Flow injection determination of methionine with amperometric detection at a stable modified electrode

A glassy carbon electrode that is modified by electrodeposition of a film comprising mixed valence ruthenium oxide stabilized by cyano crosslinlks catalyzes the oxidation of methionine at 0.92 V vs. Ag/AgCl. A linear calibration curve is obtained for flow injection amperometry over the range 0.6–180 μM with a 0.2 M K2SO4 carrier at pH 2, a 7.5 μL sample volume, and a 1.0 mL/min flow rate. The sensitivity is 20 ± 0.3 nA/μM at a 0.07 cm2 electrode. With a single surface preparation, the sensitivity of the method remains constant over more than 3 weeks with daily use. The method also was used with carriers of the composition of common liquid chromatographic eluents such as various citrate–phosphate mixtures and a 0.1 M chloroacetic acid solution at pH 3.0 in 1% acetonitrile.