A new flow-injection spectrophotometric method for the determination of glutathione (GSH), penicillamine (PEN) and tiopronin (mercaptopropionyl glycine, MPG) in pharmaceutical formulations is reported. The method is based on the reduction of Cu(II)-neocuproine reagent to Cu(I)-neocuproine by GSH, PEN or MPG in buffered medium (pH = 3) to form a stable coloured complex (λmax = 458 nm). Experimental conditions were optimized by univariate method, resulting with linear calibration curves in concentration range from 2 × 10 −6 to 3 × 10 −5 mol L −1 for GSH, 6 × 10 −7 to 4 × 10 −5 mol L −1 for PEN and 4 × 10 −7 to 4 × 10 −5 mol L −1 for MPG. The achieved analytical frequency was 180 h −1 for GSH and PEN and 120 h −1 for MPG. The proposed method was successfully applied for determination of GSH, PEN and MPG in pharmaceutical formulations, and the usual excipients in pharmaceuticals did not interfere with the analysis.
Three simple, sensitive and robust kinetic spectrophotometric methods for the determination of a novel lipophilic thiol compound, N-acetyl-L-cysteine ethyl ester (NACET), have been developed and validated. The methods are based on the reduction of Cu(II)-ligand complex to Cu(I)-ligand complex with the analyte. Studied ligands were neocuproine, bicinchoninic acid (BCA) and bathocuproine disulfonic acid (BCS). The development of chromogenic complexes was followed using kinetic setup with spectrophotometric detection at 458, 562 and 483 nm for the reactions of NACET with neocuproine, BCA and BCS, respectively. The calculated reaction orders with respect to NACET concentration were found to be 1.07, 1.01, 1.07, respectively, thus confirming a first order of reaction. The initial rate and fixed time methods were utilized for constructing calibration curves. Assays limits of detection were 1.4 × 10 -7 , 3.2 × 10 −7 and 6.0 × 10 −8 mol L -1 , respectively. The analytical performance of the methods, in terms of accuracy and precision, was established.
A novel and simple method for the determination of penicillamine (PEN), tiopronin (mercaptopropionyl glycine, MPG) and glutathione (GSH) in pharmaceutical formulations by kinetic spectrophotometry has been developed and validated. It is based on the redox reaction where the thiol compound (RSH) reduces CuII-neocuproine complex to CuI-neocuproine complex. The non-steady state signal of the formed CuI- neocuproine complex is measured at 458 nm. The initial rate and fixed time (at 1 min) methods were validated. The calibration graph was linear in the concentration range from 8.0 × 10‒7 to 8.0 × 10‒5 mol L−1 for the initial rate method and from 6.0 × 10‒7 to 6.0 × 10−5 mol L−1 for the fixed time method, with the detection limits of 2.4 × 10−7 and 1.4 × 10‒7 mol L−1, resp. Levels of PEN, MPG and GSH in pharmaceutical formulations were successfully assayed by both methods. The advantages of the presented methods include sensitivity, short analysis time, ease of application and low cost.
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