Determination of the flavonoids/antioxidant levels in Cirsium oleraceum and Cirsium rivulare extracts with cerium(IV)–rhodamine 6G chemiluminescence detection

The determination of the sum of flavonoid compounds in extracts from inflorescences (expressed as mg L−1 of apigenin) and leaves (expressed as mg L−1 of linarin) of Cirsium oleraceum and Cirsium rivulare species by flow injection system with chemiluminescence detection (FI-CL) has been carried out. The method is based on the strong enhancement by polyphenols occurring in both plants of the CL signal generated by the reaction of cerium(IV) with rhodamine 6G in a sulfuric acid medium. Under the optimized conditions, the linear working ranges of 0.1–10 and 2.5–50 μmol L−1 were obtained for apigenin and linarin, respectively. The developed method is simple, sensitive with the detection limits of 38 nmol L−1 (apigenin) and 840 nmol L−1 (linarin) and offers high sample throughput (up to 300 samples per hour). The relative standard deviation was 0.62% and 3.75% for 10 measurements of 5 μmol L−1 apigenin and linarin, respectively. The proposed method has been successfully applied to determine the flavonoids/antioxidant levels in aqueous and methanolic extracts from inflorescences and leaves of C. oleraceum and C. rivulare. A possible mechanism of the enhancement of cerium(IV)–rhodamine 6G CL system by polyphenols was briefly discussed. For comparative studies, the antioxidant activity of C. oleraceum and C. rivulare extracts was also evaluated by spectrophotometric 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical scavenging method.