Investigation of conditions allowing the synthesis of acrylamide‐based monolithic microcolumns for capillary electrochromatography and of factors determining the retention of aromatic compounds on these stationary phases

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Abstract
The influence of the cross‐linker (concentration), the porogen (lyotrophic salt) and the solvent type as well as the type and concentration of up to three “functional”, i.e., interactive monomers on the morphology and the chromatographic properties of acrylamide‐based hydrophilic monoliths are investigated. High total monomer concentrations favored polymers with a rigid rather than gel‐like structure. High cross‐linker concentrations also favor the formation of a nodular structure. The addition of a lyotrophic salt favors the formation of small nodules especially at higher monomer concentration; the pore size of the polymer can also be modulated through the salt concentration. Suitable monoliths were further investigated as potential stationary phases for capillary electrochromatography (CEC). Depending on the type and concentration of the monomers, plate numbers between 50 000 and 100 000 were routinely obtained. The standard deviation of the run‐to‐run reproducibility was below 2% and that of the batch‐to‐batch reproducibility below 5%. A set of nine hydrophobic and polar aromatic compounds (all noncharged) was used to investigate the retention mechanism. Possible candidates for chromatographic interaction and retention in these monoliths are the hydrophobic polymer backbone itself and the alkyl, carbonyl, hydroxy, amino, amide, and charged groups introduced by the various functional monomers. Judging from our results, the carbonyl and the hydroxy functions, as well as the hydrophobic polymer backbone can be supposed to be the main sites of interaction. The charged but also the alkyl functions seem to be less important in this regard. The polymerization conditions and especially the composition of the reaction mixture have a strong influence on the behavior of the final column.