Naturally present metal ions in plants could interfere with common antioxidant assays.

Most of the commonly applied assays used to assess antioxidant properties of plant extracts exploit the ability of some biologically active metabolites to participate in oxidation-reduction reactions with metal ions. On the other hand, most plants contain different chelated metal ions whose levels depend on the geographic origin, soil, and environmental pollutions. In this study the levels of redox-active metal ions in three plant sources were measured and extracts of these botanicals were treated with Chelex - an ion exchanger that is noteworthy for its ability to bind transition metal ions. The original and chelated extracts were subjected to three antioxidant assays based on single electron transfer. The results obtained showed statistically significant differences between the original and Chelex-treated extracts suggesting that the naturally present metal ions could interfere with the results of the three most commonly applied antioxidant methods.•The proposed pre-analytical procedure is simple and does not require special instrumental equipment.•Preliminary depletion of redox active metal ions, namely iron and copper ions could improve reproducibility of the analytical methods.•The method allows a more reliable comparison of antioxidant properties of particular botanical species from different geographic regions.