Fig. 4

a Δ A absorption spectrum of 40 μg/mL SF and the synthetic mixture 0f 40 μg/mL SF and 10 μg/mL VP, b first derivative of Δ A overlapped absorption spectra of 40 μg/mL SF and synthetic mix of 40 μg/mL SF and 10 μg/mL VP, c Δ A absorbance spectra of SF in 0.1 N NaOH against SF of the same concentration in 0.1 N HCl, d first derivative of Δ A absorbance spectra of SF in 0.1 N NaOH against SF of the same concentration in 0.1 N HCl