Determination of biologically active acids based on the electrochemical reduction of quinone in acetonitrile+water mixed solvent

The reduction peak of quinone in aprotic media appears at a positively shifted potential in the presence of acids, which play a role as proton sources. Since the height of the newly developed peak shows a good correlation with the concentration of the acid, it can be utilized to determine the concentration of weak acids. Although the magnitude of the peak potential shift (ΔE_p) decreases as the water content in the solution increases, the presence of a Brønsted acid still leads to a significantly large ΔE_p in an acetonitrile+water mixture (9/1, v/v) when a proper quinone derivative is chosen. ΔE_p depends on not only the acidity of proton sources but also the basicity in quinone derivatives. Among the various quinones examined in the present study, tetramethyl-1,4-benzoquinone (duroquinone) exhibits the largest ΔE_p and is found to be a suitable redox-active additive for the selective analysis of a variety of organic acids. Based on these results, conventional voltammetric techniques, e.g. differential pulse voltammetry, square-wave voltammetry and linear sweep voltammetry, can be employed to determine various biologically important acids with protonated amine or carboxylate groups. The quantitative analyses of histamine, maleic acid and pyruvic acid are successfully demonstrated in the 10^?5 M range with a high sensitivity of 17～19 μA mM^?1 by linear sweep voltammetry.