Adamantyl-group containing mixed-mode acrylamide-based continuous beds for capillary electrochromatography. Part IV: Investigation of the chromatographic efficiency dependent on the retention mode

In our previous work we have described the synthesis, characterization, and optimization of the chromatographic efficiency of a highly crosslinked macroporous mixed-mode acrylamide-based monolithic stationary phase synthesized by in situ free radical copolymerization of cyclodextrin-solubilized N-adamantyl acrylamide, piperazinediacrylamide, methacrylamide and vinylsulfonic acid in aqueous medium in pre-treated fused silica capillaries of 100 μm I.D. In the present work, we study with different classes of neutral analytes (with varied hydrophobicity) the impact of the type of retention mode (influenced by the type of analyte and the mobile phase composition) and the impact of the solute functionality on the chromatographic efficiency and peak symmetry with a monolith synthesized under optimized synthesis parameters. With this monolithic capillary high separation efficiencies (up to ca. 220,000 m−1) are obtained for the separation of different analyte classes (alkylphenones, nitrotoluenes, and phenolic compounds with k = 0.2–0.55) in the reversed-phase mode, in the normal-phase mode, and in the mixed mode. For neutral alkylanilines (k < 0.25) plate numbers of about 300,000 m−1 are routinely reached in the reversed-phase elution mode. For phenolic solutes separated in a mixed mode there is a solute-specific influence on peak symmetry and chromatographic efficiency. With increasing efficiency of the monolith, axial diffusion becomes an important mechanism of band broadening. For those peaks, which do not show a significant asymmetry (asymmetry factor ≤1.05), it is confirmed that plate heights gained via the tangent method are equivalent to those gained via moment analysis.