Determination of quercetin and its glucosides in onion by electrochemical methods

Quercetin (Q), quercetin-3,4′-diO-β-glucoside (Q3,4′G), quercetin-3-O-β-glucoside (Q3G) and quercetin-4′-O-β-glucoside (Q4′G) were determined in onion bulbs (Allium cepa) by HPLC with amperometric detection after analysis of the hydrodynamic voltammograms of flavonoid standards within the potential range of 50–1000 mV and by cyclic voltammetry (CV) method. The hydrodynamic voltammetric profiles of flavonoids showed that the peak current of Q, Q3G, Q4′G and Q3,4′G increased rapidly when the applied potential exceeded +450 mV (vs. SCE). High sensitivity and low background current were observed at the applied potential of +950 mV (vs. SCE). The lower limits of detection (LOD) were determined at signal-to-noise ratio of 3 and showed the following values: 8.05 × 10−8 M (Q), 1.08 × 10−7 M (Q3G), 1.22 × 10−7 M (Q4′G) and 2.6 × 10−7 M (Q3,4′G). The data provided by HPLC-UV-MS confirmed the presence of Q, Q3G, Q4′G and Q3,4′G in 80% methanol extracts of lyophilised onion bulbs. The CV method was applied for the qualitative assessment of onion flavonoids followed by the determination of anodic peak potential (Ea) of the standards. The qualitative analysis of onion flavonoids was based on the anodic peak current (Ia) of the extracts after external standards addition. The recorded cyclic voltammograms of the above flavonoid standards showed that all four compounds had well-defined oxidation waves with peak potentials of 310, 390, 482 and 800 mV (vs. Ag/AgCl) for Q, Q3G, Q4′G and Q3,4′G in 50 mM acetate–acetic buffer (pH 5.5) in 80% methanol, respectively. The study proved applicability of the CV method for identifying Q, Q3G, Q4′G and Q3,4′G in onion.