Prevention of a systematic underestimation of antioxidant activity in competition assays. The impact of unspecific reactions of the reactive species.

In antioxidant competition assays, an antioxidant (A) and a detector compound (D) compete for a reactive species (R). In the evaluation of these assays, it is tacitly assumed that all of R is captured by either D or A. Due to the - by definition - high reactivity of R, unspecific reactions of R are likely to occur and neglecting these reactions will result in a systematic underestimation of antioxidant activity. It was shown that in the standard hydroxyl radical scavenging assay this was indeed the case; the inaccurate mathematical evaluation resulted in an underestimation of antioxidant activity of 25% in this competition assay. The systematic underestimation of antioxidant activity can be prevented by using an adjusted Stern-Volmer equation that takes into account that only part of R is captured by D or A.