Monolithic stationary phases organic monoliths

Since 1992, a vast variety of rigid organic monolithic stationary phases with different chanistry, functionality, and column geometry has been reported for HPLC as well as CEC applications, as summarized by the number of excellent reviews [25-32,213], The development and enhancanent of monolithic stationary phases is stiU a rapidly growing area of research with scientific and industrial interest. 

Table 1.1 gives a comprehensive, albeit fragmentary, snmmary of investigated organic monolithic polymer systems (based on all different kinds of styrene, acrylate, methacrylate, (meth)acrylam-ide building blocks, as well as mixtnres thereof) together with their preparation conditions and ntilization as stationary phase. 

Due to the fact that thermally initiated free radical copolymerization is by far the most routinely employed method for fabrication of organic monolithic stationary phases, the pore formation mechanism is discussed for this particular kind of polymerization. 

Due to their defined monomodal macropore distribution (see Section 1.2.1), monolithic stationary phases, based on polymerization of organic precursors, are predestined for efficient and swift separation of macromolecules, like proteins, peptides, or nucleic acids, as their open-pore structure of account for enhanced mass transfer due to convection rather than diffusion. In fact, most of the applications of organic monolith introduced and investigated in literature are directed to analysis of biomolecule chromatography [29]. 

Svec, R, Organic polymer monoliths as stationary phases for capillary HRLC, Journal of Separation Science 27(17-18), 1419-1430, 2004. 

Li, Y., Chen, Y., Xiang, R., Ciuparu, D., Pfefferle, L. D., Horwath, C., and Wilkins, J. A., Incorporation of single-wall carbon nanotubes into an organic polymer monolithic stationary phase for mu-HPLC and capillary electrochromatography, Analytical Chemistry 77(5), 1398-1406, 2005. 

The molecular imprinting strategy can be applied for the recognition of different kinds of templates from small organic molecules to biomacromolecules as proteins. Some examples of separations investigated with MIP monoliths in CEC and LC are shown in Table 2. The influence of the imprinted monolithic phase preparation procedure and of the separation conditions on the selectivity and chromatographic efficiency have been widely studied [154, 157, 161, 166, 167, 192]. The performance of imprinted monoliths as chromatographic stationary phase has also been compared to that of the traditional bulk polymer packed column [149, 160]. It was shown that the monolithic phases yielded faster analyses and improved chiral separations. 

Faure et al. [22] described nanoelectrochromatography on poly (dimethyl)-siloxane microchips using organic monolithic stationary phases for analysis of derivatized catecholamines. Surface modification of the PDMS material was carried out by UV-mediated graft polymerization. The efficiency of the unit was ascertained by measuring theoretical plates, which were 200,000 per meter. Furthermore, the authors optimized the separation by using pinched and electrokinetic modes at different applied potential of l.OkV/cm (Fig. 7.6) and 30kV/cm (Fig. 7.7) for the pinched and electrokinetic... 

Nowadays, porous monoliths have found an extensive use in CEC of organic compounds, which represents a powerful separation tool, complementary to HPLC. CEC is a hybrid method in which the separation is performed through the phase distribution mechanisms of traditional HPLC (reversed-phase, ion exchange, etc.), while the flow of the mobile phase through column packing is affected by electroosmotic forces, as in electrophoresis. The coexistence of a stationary phase and an electric field permits separation not only of ions but also of neutral compounds, due to their different electrophoretic mobhity and different distribution between the mobile and the stationary phases. 

Cyclodextrin derivatives have been successfully used also for enantioseparations in packed capillary CEC. In this technique CyDs are used covalently immobilized to silica gel [69], to particulate silica sintered as a monolith [70], or to a monolithic organic matrix [71, 72]. In addition, CyDs have been used as dynamic modifiers of the stationary phase [73], as well as chiral mobile phase additives in combination with an achiral stationary phase [74, 75]. 

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