Extraction and on-line concentration of flavonoids in Brassica oleracea by capillary electrophoresis using large volume sample stacking

Flavonoids are bioactive compounds found in plants. Studies indicate consumption of food containing these compounds may reduce the incidences of cancer and cardiovascular diseases. In broccoli, the flavonoids are present at variable concentrations and so far have mainly been determined using high performance liquid chromatography (HPLC). This paper describes a rapid capillary electrophoresis method, involving large volume sample stacking (LVSS), suitable for the analysis of flavonoids in broccoli. Following acid hydrolysis, the two key flavonoids (kaempferol and quercetin) in a broccoli extract were concentrated on-line by LVSS prior to separation by capillary zone electrophoresis (CZE). Using an optimised method, the extract was injected for 50 s into a 50 μm (internal diameter) X 85 cm (total length) capillary followed by stacking/matrix removal at -5 kV for 83 s. The two analytes were then separated in less than 8 min by CZE using a 10 mM sodium borate buffer (pH 8.40) and a separation voltage of +30 kV at 30°C. A linear relationship in the range 1-20 ppm was observed for the method (r² = 0.9991-0.9995) with detection limits of 0.9 and 0.6 mg/kg of broccoli for kaempferol and quercetin, respectively. This method demonstrated good repeatability for the standard and extract with relative standard deviations of less than 5% for both peak area and migration time measured over five different days (n = 5). The method was successfully applied to quantitatively determine kaempferol and quercetin contents in a commercial broccoli sample as 11.8 and 14.6 mg/kg fresh weight, respectively. This result was validated by HPLC analysis and is within the ranges reported in the literature.