Eight phenolic acids were analyzed by capillary zone electrophoresis. On-line analyte preconcentration was carried out by hydrodynamic injection of large volume of sample followed by removal of the bulk of the low conductivity sample matrix by polarity switching. The optimal electrolyte system consisted of 50mM sodium tetraborate of pH 9.0 (adjusted with 0.1 M phosphoric acid) containing 2% of alpha-cyclodextrin. The separations were carried out with a fused silica capillary (effective length 50 cm, i.d. 50 microm) and monitored at 200 nm. Under optimized preconcentration conditions (sample injection 99 s at 100 mbar and the polarity switching time 1.0 min) linear calibration ranges (0.1-2.0 microg/ml, R=0.9979-0.9995), favourable limits of detection (0.01-0.025 microg/ml) and good repeatability of the peak areas (R.S.D.: 2.76-5.69%, n=6) were achieved.

Charles University Prague Pharmaceutical Faculty in Hradec Králové Department of Analytical Chemistry Heyrovského 1203 500 05 Hradec Králové Czech Republic