Repeatability of Octadecyl Methacrylate-Based Monolithic Columns for Capillary Electrochromatography

摘要

Octadecyl methacrylate-based monolithic capillary columns were prepared using octadecyl methacrylate as the monomer, ethylene dimethacrylate as the cross-linking agent, 2-acryloylamido-2-methylpropanesulfonic acid as the ionizable monomer, responsible for a negatively charged stationary phase surface and able to support a cathodic electrosmotic flow, and amyl alcohol and 1,4-butanediol as porogenic solvents. The repeatability of the morphological aspect of the monolithic material was evaluated by scanning electron microscopy (SEM). The uniformity along the same monolithic bed was also investigated by SEM. Replicate determinations of surface area and pore volume were also performed and compared. The repeatability of the obtained electrochromatographic efficiencies for separations of alkylbenzenes using the stationary phases prepared was also evaluated. A column that had a plate height of 31 mu m was tested for separation of polycyclic aromatic hydrocarbons. Even though minimal possible variation of experimental parameters was maintained, it was not possible to reproduce the stationary phases of the monolithic columns, since the relative standard deviation for the efficiencies obtained on columns prepared from the same polymerization mixture composition was 62%. The lack of repeatability for the efficiency is thought to be a result of limited homogeneity of the monolithic globular structures along the length of the majority of the prepared columns. The poor precision of preparation was justified by the copolymerization reaction that occurs in the presence of 2, 2 '-azobisisobutyronitrile as the initiator agent, whose decomposition starts at the moment that this is added to the reaction mixture.