“Quantity-effect” research strategy for comparison of antioxidant activity and quality of Rehmanniae Radix and Rehmannia Radix Praeparata by on-line HPLC-UV-ABTS assay oleh Hong-Ying Li, Jiang-Ji Fang, Hua-Dan Shen, Xue-Qiong Zhang, Xiao-Ping Ding, Jun-Feng Liu