Different values obtained by the FRAP method for the determination of slowly and rapidly reacting phenols

A number of methods have been applied to measure total antioxidant capacity (TAC), including FRAP, which is based on reducing the amount of iron ions in a complex compound. Researchers often use measurement of absorbance 10 min after mixing a sample with the FRAP reaction solution to calculate TAC. The FRAP solution has been shown to alter absorbance over time by ca 0.0010-0.0020 per hour, under storage conditions. This article intends to show that some substances do not fully or sufficiently react within the common analysis period. It is evident from the results that some substances react more quickly and others very slowly. Absorbance in relation to various phenols was measured. Compared to the levels of absorbance at 10 min, mean absorbance at 48 h was higher by 5,395% for vanillin, 426% for caffeic acid, 170% for sinapinic acid, 67% for gallic acid, 19% for syringic acid, and only by 4% for Trolox. Results for vanillin and caffeic acid indicate potential auto-catalysis.