Titanium dioxide sol-gel coated column for the capillary electrochhromatographic separation of plant phenols

In this study, titanium dioxide nanoparticles as a stationary phase in open-tubular capillary electrochromatography (CEC) was prepared. The process has been done in the following way. First, titanium isopropoxide was hydrolyzed at the pH 1.5 (4℃), then the mixture was condensed by heating at 50℃ for 7 h. A stable, clear solution was obtained. PEG 8000 (0.32 mg/ml) was then added to the TiO2 colloidal solution and concentrated under vacuum at 50℃.The high-concentration solution was introduced into the capillary column by nitrogen, then put in the oven and reacted at 200℃ for 24 h. The step made the condensation reaction of TiO2 nanoparticles with silanol groups of fused-silica capillary. EOF property was investigated in a variety of buffer solutions : /formate/Tris, phosphate and EDTA/. With formate/Tris buffer, EOF reversal at pH below 4.7 and cathodic EOF at pH above 4.7 were indicated. With phosphate and EDTA, only cathodic EOF were indicated. This is because of phosphate and EDTA buffer arosed titanium complex and yield a negative charge layer on the surface of TiO2. The CEC performance of the column was tested with plant phenols and phenolic acids. When mobile phase in phosphate (40 mM, pH 9.0), applied voltage of 15 kV, two plant phenols which are diastereomers, (+)-(2R,3S)-catechin and (−)-(2R,3R)-epicatechin, could be baseline separated. With the same column at the mobile phase of EDTA (40mM, pH 5.0), applied voltage of 20 kV, five phenolic acids, syringic acid, caffeic acid, salicylic acid, p-coumaric acid and 4-hydroxybenzoic acid could be baseline separated, the average theorical plate is 3.2 × 104 m-1 and RSD < 4% with four measurements.