure 5 Illustration of the preparation procedure for capillary columns with molecularly imprinted polymer monolith. (1) Polymerization mixture is prepared and plastic tubing is placed on both ends of a capillary derivatized with methacryloxy propyltrimethoxysilane. (2) The capillary is filled with the mixture using a syringe. (3) The capillary is sealed by placing clips on the plastic tubings. The polymerization is performed under an UV source (350 nm) at —20°C for 80 min. (4) The polymerization reaction is interrupted by flushing remaining monomer, radical initiator, and imprint molecule out of the capillary. (5) The capillary column is then ready for CEC. (Reproduced with permission from Ref. 38.)...

It is very important that no gas bubbles are trapped in the polymer monolith or in the eonnected buffer-filled capillary, since the solvent-electrolyte exchange would then be impossible. It can also be speculated that the ammonium acetate present in the pre-polymerisation mixture might interfere with the imprinting process. Nonetheless, suceessful columns have been prepared using this approach. Since the monolith is very dense, it may intuitively be eoncluded that there is a larger amount of MIP in these capillaries than in the super-porous ones described above. 

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