Open Tubular Capillary Electrochromatography Using Etched Fused-Silica Tubing Modified with Chemically Bonded Liquid Crystals

Abstract

Two liquid crystal compounds, cholesterol-10-undeceneoate and 4-cyano-4‘-pentoxybiphenyl, are attached to the inner wall of a fused-silica capillary that has been etched by ammonium hydrogen fluoride. The bonding process involves formation of a hydride layer on the etched surface via a silanization reaction with triethoxysilane followed by attachment of the liquid crystal using a hydrosilation reaction. The etched surface is characterized by photoelectron spectroscopy (ESCA) and the inner wall after being chemically modified with the liquid crystal is analyzed by diffuse reflectance infrared Fourier transform spectroscopy. The surface properties of these materials are further probed by measuring electroosmotic flow as a function of pH. The performance of the etched chemically modified capillaries as a separation medium is evaluated by electrochromatographic experiments using mixtures of proteins, pyrimidine/purine bases and a nucleoside, benzodiazepines, the synthetic and metabolic compounds of serotonin, and other small basic molecules. In all cases, peak symmetry is good and efficiency is generally considerably higher than in packed capillary CEC. In some instances, there are significant differences in selectivity between the two capillaries indicating that the type of liquid crystal (cholesteric or nematic) is an important factor in the separation mechanism. Finally, during the evaluation period for the capillaries used in this study there was no significant loss in chromatographic performance, indicating that the long-term stability of these materials is good.