Polymer-based monolithic stationary

In recent years, the interest in using porous silica and polymer-based monolithic stationary-phase media for ion chromatographic separations of inorganic and organic ions has increased.As compared to particle bed columns, monolithic columns represent a single piece of porous cross-linked polymer or porous silica. Monoliths are made in different formats as porous rods, generated in thin capillaries or made as thin membrane or disks. 

Reversed-phase chromatography is a separation method based on the hydrophobicity of the protein. In RPC, the hydrophobic stationary phase is based on silica gel or a synthetic polymer. In recent years, instead of bulk materials for column packing, polymer- or silica gel-based monolithic stationary phases have also been used [43]. 

The application of polymer monoliths in 2D separations, however, is very attractive in that polymer-based packing materials can provide a high performance, chemically stable stationary phase, and better recovery of biological molecules, namely proteins and peptides, even in comparison with C18 phases on silica particles with wide mesopores (Tanaka et al., 1990). Microchip fabrication for 2D HPLC has been disclosed in a recent patent, based on polymer monoliths (Corso et al., 2003). This separation system consists of stacked separation blocks, namely, the first block for ion exchange (strong cation exchange) and the second block for reversed-phase separation. This layered separation chip device also contains an electrospray interface microfabricated on chip (a polymer monolith/... 

Such precise control of porous properties is expected to be very useful in the design of specialized CEC columns for separation in modes other than reversed-phase. For example, size exclusion chromatography (SEC) is an isocratic separation method that relies on differences in the hydrodynamic volumes of the analytes. Because all solute-stationary phase interactions must be avoided in SEC, solvents such as pure tetrahydrofuran are often used as the mobile phase for the analysis of synthetic polymers, since they dissolve a wide range of structures and minimize interactions with the chromatographic medium. Despite the reported use of entirely non-aqueous eluents in both electrophoresis and CEC [65], no appreciable flow through the methacrylate-based monoliths was observed using pure tetrahydrofuran as the mobile phase. However, a mixture of 2% water and tetrahydrofuran was found to substan-... 

Monolithic stationary phases have emerged in the last few years as an attractive alternative to particle-packed capillary columns as a method to completely eliminate the need for bed-retaining frits and their associated problems [302,306,307]. The dimensional stability of the monolithic structures results from their rigidity and/or chemical attachment to the inner wall of the capillary. Several synthetic strategies have been described, but the most useful are based on molded porous polymers [306,326-331], molded porous sol-gel continuous beds [302], hydrothermal immobilization of packed beds [332] and particle-fixed continuous beds [302,333,334]. 

The use of monolithic columns in LC has advanced rapidly since their first introduction in the 1990s [18-21]. In contrast to capillary columns packed with particulate stationary phases, monolithic columns consist of a single continuous support. Monolithic stationary phases can be subdivided in two classes, i.e., polymer-and silica-based materials. 

The main advantages of monolithic columns are the superior separation performance and low flow resistance. In addition due to their continuous nature, frits are not required to retain the stationary phase. The production process of monolithic columns is more flexible than that of packed columns e.g., photo-polymerization can be applied to prepare monolithic structures or add selectivity locally. Both polymer- and silica-based monolithic capillary columns have been used for highly efficient separations in LC-mass spectrometry (MS) applications for proteomic research [24,25]. 

Svec, F., Monolithic stationary phases for capillary electrochromatography based on synthetic polymers Designs and apphcations, J. High Resol. Chromatogr., 23, 3, 2000. 

The production of conventional stationary phases in the form of porous polymer particle is based on suspension polymerization. Namely, the polymerization is allowed to proceed in a solvent under vigorous stirring that assures obtaining particles of the desired diameter. Since the particle size is typically in the range of a few micrometers, no problems with heat transfer are encountered. In contrast, the preparation of monoliths requires a so-called bulk polymerization. A polymer mixture consisting of monomers and porogenic solvent is mixed with an initiator. As the temperature is increased, the initiator decomposes and oligomer nu-... 

Several approaches towards monolithic GC columns based on open pore foams prepared in large diameter glass tubes were reported in the early 1970s [26,27, 110]. However, these columns had poor efficiencies, and the foams possessed only limited sample capacities in the gas-solid GC mode. Subsequent experiments with polymerized polymer layer open tubular (PLOT) columns where the capillary had completely been filled with the polymer were assumed to be failures since the resulting stationary phase did not allow the gaseous mobile phase to flow [111]. 

Hilder, E. R, Svec, R, and Frechet, J. M. J. (2004). Shielded stationary phases based on porous polymer monoliths for the capillary electrochromatography of highly basic biomolecules. Anal. Chem. 76, 3887-3892. 

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. 

PNIPAM and its copolymers are not the only options for thermoresponsive stationary HPLC phases. Poly(acrylates) and poly(methacrylates) bearing OEG groups in the side chains are known as thermoresponsive polymers that offer some advantages over PNIPAM [40]. Such polymers were recently employed for the modification of silica monoliths, which then served as stationary phase in the HPLC separation of steroids [193], Unsurprisingly, the results were qualitatively similar to those obtained for PNIPAM-based systems, but the separation of relatively hydrophilic steroids was superior. 

The methacrylate-based polymers are stable even under extreme pH conditions such as pH 2 or 12. Fig. 6.24 shows the CEC separations of aromatic acids and anilines at these pH values [14]. The sulfonic acid functionalities of the monolithic polymer remain dissociated over the entire pH range creating a flow velocity sufficient to achieve the separations in a short period of time. In contrast to the stationary phase, the analytes are uncharged, yielding symmetrical peaks. Needless to say that typical silica-based packings may not tolerate such extreme pH conditions. 

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