1,1′-[Ethenylidenebis(sulfonyl)]bis-benzene: A useful pre-chromatographic derivatization reagent for HPLC analysis of thiol drugs

A simple spectrophotometric method for the determination of captopril in pharmaceutical preparations using ammonium molybdate

A simple, rapid and sensitive spectrophotometric method for the determination of captopril (CPT) in pharmaceutical formulations is proposed. This method is based on the reduction reaction of ammonium molybdate, in the presence of sulphuric acid, for the group thiol of CPT, producing a green compound (λ max 407 nm). Beer's law is obeyed in a concentration range of 4.60 x 10-4 - 1.84 x 10-3 mol l-1 of CPT with an excellent correlation coefficient (r = 0.9995). The limit of detection and limit of quantification were 7.31 x 10-6 e 2.43 x 10-5 mol l-1 of CPT, respectively. The proposed method was successfully applied to the determination of CPT in commercial brands of pharmaceuticals. No interferences were observed from the common excipients in the formulations. The results obtained by the proposed method were favorably compared with those given by the official reported method at 95 % confidence level.

Simultaneous determination of aminothiols, ascorbic acid and uric acid in biological samples by capillary electrophoresis with electrochemical detection

A method based on capillary electrophoresis with electrochemical detection has been employed for the separation and determination of homocysteine, cysteine, reduced glutathione, ascorbic acid and uric acid. Effects of several important factors such as the acidity and concentration of the running buffer, separation voltage, injection time and detection potential were investigated to acquire the optimum conditions. The detection electrode was a 500 microm diameter platinum disk electrode at a working potential of +1.05 V (vs saturated calomel electrode). The five analytes were well separated within 10 min in a 50 cm long fused silica capillary at a separation voltage of 18 kV in a 100 mm phosphate buffer (pH 7.8). The relation between peak current and analyte concentration was linear over about 3 orders of magnitude with the detection limits (S/N = 3) ranging from 0.83 to 2.58 microm. The proposed method was successfully applied to determine cysteine, reduced glutathione, ascorbic acid and uric acid in human whole blood and rat brain tissues with satisfactory assay results and should find a wide range of bioanalytical applications.

Potentiometric determination of captopril in pharmaceutical formulations

A simple, precise, rapid and low-cost potentiometric method for captopril determination in pure form and in pharmaceutical preparations is proposed. Captopril present in tablets containing known quantity of drug was potentiometrically titrated in aqueous solution with NaOH using a glass pH electrode, coupled to an autotitrator. No interferences were observed in the presence of common components of the tablets as lactose, microcrystalline cellulose, croscarmellose sodium, starch and magnesium stearate. The analytical results obtained by applying the proposed method compared very favorably with those obtained by the United States Pharmacopoeia Standard procedure. Recovery of captopril from various tablet dosage formulations range from 98.0 to 102.0%.

Quantitative analysis of N-acetylcysteine and its pharmacopeial impurities in a pharmaceutical formulation by liquid chromatography-UV detection-mass spectrometry

A new method for the simultaneous determination of N-acetylcysteine and its pharmacopeial impurities, cysteine, cystine, N,N'-diacetylcystine and N,S-diacetylcysteine in an effervescent tablet has been developed. The method is based on on-line LC-UV-MS using a pneumatically-assisted electrospray interface (ionspray). The stability of the thiol moieties of the analytes was ensured by the acidic pH of the LC mobile phase. Quantitation of N-acetylcysteine was performed with UV detection to avoid ion-source overloading effect due to its higher concentration, whereas the impurities could be easily separated and quantified in MS. The method was validated in terms of stability, linearity, precision and accuracy.

Determination of thiols following their separation by CZE with amperometric detection at a carbon electrode

The amperometric detection (AD) employing a carbon disk electrode as a working electrode to determine the thiol compounds, including cysteine (CYS), glutathione (GSH), 6-thiopurine (TP), and methimazole (MMI), following their separation by capillary zone electrophoresis (CZE) is described in this paper. The detection potential was chosen at +1100 mV and all analytes exhibit good response at this potential on the carbon disk electrode. The reproducibility, linearity, and recovery were evaluated under the optimum conditions. The four analytes can be baseline resolved within 20 min and the detection limits reached about 10(-6) mol/l of magnitude. The method was applied to the separation and determination of the actual thiol samples and the analytical results were satisfactory.

Chiral separation of homocysteine by derivatization with 4-aminosulfonyl-7-fluoro-2,1,3-benzoxadiazole followed by capillary electrophoresis using gamma-cyclodextrin

Homocysteine was derivatized with 4-aminosulfonyl-7-fluoro-2,1,3-benzoxadiazole (ABD-F) to form an inclusion complex with cyclodextrin and to facilitate UV detection. ABD-homocysteine showed interaction with beta- and gamma-cyclodextrin in capillary electrophoresis at pH 2.25 as indicated by the decreased migration time. However, chiral separation of D,L-ABD-homocysteine was observed using gamma-CD only. Optimal separation was obtained at pH 2.25, 50 mM gamma-CD concentration, and 20 kV applied voltage. L-ABD-Homocysteine migrated faster than the D-isomer as demonstrated by a spiking experiment using dithiothreitol-reduced L-homocystine.

