Cross-linker effects on the separation efficiency on (poly)methacrylate capillary monolithic columns. Part II. Aqueous normal-phase liquid chromatography

• New polar monolithic zwitterionic polymethacrylate capillary columns were prepared.
• Polar cross-linkers provide extended HILIC range, increased efficiency and resolution.
• The column preparation is highly reproducible.
• 70,000 theoretical plates/m were achieved for isocratic separation of small molecules.
• The columns show excellent separation of very polar phenolic acids.

We synthesized seven different polymethacrylate monolithic capillary columns using N,N-dimethyl-N-metacryloxyethyl-N-(3-sulfopropyl) ammonium betaine functional monomer (MEDSA) and various cross-linking monomers differing in the polarity and size. The efficiency of monolithic columns for polar low-molecular compounds in the aqueous normal-phase (HILIC) mode depends rather on the polarity, than on the size of the cross-linker molecules. Cross-linking molecules, which exhibit high polarity can produce poly(methacrylate) monoliths with an increase in pore sizes below 50 nm. Columns prepared with pentaerythritol triacrylate or bisphenol A dimethacrylate cross-linkers show large inner pore (mesopore) porosity, and provide poor efficiency, whereas columns with trioxyethylene dimethacrylate (TriEDMA) and tetraoxyethylene dimethacrylate (TeEDMA) cross-linkers showed poor permeability. Columns prepared using dioxyethylene dimethacrylate (DiEDMA), and especially glycerolate dimethacrylate (BIGDMA) cross-linkers, showed best efficiency, with more 60,000–70,000 theoretical plates/m, almost twice in comparison to (poly)methylene dimethacrylate (HEDMA) in the HILIC mode. All columns show dual retention mechanism and can be used for separations of low-molcular compounds such as phenolic acids in the HILIC mode in acetonitrile-rich mobile phases and in the reversed-phase mode in mobile phases with higher concentrations of water. Some columns show broad pore distribution and can be used for size-exclusion chromatography of non-polar polymers in tetrahydrofuran.