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Investigation of neutral monolithic capillary columns with varying n‐alkyl chain lengths in capillary electrochromatography

Two different neutral nonpolar monolithic columns series (designated as A and B columns series) each consisting of three columns at varying n‐alkyl chain length were prepared by the copolymerization of the functional monomers C8‐methacrylate, C12‐acrylate, or C16‐methacrylate with the cross‐linking monomer pentaerythritol triacrylate (PETA) to yield monoliths with surface bound C8, C12, and C16 chains. In the A columns series, the composition of the functional monomers and crosslinker was adjusted to yield comparable chromatographic retention regardless of the alkyl chain length. In the B columns series, the composition of the functional monomers and crosslinker was kept constant yielding chromatographic retention, which increased as expected in the order of increasing the n‐alkyl chain length. Due to their direct influences on the monolith porosity and retention energetic, the nature and composition of the monomers at a given porogen composition have largely affected the solute's mass transfer characteristics and sorption kinetics, as assessed by the van Deemter plots and separation efficiencies. The C16‐monolith of the A series yielded the highest separation efficiency toward small solutes, but the A columns series were inadequate for protein separation. The C8‐monolith of the B series provided the best separation efficiency for proteins while for tryptic peptide mapping, the C16‐monolith of the A series seems to provide the best separation. For large protein molecules, the energetically “softer” C8 surface allowed faster sorption kinetics and in turn improved efficiency, while an energetically “harder” C16 surface favored better separation of the smaller size peptide solutes.