Simultaneous determination of tiopronin and its metabolites in rat blood by LC-ESI-MS-MS using methyl acrylate for stabilization of thiol group

A sensitive and reproducible HPLC-electrospray tandem mass spectrometric method has been developed for the analysis of tiopronin (TP) and its metabolites, 2-mercaptopropionic acid (2-mpa) and S-methylated TP (SA13), in rat blood using methyl acrylate (MA) for the stabilization of a thiol group. The thiol groups of TP and 2-mpa in rat blood were immediately derivatized by the addition of MA-acetonitrile solution in 0.1 M Tris HCl (pH 9.1). The purification of the derivatives was accomplished by a simple liquid-liquid extraction procedure involving protein precipitation step. The analysis was performed on a Zorbax SB-C18 analytical column by a gradient elution with methanol-0.05 M acetic acid (15:85 and 7:3, v/v). Negative ion electrospray ionization with selected reaction monitoring was employed for the detection of analytes. Linearity of calibration was observed over the range of 0.5-1000 ng/ml for TP and 2-mpa, and 2-1000 ng/ml for SA13. The intra- and inter-assay variability for all analytes at the limit of quantitation (LOQ) level ranged from 5.47 to 16.75% and 4.95 to 7.23%, respectively. The LOQs estimated for TP, 2-mpa and SA13 were 0.5, 0.5 and 2 ng/ml, respectively. This assay method was successively applied to a pharmacokinetics study after an oral administration of TP (10 mg/kg) to rats.

Utilization of carbon disulphide for the analytical determination of betahistine hydrochloride and captopril in their pharmaceutical preparations

Spectrophotometric, atomic absorption spectrometric and high performance liquid chromatographic (HPLC) procedures have been developed for the determination of betahistine hydrochloride and captopril. The three procedures are based on the reaction of the drugs with carbon disulphide in alkaline medium with the formation of the dithiocarbamate or the trithiocarbonate derivative of betahistine (BHT) and captopril (CAP), respectively, then subsequent chelation with divalent metals. The absorbance measurement of the formed chelates or of the metal moiety of chelates was used as the basis for the spectrophotometric and atomic absorption spectrometric determinations. The formed complexes have been used as pre-column derivatizing procedure for the HPLC determination of the two drugs. The different experimental conditions were optimized. The calibration graphs were linear over the applicable concentration ranges. The proposed procedures were applied successfully for the determination of the two investigated drugs in their tablets dosage form.

Determination of captopril and its degradation products by capillary electrophoresis

Captopril, an antihypertensive agent, and its degradation products have been quantified in pharmaceutical formulations by capillary zone electrophoresis (CZE). A method using cetyltrimethylammonium bromide (CTAB) added to a sodium phosphate buffer (pH 5.5; 100 mM) as running buffer and using N-acetyl-L-tyrosine as an internal standard has been developed and validated for the quantitative determination of captopril in tablets. The method is an indicator of compound stability and can also be applied to the purity control of the raw material and for the determination of the degradation products. For this purpose, salicylic acid is used as an internal standard.

Analytical study of penicillamine in pharmaceuticals by capillary zone electrophoresis

The ability of capillary zone electrophoresis in the development of analytical methods devoted to the quality control of the thiol drug penicillamine is shown. Using 50 mM phosphate running buffer (pH 2.5), good quantitations of underivatized penicillamine and its disulfide were achieved; detection at 200 nm allowed checking the presence of the disulfide impurity in pharmaceuticals. The use of 1,1-[ethenylidenebis(sulfonyl)]bis-benzene as a thiol specific reagent resulted in an increased sensitivity for the quantitation of D-penicillamine (limit of detection at 200 nm wavelength was 1.5 microM). Introducing beta-cyclodextrin as chiral selector in the running buffer, enantioseparation of D-L-penicillamine was obtained; for this purpose (+)-camphor-10-sulfonic acid, a chiral ion-pairing reagent, was found to be an essential additive in obtaining a baseline separation. The resulting enantioseparative system was validated in order to evaluate the presence of the toxic L-penicillamine enantiomer in pharmaceutical samples.

Simultaneous high-performance capillary electrophoresis analysis of the reduced and oxidised forms of ascorbate and glutathione

We describe here a procedure for the simultaneous analysis of the oxidised and reduced forms of the major cellular hydrophillic antioxidants, ascorbic acid (vitamin C) and glutathione (gamma-L-glutamyl-L-cysteinylglycine), by high-performance capillary electrophoresis. Separations are performed in uncoated fused-silica capillaries using 200 mmol/l borate pH 9.0, containing 20% (v/v) acetonitrile as the background electrolyte with fixed-wavelength UV absorbance detection at 185 nm. The influence of pH, organic solvent and other additives on the resolution of these compounds is described and we show that the optimised protocol is capable of simultaneously resolving other thiol components including, N-acetylcysteine and methyl-S-glutathione. The method is suitable for the analysis of these antioxidants in Arabidopsis and Nicotiana leaf tissue and is compatible with the use of the high ionic strength, acidic extraction solvents which are necessary to quench the redox equilibria of these labile components.

Determination of glutathione, cysteine and N-acetylcysteine in rabbit eye tissues using high-performance liquid chromatography and post-column derivatization with 5,5'-dithiobis(2-nitrobenzoic acid)

A high-performance liquid chromatographic method to determine glutathione, cysteine and N-acetylcysteine in rabbit retina, vitreous and lens has been developed. The thiols are separated using a 25 x 0.46-cm octadecylsilane column with 0.5 M phosphate buffer, pH 3, as mobile phase. The detection, at 412 nm, involves a post-column derivatization with 5,5-dithiobis(2-nitrobenzoic acid) in presence of cationic micelles of hexadecyltrimethylammonium bromide that enhances the sensitivity. The detection limits are 0.21, 0.92 and 0.61 mumol/g wet sample for glutathione, cysteine and N-acetylcysteine, respectively.