Chromatographic and electrochemical determination of quercetin and kaempferol in phytopharmaceuticals

An NP-HPLC method both with diode-array (DAD) and electrochemical detection (ED) was developed and validated for the determination of quercetin and kaempferol, the principal active constituents in phytopharmaceuticals of Ginkgo Biloba. Calculated retention of the two flavonoids was contrasted with experimental values in five different reversed phase columns for methanol-water, acetonitrile-water, THF-water and dioxane-hexane binary mixtures as mobile phases. The capacity factor k, selectivity α and asymmetry factor F were evaluated and compared in DAD-RP-HPLC, DAD-NP-HPLC, ED-RP-HPLC and ED-NP-HPLC. The methods were used for the quantitative analysis of acid hydrolyzed extracts of tablet phytopharmaceuticals. Calibration curves were linear within the range 10 and 40 μg ml−1 for the DAD and 10-270 μg ml−1 for the ED, whereby limits of detection ranged from 0.5 μg ml−1 (quercetin) to 0.1 μg ml−1 (kaempferol). The electrochemical method based on differential pulse voltammetry (DPV) with a C-PVC electrode resolved the quercetin and kaempferol peaks and exhibited a two orders higher sensitivity in comparison with a carbon fibber electrode. DPV calibration curves were linear within the range 96-300 μg ml−1 for quercetin and 68-960 μg ml−1 for kaempferol. The respective oxidation peaks appeared at 462 and 518 ± 2 mV and were used in the direct determination of quercetin in extracts of commercial phytopharmaceuticals.