Modification of DC polarographic antioxidant assay—Application to aromatic plants and their active principles

Abstract

A direct current (DC) polarographic assay based on the decrease of anodic current of HydroxoPerhydroxoMercury (II) Complex (HPMC) formation in H2O2 solution in Clark Lubs (CL) buffer (pH 9.8) as working solution was previously applied to determine antioxidant (AO) capacity of water‐soluble compounds. Here, the applicability of HPMC assay was extended into samples poorly soluble in water such as the essential oils and extracts of Lamiaceae and Apiaceae species obtained by ultrasound‐assisted maceration (UAM) and Soxhlet extraction (SE) with 70 and 96% ethanol as well as individual compounds present. The influence of solvents miscible with water on HPMC anodic current was studied, and working solution was optimized. A modified HPMC method was applied to measure AO capacity of essential oils and extracts as well as compounds identified using gas chromatography‐mass spectrometer (GC‐MS) and gas chromatography with flame‐ionization detection (GC‐FID) (terpenoids and catechols) or known to be present (phenolic acids and flavonoids). A 1:1 mixture of CL buffer and ethanol was chosen as working solution. AO capacity of essential oils and extracts, established by modified HPMC assay, was in range of 1.2‐2.0 and 9.2‐38.4 (%/mL) while terpenoids and phenolics in range of 0.1‐0.5 and 13.5‐38.5 (%/mol), respectively. The superior AO capacity was ascribed to essential oils and extracts of Lamiaceae species and to extracts obtained with 70% ethanol. A higher AO capacity of extracts in comparison to essential oils was corroborated with a difference in phenolics and terpenoids capacity.