Selective determination of tryptophan by using a carbon paste electrode modified with an overoxidized polypyrrole film

We report on the selective determination of tryptophan, using a carbon paste electrode coated with an overoxidized polypyrrole film. Out of 21 protein amino acids, only tryptophan and tyrosine exhibited an oxidative voltammetric response with this electrode. Tryptophan, which was preferentially concentrated to the electrode under an open circuit condition, was determined by the stripping voltammetric technique with a linear response range of 10-100 AM. For the determination of 10 muM tryptophan, interference from a 15-fold excess of tyrosine gave an positive error of 6%, while the other amino acids did not exhibit any detectable interference.