New mixed-mode methacrylate-based polymeric monoliths prepared via complexation with cyclodextrins employed as stationary phases for capillary electrochromatography

Highly crosslinked, macroporous mixed-mode monolithic stationary phases were synthesized for capillary electrochromatography (CEC). Free radical copolymerization was performed in aqueous solution with a cyclodextrin-solubilized hydrophobic monomer, a water-soluble crosslinker (piperazinediacrylamide), and a negatively charged monomer (vinylsulfonic acid) in bind silane-pretreated fused-silica capillaries. The prepared monolithic stationary phases were used for the separation of neutral hydrophobic (alkylphenones) and neutral polar (phenolic and nitrotoluene solutes) analytes by CEC. Chromatographic properties of the synthesized monoliths were studied with aqueous and non-aqueous mobile phases with hydrophobic and with polar analytes. Due to the amphiphilic nature of the polymers synthesized, the elution orders obtained correspond to the reversed-phase mode and to the normal-phase mode depending on the polarity of the mobile phase. However, observations made with polar solutes and polar mobile phase can only be explained by a mixed-mode retention mechanism. Comparison of retention data with those of a commercial octadecyl silica gel HPLC column reveals that the hydrophobicity of alkylphenones (expressed as methylene selectivity) of the monolithic capillaries prepared in this study is very similar to that of routinely used octadecyl silica gels. The potential of methacrylate-based mixed-mode monolithic stationary phases bearing sulfonic acid groups for the separation of positively charged analytes (alkylanilines, amino acids, and peptides) by CEC is investigated. The retention mechanism of these charged solutes on these negatively charged mixed-mode stationary phases is investigated by studying the influence of mobile phase and stationary phase parameters on the corrected retention factor. Results show that both hydrophobic and ion-exchange interactions contribute to the retention of these analytes. The quantitative relationship between hydrophobic and ion-exchange interactions is investigated by comparing two different retention models for charged analytes on a mixed-mode stationary phase: the one site and the two-site model