Flow‐injection amperometry of cysteine and glutathione at an electrode modified with a ruthenium‐containing inorganic film

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Abstract
A film that contains ruthenium oxide of mixed valences and cyanide (mvRuCN) on a glassy carbon electrode promotes the diffusion‐controlled oxidation of cysteine and glutathione at 0.92 V vs. Ag/AgCl. These mediated oxidations allow the determination of these thiols by flow‐injection amperometry in pH 2 solutions of 0.2 or 0.02 M K2SO4. Calibration curves are linear over about three orders of magnitude, and detection limits are in the submicromolar range with 7.5 μL injections. The sensitivity for cysteine is about 50 nA/μM with a flow rate of 1.0 mL/min. The throughput calculated on the basis of peak widths was 140 samples/h. Experimentally, with a manual injector, 60 samples/h were routinely analyzed. The sensitivity for glutathione in 0.2 M K2SO4 at pH 2 is somewhat lower, 36 nA/μM. A single electrode was used for hundreds of injections over a 2‐week period with no change in sensitivity. The mvRuCN‐coated electrode is operated in a potentiostatic mode; reactivation of the surface between series of experiments was not necessary. The electrode was stored at open circuit in contact with an electrolyte solution when not in use.

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