VOLTAMMETRIC BEHAVIOR OF MORPHINE AT A GLASSY-CARBON ELECTRODE AND ITS DETERMINATION IN HUMAN SERUM BY LIQUID-CHROMATOGRAPHY WITH ELECTROCHEMICAL DETECTION UNDER BASIC CONDITIONS

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 x 10(-13) mol of morphine injected. The detector gave a linear response from 1.2 x 10(-12) to 4.0 x 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 x 10(-7) mol dm-3. The relative standard deviation was 4.95% at 0.75 x 10(-7) mol dm-3 and 3.73% at 3.0 x 10(-7) mol dm-3.