Voltammetric behaviour of all-trans-retinol (vitamin A1) at a glassy carbon electrode and its determination in human serum using high-performance liquid chromatography with electrochemical detection

Abstract

Cyclic voltammetry has been used to investigate the oxidation of all-trans-retinol (vitamin A₁) at a planar glassy carbon electrode. The electrode reaction was found to be dependent on the percentage of methanol, pH and ionic strength of the acetate buffer. The maximum electrochemical signal was obtained with a supporting electrolyte containing 95% methanol-0.075 M acetate buffer (pH 5.0). The oxidation was found to occur in two steps and to be irreversible; the reactant was found to undergo adsorption at the glassy carbon surface. The final product may also be adsorbed; this did not affect the signal obtained with a glassy carbon electrode in the wall-jet configuration, but did affect seriously that obtained with a thin-layer cell. Liquid chromatography with electrochemical detection (LCEC) was performed with the optimum supporting electrolyte as the mobile phase; the calibration graph was linear in the range 0.05–22.0 ng injected. The LCEC method was shown to be about four times more sensitive than liquid chromatography with UV detection; the former was applied successfully to the determination of normal and depressed circulating levels of the vitamin in human serum samples.