Antioxidant Activity of Selected Thyme (Thymus L.) Species and Study of the Equivalence of Different Measuring Methodologies.

This study presents the results of comparative evaluation of the antioxidant activity of the phenolic fraction exhaustively extracted with aqueous methanol from 18 different thyme (Thymus L.) specimens and species. This evaluation is made with use of the same free radical source (DPPH• radical), three different free radical scavenging models (gallic acid, ascorbic acid, and Trolox), and three different measuring techniques (the dot blot test, UV-Vis spectrophotometry, and electron paramagnetic resonance spectroscopy, EPR). A comparison of the equivalence of these three different measuring techniques (performed with use of hierarchical clustering with Euclidean distance as a similarity measure and Ward's linkage) is particularly important in view of the fact that different laboratories use different antioxidant activity measuring techniques, which makes any interlaboratory comparison hardly possible. The results obtained confirm a semiquantitative equivalence among the three compared methodologies, and a proposal is made of a simple and cost-effective dot blot test that uses the DPPH• radical and provides differentiation of antioxidant activity of herbal matter comparable with the results of the UV-Vis spectrophotometry and EPR.