A critical evaluation on the reliability of two aluminum chloride chelation methods for quantification of flavonoids

Flavonols kaempferol, quercetin, myricetin and gossypetin, and flavones apigenin, acacetin, luteolin, orientin and tricin, are subjected to two AlCl3 spectrophotometric methods used for determination of total flavonoid content. The method developed by Woisky and Salatino involves addition of AlCl3 solution to the flavonoid solution and recording of optical density at 420 nm. All flavonols except kaempferol have absorption maxima above 440 nm and so readings at 420 nm are erroneous. Among flavones, all except for luteolin and orientin, have absorption maxima below 400 nm. Further, addition of CH3COOK and recording the absorbance at 415 nm, as modified by Chang et al., works well for flavonols kaempferol, quercetin and myricetin, but not for gossypetin. The flavones luteolin and orientin absorbed above 400 nm, whereas all others absorbed below 400 nm. Examination of the results of both methods indicates they are inadequate, and should not to be considered as universal and standard methods for total flavonoid determination.