Flavonoid metabolites in human plasma and urine after the consumption of red onions: analysis by liquid chromatography with photodiode array and full scan tandem mass spectrometric detection.

Blood was collected from six human volunteers 1 h after the ingestion of lightly fried red onions, which are rich in anthocyanins and flavonols. Urine excreted 0-4 h after ingestion was also collected. Plasma and urine were analysed by reversed-phase HPLC with photodiode array and tandem mass spectrometric detection. Twenty-three flavonols as a range of mixed sulphate, methyl, glucuronide and glucoside derivatives of quercetin were detected. The availability of reference compounds facilitated the identification of quercetin-3-glucuronide. quercetin-3'-glucuronide, quercetin-4'-glucuronide, quercetin-3'-sulphate and isorhamnetin-3-glucuronide in samples from all volunteers. Samples from one volunteer also contained trace amounts of quercetin-3,4'-diglucoside, quercetin-3-glucoside, isorhamnetin-3-glucoside and the aglycone quercetin. Despite a high dosage, neither anthocyanins nor anthocyanin metabolites accumulated in either plasma or urine in detectable quantities.