Fast screening of total flavonols in wines, tea-infusions and tomato juice by flow injection/adsorptive stripping voltammetry

A novel approach for a fast screening of total flavonols (quercetin, kaempferol, myricetin) in wines, tea-infusions and tomato juice using adsorptive stripping voltammetry in a flow injection system is described. The proposed method is based on the property of flavonols to be pre-concentrated on carbon paste electrodes where diphenylether is used as the pasting liquid. The samples are subjected to an acid hydrolysis step and 1 mL sample of the diluted hydrolysate is injected in the flow injection apparatus. After 2 min pre-concentration period (under reciprocated flow) the electrode is washed with Britton-Robinson buffer, pH 5 solution and an anodic voltammetric scan is applied to produce an anodic current peak (analytical signal). The lower determination limits for kaempferol, quercetin and myricetin in pure aqueous solutions were 20, 9 and 25 μg L−1. Mixtures of flavonols resulted into a single anodic peak, which can be used for the calculation of the total flavonols concentration expressed in terms of “quercetin equivalent”. The measurement throughput was about 13 h−1 and the overall reproducibility of measurements was 3-6%. The total concentration of quercetin equivalent in eight tested samples was found in the range 3-60 mg L−1 and the results compare favorably with those obtained by an LC procedure.