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Determination of quercetin in human plasma was carried out by semi-micro high-performance liquid chromatography with electrochemical detection. Peak heights for quercetin were found to be linearly related to the amount of each quercetin injected from 1.5 to 750 pg. The detection limit (signal-noise ratio, S/N = 3) of the present method for quercetin was 0.3 pg. Glucuronic and sulfate forms of quercetin in plasma were hydrolyzed enzymatically using beta-glucuronidase and sulfatase, respectively. Quercetin in plasma and the hydrolyzed solution were extracted with ethyl acetate and determined by the present method. The time courses of concentrations of quercetin in human plasma showed maxima at 1-1.5 h after ingestion of 340 mL of commercial canned green tea.

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