Voltammetric determination of lapachol in the presence of lapachones and in ethanolic extract of Tabebuia impetiginosa using an epoxy-graphite composite electrode

Abstract

Square-wave anodic voltammetry (SWV) was used to determine lapachol using a laboratory-made graphite-epoxy electrode. The electrolytic medium consisted of an aqueous solution containing cationic surfactant CTAB (1.2×10−4molL−1), buffer phosphate (4.0×10−2molL−1; pH6.0) and KNO3 (1.0molL−1). The cationic surfactant improved diffusion and interaction of the analyte with the electrode, producing a reversible process that improved the total current measured by SWV. Lapachol signal was measured at −470mV after pre-concentration at 400mV during 140s, using frequency of 30Hz, pulse amplitude of 40mV with step potential of 20mV. The instrumental limit of detection was 0.029mgL−1 and the dynamic linear range covered two decades. In the presence of β-lapachone (structural isomer) selectivity was achieved by first order derivative of the SWV signal. Lapachol determination in ethanolic extract of heartwood of Tabebuia impetiginosa was accomplished after a simple thin-layer chromatographic separation of the analyte from components of the complex sample. The results agreed (at 95% confidence level) with the ones achieved by using high performance liquid chromatography with absorciometric detection.