2.4. Preparation of DPPH and ABTS radical solutions
2.4.1. DPPH radical solution
DPPH powder was dissolved in 500 mL methanol and then used to prepare the concentration-gradient 0.20, 0.10 and 0.05 mM, respectively, for optimization of DPPH radical solution concentration in on-line HPLC-DPPH-FRSD system and for off-line DPPH radical scavenging assay. The prepared solutions were stored at 4 °C in dark.