The role of 3,4-dihydroxyphenylacetic acid adsorption in the oxidation of homovanillic acid at a glassy carbon rotating disc electrode

Abstract

Homovanillic acid (HVA) is an important biomarker for neurological disorders. Traditional methods for detection of this compound are chromatographic methods coupled with fluorescence, mass spectrometric, or electrochemical detectors. However, voltammetric methods can be useful because of their low cost, easy automation, portability, and sensitivity. 3,4-Dihydroxyphenylacetic acid (DOPAC) is formed during electrochemical oxidation of HVA. The formation of this compound could interfere with the quantification of HVA using electrochemical methods. This work aims at describing the behavior of the intermediate species generated in the oxidation of HVA using cyclic voltammetry (CV), linear staircase voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) with a glassy carbon rotating disc electrode (GC RDE). The importance and desirability of detailed investigation of surface phenomena connected with electrochemical oxidation used for electroanalytical purposes are thus clearly demonstrated. The EIS measurements indicate that the intermediate DOPAC formed by the oxidation of HVA can adsorb on the surface of a glassy carbon electrode (GCE). For the static GCE, a semi-finite linear diffusion was observed and for the GC RDE, a finite-length diffusion was verified. In the case of the semi-finite linear diffusion, EIS measurements revealed that the resistance to charge transfer decreases from 100 to 50Ω, and the capacitance of the double layer increases from 1.44 to 1.66μF with increasing concentration of HVA. However, LSV results showed that Levich equations for electrochemical reduction of K3[Fe(CN)6] probe in basic medium are ilim=−0.157 ω1/2−0.006 (R2=0.9999) in the absence of HVA (i.e. without DOPAC adsorption at GCE) and ilim=−0.154 ω1/2−0.021 (R2=0.9998) in the presence of HVA (i.e. with DOPAC adsorption at GCE). The small difference in these two equations proves a negligible influence of DOPAC adsorption on the quantification of HVA using stationary methods.