Lee Wah LIM$^1$,$^2$
1Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
2International Joint Department of Materials Science and Engineering between National University of Malaysia and Gifu University, Graduate School of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
Monolithic columns, which consist of μm-sized skeletons and a double-pore (through-pore and meso-pore) structure, could offer high separation efficiency with ultra-low flow resistance, and they have attracted much attention since their introduction in the early 1990s [1-2]. Generally, monolithic columns can be divided into two types, i.e. silica- and organic polymer-based. The silica monolith has some advantages over the polymer monolith such as good mechanical strength, well-controlled pore structure and high column efficiency especially for small molecules. On the other hand, polymer monolith is robust over a wide range of pH and it is easy to prepare. In recent years, hybrid types of organic-inorganic materials, which seem to have combined the advantages of these two, are also being widely investigated. In this study, we focused our attention on the rapid and simple fabrication (synthesis) of both silica- and organic polymer-based monolithic capillary columns via one-pot approach, and the experimental conditions were optimized. The preparation time was drastically reduced when using a microwave synthesis device.
The utilized microwave device (Wave Magic, model MWO-1000S) has a controllable output range from 50 to 500 W, and a controllable temperature range between room temperature + 10-250°C. The reaction solution was filled into fused-silica capillary tubing and the column was then irradiated at different time under various temperature and output control of the microwave device. The morphology of the monoliths was observed with a scanning electron microscope (SEM) and was compared to those monoliths that were fabricated under normal thermal conditions. The results showed that the degree of output of the microwave device had direct influence on the morphology as well as size of the skeleton backbone of the monoliths. In addition, when a methacrylate-based polymer was used, the capillary column could be fabricated within less than 15 min, which is ca. 100-fold faster than the conventional thermal polymerization in a water-bath, and the theoretical plate number, i.e. N is calculated to be approximately 50,000 plates/m for a normal liquid chromatographic separation.
In conclusion, the use of microwave-assisted synthesis has been proven to be effective in certain organic reactions; however, the results are somewhat not reproducible especially for the silica-based materials. In addition, the “real” mechanism behind it is still remain inexplicable. Nevertheless, microwave-assisted synthesis is an attractive and yet challenging alternative as far as the rapid fabrication of capillary monolithic columns is concerned without compromising their separation efficiency. Further studies involving one-pot synthesis of ion-exchange monolithic polymer-based capillary columns are still undergoing.
Keywords:
Microwave-assisted synthesis; Monolithic capillary column; Facile preparation; Capillary liquid chromatography