Sensitive and selective determination of phenolic compounds from aromatic plants using an electrochemical detection coupled with HPLC method.

Introduction: Phenolic compounds contained in essential oils from plants are responsible for their anti-oxidant capacity. The natural extract from each aromatic plant is characterised by a typical ratio of phenolic components, so each one of the essential oils shows different properties.

Objective: The development of a simple reversed-phase high-performance liquid chromatographic (RP-HPLC) method for the determination of phenolic compounds from aromatic plants using spectrophotometric detection with a diode-array and electrochemical detection with amperometric and coulometric detectors.

Methods: Chromatographic conditions are optimised to separate vanillin, eugenol, thymol and carvacrol using spectrophotometric detection. Acetonitrile and methanol are studied as mobile-phase organic modifiers. The hydrodynamic curves are obtained for both electrochemical detection modes and the principal values of merit are calculated. The proposed methodology is applied to determine the four analytes in real samples.

Results: The shortest elution times and the highest electrochemical signals are achieved using 65% methanol solution in 0.1 mol/L acetic acid-acetate buffer as the mobile phase. Potential values of 0.925 V for amperometric detection and 0.500 V for coulometric detection are chosen as working potentials. The limits of detection (LOD) for the compounds studied ranged between 9.7-17 µg/L and 0.81-3.1 µg/L in amperometric and coulometric detection modes, respectively. In general, the obtained LODs are better than those previously reported.

Conclusion: The low LODs obtained using coulometric detection make this methodology very competitive and adequate for quality control of these phenolic compounds in comparison with others, such as GC-MS, that are more expensive and complicated to use than the RP-HPLC method with coulometric detection.