Resolution by polarographic techniques of the ternary mixture of captan, captafol and folpet by using PLS calibration and artificial neuronal networks.

The simultaneous polarographic determination of the ternary mixture of captan-captafol and folpet is studied. The polarographic signals of these compounds in their mixture show a high overlapping. For this reason different chemometric methods such as PLS, PCR and artificial neuronal network (ANN) have been utilized for the simultaneous determination of these compounds in mixtures. The calibration model is built from solutions containing river water of known pesticide concentrations and the signals obtained by Sampled DC and DPP (differential pulse polarography) have been used. The analysis of both synthetic and real samples (river water) has been carried out by PLS with satisfactory results in most cases. It is possible to determine 0.25 ppm of each pesticide in river water samples after a preconcentration step by extraction into diethyl ether. ANN has also been applied to improve the results obtained by the PLS tool when the sampled DC current is recorded or when liquid-solid extraction with C18 cartridges is performed.

Reagentless enzyme electrode based on phenothiazine mediation of horseradish peroxidase for subnanomolar hydrogen peroxide determination.

The development and characterization of a highly sensitive enzyme immobilized carbon based electrode for the determination of subnanomolar concentrations of hydrogen peroxide in aqueous samples is described. The biosensor consists of horseradish peroxidase (HRP) immobilized in solid carbon paste along with a suitable redox mediator. The latter allows the acceleration of the electroreduction of HRP in the presence of hydrogen peroxide. Several phenothiazines as mediators are investigated in a comparative manner and with respect to dimethylferrocene using cyclic voltammetry and amperometry. Insolubilization of the HRP in the solid carbon paste is achieved by cross-linking the enzyme with glutaraldehyde and bovine serum albumin. Several experimental parameters such as pH, mediator and enzyme content are considered. The hydrogen peroxide determination is better carried out in 0.1 M acetate buffer, pH 4.5, by amperometry at an applied potential of 0.0 V versus Ag/AgCl, 3 M NaCl concentration and by using the phenothiazine base as redox mediator. The biosensor response is linear over the concentration range 2 nM-10 microM with a detection limit of 1 nM. The linear range of the hydrogen peroxide response without a mediator in the biosensor is found between 2 and 40 microM. The biosensor can be used for more than 180 measurements. Additional modification of the electrode by incorporation of Nafion SAC-13 microparticles in the solid carbon paste allows detection of concentrations of hydrogen peroxide as low as 0.1 nM.

Fluorimetric determination of tin and organotin compounds in hydroorganic and micellar media in the presence of 8-hydroxyquinoline-5-sulfonic acid.

The fluorescence of tin(IV) complexed by 8-hydroxyquinoline-5-sulfonic acid (8-HQSA) has been studied in both aqueous and hydroorganic (acetate buffer and dimethylsulfoxide) media. Several experimental parameters such as pH, DMSO/water ratio and reactant concentration have been investigated to increase the fluorescence of the tin(IV)-8-HQSA complex. A linear relationship between tin(IV) concentration and fluorescence intensity was observed between 1.7 and 20 microM). Mechanistic and quantitative studies in the presence of surfactants have been performed. Judiciously selected micellar media permitted solubilisation and quantitation of tin(IV) as well as dibutyltin compounds. A linear relationship between concentration and fluorescence intensity was found for mono-, di- and tributyltin with detection limits of 0.1 microM, 0.7 microM and 1 microM, respectively.

Electrochemical analysis of surfactants: An overview.

This work presents an overview of electrochemical techniques, namely potentiometry, amperometry, tensammetry, electrocapillary measurements and biosensors, recently applied for the determination of surfactants.

Electrochemical behavior of zopiclone.

The electrochemical properties of zopiclone, an anxiolytic and hypnotic drug, have been investigated by different techniques. The compound is reduced in two 2-electron steps in the pH range 0-12. The first step, which corresponds to the reduction of the pyrazine ring, is reversible in acidic and neutral solutions. Strong adsorption phenomena accompany the reduction process in acidic and neutral media. Zopiclone can be quantitatively measured over the entire pH range using DC polarography. However, the use of differential pulse and square-wave modes for quantitative measurements is more limited due to a slope modification in the current-concentration relationship. Adsorptive stripping voltammetry can be applied to the determination of low levels of the drug at pH 9, but only short deposition times may be used because large amounts of material accumulated under stirring conditions due to fast adsorption kinetics are rapidly released from the electrode surface. Detection limits are 1 x 10(-7)M and 2 x 10(-10)M for polarography and adsorptive stripping voltammetry, respectively. Only the first wave is of analytical interest for both techniques.

