The performance and analytical characteristics of a glassy carbon glutaraldehyde immobilized glucose oxidase electrode have been established with regard to the direct detection of hydrogen peroxide produced from the reaction of glucose with oxygen. Measurements were performed at + 1.1 V vs. SCE, and selectivity was obtained by casting the surface with a cellulose acetate membrane. Results compared favorably with the classical platinum-enzyme probe. The mechanism of ascorbic acid interference in hydrogen peroxide detection is reported. Mediated detection was also investigated for oxidase enzymes (glucose oxidase and xanthine oxidase) immobilized on the bare glassy carbon electrode. The probes were characterized using a specific enzyme mediator in solution (phenazine methosulfate or dichlorophenol-indophenol) plus hexacyanoferrate(III) as an electrochemical mediator. The electrode was poised at + 0.36 V vs. SCE for the detection of hexacyanoferrate(II). The advantages of this dual mediator configuration include high stability and sensitivity of the electrochemical signal and the ability to use less positive potentials for increased selectivity. Application to other enzymes, such as hydrogenases, using such a binary redox configuration is suggested.

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 × 10−7M and 2 × 10−10M for polarography and adsorptive stripping voltammetry, respectively. Only the first wave is of analytical interest for both techniques.

The electrochemical behaviour of tizanidine [5-chloro(DELTA-2-imidazolinyl-2-amino)-4-benzothiadiazole-2,1,3], a centrally-active skeletal muscle relaxant has been investigated in aqueous media at the carbon paste electrode (CPE). Cyclic voltammetry at different pH values, controlled potential coulometry and comparative studies on three structurally related molecules have permitted identification of the oxidation site of tizanidine and suggest possible oxidation products in acidic media. The electrochemical reduction at the CPE occurred in one irreversible step and the reduction product (diamine derivative) was detected and characterized on the positive going scan in cyclic voltammetry. Quantitative measurements of tizanidine within the range 2 x 10(-5) M and 1 x 10(-4) M have been realized at the CPE using the differential pulse technique.

Three types of polymeric electrodes were using a poly(vinyl chloride) film plasticized with a dioctyl phthalate-nitrobenzene mixture and containing an ion pair of the drug tizanidine. The membrane of the so-called conventional electrode, which includes an internal Ag/AgCl reference, was immobilized on a micropipette tip. A single-layer electrode was prepared by coating a graphite rod with the membrane, while the graphite rod of a two-layer electrode was previously coated with a poly(4,4′-biphenol) film before the sensitive membrane was deposited. These electrodes exhibit a Nernstian response in the concentration range 5 x 10−6−1 x 10−2 M with a slope between 55 and 57 mV per decade. The response is not affected by pH between 3 and 7. The influence of several parameters involved in the construction of these electrodes on their lifetime is discussed, the highest durability being observed for the conventional electrode. Selectivity coefficients against various organic and inorganic cations were evaluated. The conventional electrode was applied to the determination of tizanidine in drug formulations using direct potentiometry.

Le comportement électrochimique de la mitoxantrone à l'électrode de mercure a été étudié en technique chronocoulométrique en faisant appeal aux méthodes polarographiques conventionnelle et impulsionnelle. La vague polarographique biélectronique enregistrée est réversible et son potentiel de demi-vague ainsi que son intensité dépendent du pH. Les mesures de capacité d'électrode mettent en évidence l'adsorption des formes oxydée et réduite de la mitoxantrone à la surface du métal. A pH = 1 et en technique impulsionnelle (intégration des courants de l à 41 ms aprés le saut de potentiel) la vague principale est suivie d'une vague totalement dissymétrique qui résulte de la réduction du film adsorbé au potentiel initial. Cette dernière vague est contrôlée par un p[rocessus de nucléation et croissance bidimensionnelle.

