Monolithic stationary phases silica

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. 

In another approach, Breadmore et al. developed silica-based monolithic stationary phases suitable for microchip electrochromatography. They modified the procedure originally developed... 

Monolithic CEC columns can be formed from organic porous polymeric monoliths or porous silica sol-gel monoliths. Other monolithic CEC column types, such as immobilized particles, have also been demonstrated but have not shown the potential of the first two types. The monolithic stationary phases are created in situ by polymerization reactions under controlled conditions, such that a porous bed is created. 

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 major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

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/... 

Lubda, D., Cabrera, K., Nakanishi, K., Lindner, W. (2003). Monolithic silica columns with chemically bonded b-cyclodextrin as a stationary phase for enantiomer separations of chiral pharmaceuticals. Anal. Bioanal. Chem. 377, 892-901. 

FIGURE 8.6 SEM of an organo-silica hybrid monolith prepared with the precursor A-octadecyldimethyl[3-(trimethoxysilyl)propyl] ammonium chloride to provide reversed phase stationary phase and anodic EOF. (a) Cross-sectional view (magnification 1800 times) and (b) longitudinal view (magnification 7000 times). (Reprinted from J. D. Hayes, A. Malik, Anal. Chem., 72 4090 (2000). With permission. Copyright American Chemical Society 2000.)... 

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