(Normal-phase) capillary chromatography using acrylic polymer-based continuous beds

Microchromatographic separations of polar aromatic compounds (pyridine, 4-pyridylmethanol, 4-methoxyphenol, 2-naphthol, catechol, hydroquinone, resorcinol, 2,7-dihydroxynaphthalene) using continuous beds are described. The columns were prepared by a simple one-step in situ polymerization procedure: a solution of acrylic monomers, including the cross-linking agent piperazine diacrylamide, was polymerized in a fused-silica capillary pretreated with 3-(trimetoxysilyl)propyl methacrylate. The continuous bed formed contained a network of channels and was attached covalently to the wall of the silica capillary (100 mu m LD.) via its methacrylate groups. Therefore, the frit used in conventional, packed columns could be omitted. The separation mechanism is discussed, particularly with regard to whether the so-called aromatic adsorption to the matrix itself is involved, an interaction first described by Gelotte [1] (the ligands, isopropyl and sulfonate groups, are not required for separation). This discussion is relevant to the question of whether the separation technique described should be classified as normal-phase or adsorption chromatography. The mobile phase from the HPLC pump was split via an open capillary to get a flow rate through the continuous bed of about 100 nl/min. The beds were tested up to a pressure of 150 bar (8.8 bar/cm). A continuous bed synthesized at a relatively low molar fraction of the cross-linker in the monomer mixture (16.5%) and high total concentration of the monomers (31.9% (w/v)) afforded the highest efficiency for the separation of the polar organic compounds. Plate numbers up to 150000 m(-1) were obtained and the run-to-run reproducibility was high. The selectivity of the separations was adjusted by changing the composition of the mobile phase (hexane-ethanol-methanol). The sample was applied by a diffusion-based injection technique. (C) 1999 Elsevier Science B.V. All rights reserved.