Monolithic Anion-Exchange Media

As already outlined in Section 3.4.1.6, monolithic separation media have made a significant impact on separation science because of the advantages they offer over packed columns. This research resulted in two types of monolithic 

The most widely used polymer monoliths are poly(styrene-co-divinylben-zene)-based monoliths and moderately polar monoliths based on methacrylic acid esters. While the first ones are strongly hydrophobic materials prepared by polymerization of styrene and its derivatives with divinylbenzene as the cross-linking agent [323,324], methacrylate-based monoliths are synthesized by polymerization of butyl methacrylate or other methacrylic acid esters with ethylene dimethacrylate as the cross-linking agent [325,326]. These types of monolithic media in capillary format are the main focus of a very detailed review by Urban and Jandera [327]. 

The preparation of methacrylate-based monoliths has been described in great detail by Vlakh and Tennikova [328] it is relatively straightforward. The column is filled with a polymerization mixture, sealed at both ends, and the polymerization is initiated by heat or UV radiation. The functional monomer in the polymerization mixture controls the polarity of the final monolithic material, while the cross-linking agent (a monomer with two or more double bonds) and the poro-gen solvent determine the size and the distribution of the pores. Polymerization is initiated by azo compounds such as 2,2 -azo-bis-isobutyronitrile (AIBN), which requires temperatures between 60 and 80 °C to decompose with the formation of free radicals. 

When polymerization is finished, the column is washed with an appropriate solvent to remove the porogens and any other soluble compounds including unreacted monomers. 

The Dionex ProSwift SAX-IS and WAX-1 S anion-exchange monoliths (see Section 4.8.2 for SCX-1S and WCX-1S cation-exchange monoliths) are specifically designed to provide high-resolution, high-efficiency separations of proteins at elevated flow rates with a loadability exceeding that of nonporous beads. 

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The epoxide groups of a polyglycidyl methacrylate-co-ethylene dimethacrylate monolith readily react with diethylamine to create a weak anion exchange column. This monolithic column is characterized by fast mass transport kinetics leading to excellent resolution. The columns seem to be stable, and the specific capacity is 40-mg/g medium for ovalbumin [60,62]. More recently, graft polymerization within... 

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