Voltammetric behaviour of morphine at a glassy carbon electrode and its determination in human serum by liquid chromatography with electrochemical detection under basic conditions

Abstract

The electrochemical oxidation of morphine was studied at pH values of between 7.00 and 12.00 by cyclic voltammetry and chronoamperometry at a planar glassy carbon electrode. The peak potential was dependent on pH over the range 7.00–9.75; it was independent of pH above the latter value, indicating a pK_a value of 9.75. The peak current was found to be independent of pH, ionic strength of phosphate buffer (0.02–0.1 mol dm^–3) and percentage of acetonitrile (0–40% v/v). The oxidation was found to occur in three steps; these are considered to result from a one-electron oxidation of the phenoxide group, followed by a one-electron loss from the oxidation product, pseudomorphine, and finally a two-electron loss from a tertiary amine group. A simple method of analysis by high-performance liquid chromatography was developed which employed a column packed with a reversed-phase, pH-stable, octadecylsilane-modified silica. Separation was achieved with a mobile phase containing 20% v/v acetonitrile in 0.05 mol dm^–3 phosphate buffer, pH 11.0. Amperometric detection was carried out with an applied potential of +0.45 V versus Ag–AgCl. The detection limit was 1.24 × 10^–13 mol of morphine injected. The detector gave a linear response from 1.2 × 10^–12 to 4.0 × 10^–10 mol of morphine injected. The extraction method required 0.5 ml of serum, and no solvent evaporation was needed. The recovery of morphine was 80.9%. The method gave a linear response to at least 15.0 × 10^–7 mol dm^–3. The relative standard deviation was 4.95% at 0.75 × 10^–7 mol dm^–3 and 3.73% at 3.0 × 10^–7 mol dm^–3.