Determination of thiocompounds by liquid chromatography with amperometric detection at a Nafion/indium hexacyanoferrate film modified electrode

Polynuclear indium(III) hexacyanoferrate(II,III) thin films are electrodeposited on a glassy carbon (GC) electrode through potential cycling between 1.1 and 0.0V vs. SCE in a fresh modification solution containing potassium, indium(III) and hexacyanoferrate(III) ions. During the negative scans, the cationic In³⁺ species interact with the simultaneously generated anionic Fe(CN)^4-₆ to yield a sparingly soluble mixed-valent coating on the electrode. Coated with Nafion film, the chemically modified electrode (CME) shows a better stability. The CME shows a single set of reversible, well-defined peaksc significantly separated from the solution responses. The apparent surface coverage is found to be 6.3x10^-9mol/cm². This CME exhibits an efficient catalytic activity towards the rapid oxidation of thiocompounds in cyclic voltammetry. Cysteine (Cys), glutathione (GSH), 6-thiopurine (6-TP) and methimazole (MMI) have good and stable current responses at the CME in liquid chromatography with electrochemical detection (LC-ECD). The linear ranges are over three orders of magnitude and the detection limits are 1.6pmol for Cys, 20pmol for GSH, 20pmol for 6-TP and 1.6pmol for MMI. The method has been successfully applied to assess the purity of methimazole, the contents of cysteine in human urine and other thiocompounds in artificial urines. Copyright (C) 1999 Elsevier Science B.V.