DETERMINATION OF NANOMOLAR LEVELS OF HISTIDINE BY DIFFERENTIAL-PULSE ADSORPTIVE-CATHODIC STRIPPING VOLTAMMETRY OF ITS COPPER(II) COMPLEX

Histidine was determined at the 5 × 10-9-1.6 × 10-7 M level by differential-pulse adsorptive stripping voltammetry at a hanging mercury drop electrode using the reduction peak of its copper(II) complex at -0.27 V versus Ag - AgCl obtained in pH 9.2 borate or hydrogen carbonate buffer: freecopper(II) is reduced at -0.10 V. The copper(II) - tryptophan complex is also adsorbed, is reduced at -0.20 V and interferes when present at five times the concentration of the histidine complex. Other amino acids do not interfere. The copper(II) complexes of cysteine and cystine adsorb and are reduced at significantly more negative potentials than the histidine complex; these amino acids can be determined simultaneously with histidine. Surfactants interfere when present at high concentrations.