Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals and chain reactions that may damage the cells of organisms. Antioxidants such as thiols or ascorbic acid (vitamin C) may act to inhibit these reactions. To balance oxidative stress, plants and animals maintain complex systems of overlapping antioxidants, such as glutathione. Measurement of the total non-enzymatic antioxidant capacity (TAC) of biological samples is indicative of their ability to counteract oxidative stress-induced damage in cells. TAC is used to provide insights into the development and treatment of oxidative-stress related disorders. In the Total Antioxidant Capacity Assay Kit, either the concentration of the combination of both small molecule and protein antioxidants, or theconcentration of only small molecule antioxidants can be determined. Under acidic condition, Fe3+-TPTZ is converted to Fe2+-TPTZ by both small molecules and proteins. The reduced Fe2+ ion chelates with a colorimetric probe, giving a broad absorbance peak at 593nm, which is proportional to the total antioxidant capacity. Because of acidic conditions,and the total plasma concentration of iron ion and ferrous ion in serum samples is usually lower than 10 μM, some endogenous interference factors canbeinhibited.