DETERMINATION OF LIPOIC ACID USING SEMI-DIFFERENTIAL CATHODIC STRIPPING VOLTAMMETRY IN THE PRESENCE OF COPPER(II)

Lipoic acid was determined at the 1 X 10(-8)-1 X 10(-6) mol dm(-3) level by semi-differential cathodic stripping voltammetry with a hanging mercury drop electrode in acetate buffer (pH 4.6) containing copper(II). The optimized conditions include a copper concentration of 1 X 10(-5) mol dm(-3) and an accumulation potential of -0.30 V. The cathodic stripping peaks for lipoic acid obtained in the presence of added copper(II) are usually narrower and higher and are located at more negative potentials than the peaks obtained in the absence of copper(II). Hence, the peak obtained with copper(II) present is considered to be as a result of the reducing reaction of the copper(I) complex formed with dihydroiipoic acid. The possible interference from several trace-metal ions, anions and major amino acids was tested. The detection limit is 8 x 10(-9) mol dm(-3) lipoic acid after a 60 s accumulation.