Adsorptive voltammetry and hydrolysis kinetics of loprazolam mesilate.

Loprazolam is determined by square-wave adsorptive stripping voltammetry in 0.04M ammonium chloride at pH 4.0, with an accumulation potential of -0.25 V vs. Ag/AgCl/KCl(s), at which the nitro group is reduced to a hydroxylamine group, with adsorption of the product. Cathodic stripping results in reduction of the azomethine bond of the adsorbed product. With a deposition time of 120 sec the detection limit is 2.5 x 10(-10)M. The relative standard deviation is 1.7% for 5 x 10(-8)M loprazolam (60 sec deposition). Reversible hydrolysis of the azomethine group occurs in sulfuric or hydrochloric acid. The reaction is initially first-order, followed by an apparent second-order reaction. First-order rate constants and half-lives are reported for 0.1-1M sulfuric acid and 0.02M hydrochloric acid media and compared with the values for nitrazepam hydrolysis.

Adsorptive stripping voltammetry applied to drug analysis: a powerful tool.

A brief review of the principles and instrumentation of adsorptive stripping voltammetry is presented and the advantages of the method are described. As for many highly sensitive techniques applied to the analysis of complex media, severe interferences may occur. Different approaches can be used to circumvent these problems, exemplified by several applications in biological fluids. The application of modified electrodes to enhance selectivity is discussed.

Electrochemical study of carminomycin at solid electrodes.

The electrochemical behaviour of carminomycin at carbon-paste, glassy-carbon and composite carbon-polymer electrodes has been compared. Three electroactive sites on the molecule have been identified. Two sites are involved in the oxidation processes, the first corresponding to the hydroquinone structure in the C-6/C-11 positions, the second attributed to the C-4 phenolic function. Applying a negative-going scan allows reduction of the quinone moiety in the C-5/C-12 positions. Both the quinone and hydroquinone functions exhibit quasi-reversible electrochemical reactions in acidic medium, becoming irreversible in alkaline solution. Adsorption phenomena accompany both the oxidation and reduction processes, primarily in neutral and alkaline media. The adsorption can be decreased by using the composite electrode, which is less sensitive to surface phenomena. This working electrode is proposed as the most suitable for the determination of carminomycin at pH 1. A linear calibration plot can be obtained over the range 1 x 10(-6)-1 x 10(-4)M, with a limit of detection of 6 x 10(-8)M.

Square-wave adsorptive stripping voltammetry of menadione (vitamin K(3)).

Menadione (vitamin K(3)) undergoes a reversible two-electron transfer involving the quinone structure in acidic medium. As demonstrated by using cyclic voltammetry, the reduced form is more strongly adsorbed than the oxidized one. Stripping voltammetry of an adsorbed layer has been applied to the determination of this molecule after preconcentration of the reduced compound and scanning the potential towards less negative values. Adsorption, which is highly effective when stirring is used, approaches an equilibrium process in quiescent solution, as evidenced by a loss of part of the adsorbed material when stirring is stopped. A square-wave mode has been selected owing to its high sensitivity (the current is 20 times that for the differential pulse mode), but also to its high scan-rate, which minimizes the slow desorption process occurring during the scan. A concentration range from 2 x 10(-10) to 5 x 10(-7)M is easily investigated, the detection limit being 1.3 x 10(-10)M. The influence of several operational parameters has also been considered.

[Not Available]. / Electrochimie de substances médicamenteuses en utilisant de nouveaux modèles d'électrodes modifiées.

The analysis of pharmacologically active compounds using electrochemical techniques with modified electrodes is reviewed. Several physical and chemical methods for the immobilization of materials on to electrodes are available, and the resulting surfaces can be examined microscopically . Examples of these procedures and their analytical applications are given.

Electrochemical behaviour of some neuroleptics: Haloperidol and its derivatives.

The electrochemical characteristics of Haloperidol and related compounds, representative neuroleptics of the butyrophenone family, have been investigated as a function of pH and concentration by direct-current, alternating-current and differential-pulse polarography and cyclic voltammetry at a hanging mercury drop electrode. A single cathodic wave representing an irreversible two-electron reduction is obtained, and its half-wave potential differs from that characteristic of aromatic ketone reduction. Adsorption processes disturb the wave behaviour and an adsorption prewave is observed at high concentrations. Quantitative measurements were successful in the concentration range 1 x 10(-4)-1 x 10(-6)M (0.4 mg/l.), the lower concentration representing the detection limit by differential-pulse polarography.