The PromoKine Total Antioxidant Capacity (TAC) Assay Kit is a colorimetric assay that is based on copper reduction by antioxidants. It measure either the combination of both small molecule antioxidants and proteins or small molecules alone in the presence of a proprietary Protein Mask. In the presence of antioxidants, the Cu2+ ion is converted to Cu+ (by both small molecules and proteins). However, the Protein Mask prevents Cu2+ reduction by proteins, enabling the analysis of only the small molecule antioxidants. The reduced Cu+ ion is chelated with a colorimetric probe giving a broad absorbance peak around 570 nm, proportional to the total antioxidant capacity.