An Inhibitory Kinetic Method for the Methionine Determination

A fast and reproducible method is proposed for the kinetic determination of methionine (MET). The method depends on the inhibitory property of methionine, which reduces the Hg2+ catalyzed imitation of cyanide from [Ru(CN)6]4- with pyrazine via forming a stable complex with Hg2+. Spectrophotometric measurements were carried out by recording the absorbance at 370 nm (λmax of [Ru(CN)5Pyrazine]3- complex) at a fixed time of 12 min under the optimized reaction conditions with [Pyrazine] = 6.5 × 10-4 M, I = 0.05 M (KNO3), Temp. = 45.0 ± 0.2 oC, pH = 4.00 ± 0.03, [Hg2+] = 7.5 × 10-5 M and [Ru(CN)6 4-] = 5.25 × 10-5 M [46]. With the proposed method, methionine can be determined quantitatively down to 2.5 × 10-6 M. This methodology can be effectively used for the rapid quantitative estimation of MET in the pharmaceutical samples with good accuracy and reproducibility.