Previous voltametric experiments have shown the electrooxidation of cisplatin (cis-diamminedichloroplatinum II, CDDP) and its hydrolysis products at the glassy carbon electrode. Taking into account the advantages of the oxidative detection mode(rather than reduction) and by using an easy to handle wall-jet configuration, the sensitive HPLC determination of cis-platin has been successfully performed in biological samples. Several counter-ions generally employed in ion-pair chromatography have been investigated in order to optimize the analysis of CDDP in terms of separation efficacy, retention time and compatibility with the electrochemical detection mode.The cationic detergent cetyltrimethylammonium chloride(CTACl) has been found to be the most appropriate ion-pairing reagent for the determination of intact CDDP in spiked plasma.The limit of detection was 30 ng/ml and 150 ng/ml in aqueous and plasma samples, respectively. The recovery of cis-DDP in plasma was 86 +/- 5 % . At a concentration of 10 mug/ml, the coefficient of variation was 6 % (within-day) .

The electrochemical behavior of mitoxantrone at the mercury electrode has been studied chronocoulometrically using direct and pulse polarographic techniques. The recorded two-electron wave is reversible and both its half-wave potential and its intensity depend on pH. Electrode capacity measurements demonstrate the adsorption of both the oxidized and the reduced forms of mitoxantrone. Using the pulse method at pH 1 (current integration from 1 to 41 ms), the main wave is followed by a dissymmetrical wave resulting from the reduction of the film adsorbed at the initial potential. This last wave is controlled by a nucleation and bidimensional growth process.

The amperometric response of a mixed-glucose oxidase-redox mediator-carbon paste electrode, permanently poised at the operating potential, has been investigated in phosphate buffer as a function of glucose concentration over a period of four weeks. Results of the cyclic voltammetric data of the amphiphilic ferrocene mediator and the high positive operating potential applied (+ 0.6 V vs S.C.E) contributed significantly to the interpretation of the observed phenomena. The evolution of the apparent Michaelis-Menten constant (exponential decline) and the slope of the response (cyclic fluctuation) over the entire electrode lifetime support the suggestion that mediator leaching and gradual surface erosion mainly limit the analytical performance of the biosensor.

Polarographic reduction of minoxidil (1 x 10(-5) - 5 x 10(-4) M) and spectrophotometric study were carried out at different pH!s pK values of the acid-base equilibria were calculated using both techniques. In addition, detection limits and sensitivity were determined using electrochemical (taste, differential pulse and pseudoderivative pulse polarography) and spectrophotometric techniques. A simple method is described for the determination of this compound in pharmaceutical formulations. Electrochemical oxidation was also performed at carbon paste and at lipid modified carbon paste electrodes.

The construction and performance of ion-selective membrane electrode for neuroleptic tranquilizers by coating a graphite rod with a film of polyvinylchloride plasticized with a mixture of dioctylphtalate-nitrobenzene and incorporated by the phosphotungstate as counter-ion is described. The electrode exhibites a Nernstian response in the range of pH 3 to 4 for the fluphenazine, between pH 3.5 to 5.5 for the methixene and pH 5.1 to 6.3 for the dibenzepine. The ion-selective electrode slopes were into the range 53-55 mV per concentration decade over 10(-3) - 10(-6) M. Selectivity coefficients were determined for organic substances containing the pyrrolidin, piperazin and piperidin. Dibenzo compounds present a certain degree of interferences but the benzodiazepins family do not interfere. Direct potentiometry is used to determine these compounds in pharmaceutical preparation with accurate results.

Glucose oxidase is immobilized onto a cellulose acetate membrane by glutaraldehyde linkage, and the membrane is used to cover the platinum electrode of a hydrogen peroxide sensor. A silanized polycarbonate membrane then covers the enzyme layer, and extends the linear calibration range to higher concentrations. The sensor, when incorporated into a flow-injection system, allows the determination of glucose at levels up to 1 M in soft drinks at a rate of 60 samples h−1 without sample dilution.

The modification of carbon-paste electrodes by incorporation of the enzyme glucose oxidase (GOD) is described. The resulting probes can be operated as amperometric glucose sensors in the presence or absence of a mediator (1,1'-dimethylferrocene) mixed into the paste. Extended linear calibration ranges have been obtained up to 90 and 5OmM glucose respectively. The electrode responses were rapid, reaching steady-state values within 30-40 sec. Advantages of using a GOD-paste formulation are suggested. Plasma glucose assays were correlated with spectrophotometric determinations based on glucose oxidase (y = 1.07x - 0.16, r = 0.973, n = 17).

