On the separation of small molecules by means of nano-liquid chromatography with methacrylate-based macroporous polymer monoliths

Macroporous monolithic poly(butyl methacrylate-co-ethylene dimethacrylate) stationary phases were synthesized in the confines of 100 microm I.D. fused-silica capillaries via a free radical copolymerization of mono and divinyl monomeric precursors in the presence of porogenic diluents. These columns were used in order to determine their suitability for the reversed-phase separation of small molecules in isocratic nano-LC mode. Carefully designed experiments at varying realized phase ratio by a terminated polymerization reaction, as well as content of organic modifier in the mobile phase, address the most significant parameters affecting the isocratic performance of these monoliths in the separation of small molecules. We show that the performance of methacrylate-based porous polymer monoliths is strongly affected by the retention factor of the analytes separated. A study of the porous and hydrodynamic properties reveals that the actual nature of the partition and adsorption of the small analyte molecules between mobile and stationary (solvated) polymer phases are most crucial for their performance. This is due to a significant gel porosity of the polymeric stationary phase. The gel porosity reflects stagnant mass transfer zones restricting their applicability in the separation of small molecules under conditions of strong retention.