Electrocatalytic tetracycline oxidation at a mixed-valent ruthenium oxide--ruthenium cyanide-modified glassy carbon electrode and determination of tetracyclines by liquid chromatography with electrochemical detection

Mixed-valent films of ruthenium oxide-ruthenium cyanide were electrodeposited onto glassy carbon and characterized for the electrocatalytic oxidation of tetracycline. The currents produced by tetracycline were higher than from previously reported electrode modifications or pretreatments. In H(2)SO(4) pH 1.0 + 0.5 M K(2)SO(4), the second-order rate constant for the reaction between tetracycline and the Ru(III/IV) couple of ruthenium oxide was 3 x 10(5) +/- 1 x 10(5) mol(-1) cm(3) s(-1), and the rate of charge diffusion through the films was 4.5 x 10(-7) +/- 3.5 x 10(-7) cm(2) s(-1). Reaction was localized at the film-solution interface. When used as detectors in liquid chromatography (in H(3)PO(4) pH 2.5 + 0.1 M KH(2)PO(4) + 20% CH(3)CN, E = 1.10 V vs SCE), the electrodes gave limits of detection (>3 S/N) of 0.1 ppm for tetracycline and oxytetracycline and 0.5 ppm for doxycycline and chlorotetracycline. These limits were suitable for FDA and Codex Alimentarius guidelines for tetracyclines in food. Recoveries of the four tetracyclines from sea and freshwater shrimp were in the range 73-111%, which was higher or similar to the previously reported recoveries from shrimp.