New polymeric electrodes responding to the cationic forms of tetracaine (TC), lidocaine (LD), and procaine (PC) were constructed by incorporating their ion-pair complexes (the salts of TC, LD and PC with phosphotungstic acid) into ethylene-vinyl acetate (E/VAC) copolymer. Other ion pairing agents investigated were silicotungstate and tetraphenylborate. The phosphotungstic acid resulted in the best linear and Nernstian response. A 1:1 (v/v) mixture of dioctyl phthalate (DOP) and nitrobenzene (NB) was used as plasticizer. The electrodes exhibited linear response over the concentration ranges 10(-2)-5.6 x 10(-6), 10(-2)-2.5 x 10(-5) and 10(-2)-1.8 x 10(-5) M of TC, LD and PC, respectively. pH did not affect the electrode performances within the ranges 2.7-6.3, 2.6-6.7 and 2.8-7.5 for the three electrodes, respectively. Interferences are negligible for many organic base and alkali metal cations. Cations of similar structure interfere with LD and PC, but not appreciably with TC. Direct potentiometry was used to determine these compounds in pharmaceutical preparations with accurate results.

A new polymer matrix based membrane electrode with an ion-exchanger responding to calcium was constructed by dissolving the copolymer ethylene-vinyl-acetate together with the ion-exchanger in chloroform in the presence of a mixture of dioctylphthalate-nitrobenzene as plasticizer. The ion-exchanger used as the electroactive component was calcium didecyl phosphate in di-(n-octylphenyl) phosphonate (Orion). This electrode exhibited near-Nernstian response over the concentration range 10(-1)-4 x 10(-6)M calcium. The pH did not affect the electrode performance within the range 8-11. Response time varied from 15 to 120 sec and the lifetime exceeded six months. The membrane is subject to static charge buildup, but this is avoided by controlling the level of dryness of the membrane. Selectivity coefficients determined for both monovalent and divalent cations showed negligible interference by most of these ions. The electrode was applied successfully to the determination of calcium in commercial mineral waters.

The electrochemical behavior of Tizanidine has been studied using dc, ac, and pulse polarographic techniques. Reduction of the compound occurs through an electrochemical mechanism involving a six-electron transfer. Autoinhibition of the process resulting from the adsorption of species affects the reduction rate constants. At the electrode surface, adsorption proceeds through a competition between the oxidized and reduced forms.

AbstractAn overview on pharmaceuticals treated by electroanalytical techniques is presented. The review covers the period between 1987 and 1990 and includes more than 100 references. It includes the use of mercury and solid modified electrodes. Recent trends are discussed. The coverage is not exhaustive.

La mesure des excès superficiels des particules de quinine et de flumazénil adsorbées à lélectrode de mercure est réalisée en tecnique de saut potentiostatique, le potentiel initial étaNT situé dans la prÉvague d'adsorption et le potentiel de réduction dans la région du courant limite de la vague principale. Les charges transitoires portées en fonction de la racine carrée du temps de mesure présentent aux temps courts un comportement non linéaire qui est interprété en termes d'une surconcentration locale en particules au plan x = 0 de l'électrode. Cette superconcentration résulte de la dÉsorption rapide de la particule qui précède sa réduction au potentiel de la vague principale.

A lipid-modified carbon-paste electrode is prepared by mixing phospholipids with the carbon-paste matrix. The resulting electrodes have polar head-groups which allow interactions with positively charged drugs and improved preconcentration/extraction steps. The accumulation of the antitumor drug, marcellomycin, is enhanced in the presence of lipids, giving an 8-fold enhancement of current. The electrode response has been optimized with respect to paste composition, nature of the lipid, pH, temperature, and stirring time. A mechanism for marcellomycin accumulation is proposed, based on electrochemical and UV-visible spectrometric measurements as a function of pH. The electrode response is linearly related to the marcellomycin concentration within the range 1 x 10(-8)-6 x 10(-6)M. Known amounts of marcellomycin added to a urine sample diluted sixfold with water have been measured by use of the medium-exchange technique.