Acrylamide-based monoliths as robust stationary phases for capillary electrochromatography

A method is described for the synthesis of rigid, macroporous polymers (mon oliths) to be used as stationary phases in capillary electrochromatography (CEC). The procedure reproducibly results in columns with good mechanical a nd chemical stability. Once the procedure was optimized, it yielded the des ired CEC columns in nearly 100% of the cases. The batch-to-batch standard d eviation of the migration of the electroosmotic flow (EOF) marker for nine randomly chosen columns was 5%. The polymerization is carried out inside th e capillary, an aqueous phase is used as solvent. Monomers based on acrylam ides with varying hydrophilicity were used to introduce the interactive moi eties together with piperazine diacrylamide as cross-linker and vinylsulfon ic acid as provider of the charged, EOF-producing moieties. The pore size o f the monoliths was adjusted by adding varying amounts of ammonium sulfate to the reaction mixture. In this manner, the average pore size of a given m onolith could be reproducibly adjusted to values ranging from 50 nm to 1.3 mum. The procedure was optimized for four particular types of monoliths, wh ich differed in hydrophobicity. The latter was adjusted by introducing suit able co-monomers, such as alkyl chain-bearing molecules, into the monolithi c structure. Attempts to systematically investigate the chromatographic beh avior of the monolithic stationary phases were made, using a model mixture of aromatic compounds as sample. The standard deviations for the run-to-run reproducibility of the retention times for unretained and retained analyte s were <1.5%. Flat Van Deemter curves were measured even at elevated flow-r ates (2 mm/s). Plate heights between 10 and 15 <mu>m were measured in this range. The retention order was taken as the principal indication for the ch romatographic mode. The separation was found to be governed neither by pure reversed-phase nor by pure normal-phase chromatography, even on monoliths, where large amounts of C, ligands had been introduced. (C) 2001 Elsevier S cience B.V. All rights reserved.