Depletion of high-abundance flavonoids by metal complexation and identification of low-abundance flavonoids in Scutellaria baicalensis Georgi

•Ethanol extract of Radix Scutellariae was treated with zinc acetate.•High-abundance flavonoids were depleted efficiently by complexation.•Low-abundance constituents were recovered well in the supernatant.•The supernatant was analysed by HPLC ESI MS/MS.•The number of compounds identified increased 2.5 times compared to that in an untreated sample.

The complexation of metal cations and flavonoids with 5-hydroxyl or ortho-hydroxyl groups was successfully used for high-abundance flavone depletion from a botanical extract in this study. Due to their structural differences, five of the most highly abundant constituents, baicalin, wogonoside, baicalein, wogonin and oroxylin A, were successfully depleted from the ethanol extract of Radix Scutellariae. The depletion rates were approximately 99%, 85%, 99%, 70% and 76%, respectively. The recoveries of low-abundance constituents were very strong (approximately 70–100%). The efficiency of the low-abundance compounds’ identification by high performance liquid chromatography electrospray tandem mass spectrometry (HPLC ESI MS/MS) was remarkable after the high-abundance constituents were removed. The number of compounds identified from the HPLC MS/MS data was 250% greater than the number of compounds identified in the untreated total extract. One hundred seventeen flavonoids were identified in the ethanol extract of Radix Scutellariae using this method, which was much greater than the number identified in previous studies without high-abundance constituent depletion. Among them, 13 sulphated flavonoids were identified. These low-abundance sulphated flavonoids can barely be detected in untreated total extracts. To the best of our knowledge, this is the first reported evidence that sulphated flavonoids have been identified from Radix Scutellariae. This method will facilitate the removal of high-abundance flavonoids and the identification of low-abundance compounds in botanical extracts.