Voltammetric Simultaneous Quantification of Lapachone Isomers In Plant Extracts Using Graphite–Epoxy Composite Electrode

The method developed for the determination of lapachone isomers ( α ‐ and β ‐lapachones) involves the use of square wave voltammetry (SWV) with an electrode based on epoxy–graphite composite. The electrolytic aqueous solution contained a cationic surfactant (CTAB), phosphate buffer (pH 6.0), and KNO 3 . The addition of CTAB enhanced analyte diffusion into the electrode–solution interface, improving detection through SWV. Under chosen conditions, β ‐lapachone and α ‐lapachone respectively present reversible and quasireversible processes. The analytical signals were detected at the specific potentials of −370 mV ( α ‐lapachone) and −190 mV ( β ‐lapachone) using 140 s preconcentration at +400 mV. The SWV parameters used include 30 Hz frequency, 40 mV pulse amplitude, and 20 mV potential step. Instrumental detection limits were 2.4 × 10 −7 mol L −1 and 1.4 × 10 −7 mol L −1 respectively for α ‐lapachone and β ‐lapachone. Lapachol and, in a lesser extent, sulfonated β ‐lapachone interfere with both analyte signals, requiring liquid–liquid extraction prior to the determination of α ‐lapachone and β ‐lapachone in ethanolic plant (heartwood of the Tabebuia impetiginosa ) extract. The results obtained using SWV agreed with those achieved by high‐performance liquid chromatography.