Paulino Tuñón Blanco

Electrochemical behaviour of clenbuterol at nation-modified carbon-paste electrodes

A detailed study of the electrochemistry of clenbuterol at bare carbon-paste electrodes (CPEs) has been carried out. Results showed that clenbuterol undergoes an ECE process. This compound is irreversibly oxidised at high potentials, resulting in the formation of a product which demonstrates quasi-reversible electrochemical behaviour at less positive potentials. The amount of this chemical product formed is very pH-dependent. Investigations into the electrochemical behaviour of clenbuterol at Nafion-modified CPEs were also made. The use of a thin Nafion film cast over the CPE resulted in a large increase in peak current over bare electrodes. Linear accumulation occurred with time, the linear range increasing with decreasing concentration. This allowed the detection of low concentrations of clenbuterol. Diffusion proved to be the rate-controlling process of clenbuterol through the Nafion membrane.

Voltammetric study of salbutamol, fenoterol and metaproterenol at unmodified and Nafion-modified carbon paste electrodes

An electrochemical study of three important β-agonist drugs at unmodified and Nafion-modified carbon paste electrodes was carried out. All the compounds are oxidized irreversibly at high positive potentials at a bare carbon paste electrode, giving rise to sharp, well-defined peaks. A secondary oxidation process was observed at pH values above 6.0. The rate-determining step was investigated for each compound at two concentration levels. Electrochemical activation procedures were optimized to ensure reproducible signals for the construction of calibration graphs. The modification of the carbon paste surface with a Nafion film allowed a preconcentration process to take place for all compounds, such that higher sensitivities were achieved compared with the bare surface. Such modifications resulted in limits of detection for the compounds down to 2.5 × 10–8 mol dm–3. Optimum accumulation was obtained at low pH values (2–3). Cyclic voltammetry at two Nafion film thicknesses demonstrated that a diffusion-controlled process exists within the Nafion layer. Salbutamol showed a higher affinity for Nafion than the other two compounds, both in terms of longer linear accumulation and linear accumulation at higher concentrations.

ELECTROCHEMICAL PRETREATMENT OF CARBON FIBRE MICROELECTRODES FOR THE DETERMINATION OF FOLIC ACID

Abstract An electrochemical pretreatment regime was optimised for the determination of folic acid at cylindrical carbon fibre microelectrodes. The effects of pretreatment on the functional characteristics of the microelectrode were examined. The pretreated electrode gave rise to higher faradaic and capacitive currents, presumably due to the presence of new surface functionalities. Differential pulse voltamrnograms of folic acid recorded in 0.1 M perchloric acid at the activated electrode exhibited a very well defined reduction peak resulting in a detection limit of 1×10−8 M with a linear range of 2×10−8 M to 1×10−6 M folic acid.

Determination of dimethylamine in groundwater by liquid chromatography and precolumn derivatization with 9-fluorenylmethylchloroformate☆

A reversed-phase liquid chromatographic method involving precolumn derivatization with 9-fluorenyl-methylchloroformate (FMOC) has been developed for the determination of dimethylamine (DMA) in groundwater. FMOC reacts rapidly with DMA under mild conditions, and the derivative is separated from matrix components and FMOC degradation products on a C18 column using isocratic elution with a mobile phase of 0.03 M acetate buffer (pH 4.0)-acetonitrile (30:70, v/v). Mean recovery was 98.5±4% and a detection limit of 4.5·10−7 M, corresponding to an injected amount of 3.6 pmol, was estimated using fluorimetric detection with excitation and emission wavelengths of 265 and 310 nm, respectively. No matrix interferences were found even when highly coloured and/or cloudy samples were examined. Several mixtures containing eleven amines, diamines and amino acids can be resolved using this method.

Voltammetric assay of heroin in illicit dosage forms

The oxidation of heroin on a carbon paste electrode has been studied by using voltammetric techniques under both semi-infinite linear diffusion and hydrodynamic conditions. By employing a simple and rapid in situ pre-treatment of the electrode, excellent reproducibility of the current signal was obtained. Subsequently, the current was measured and the concentration of total heroin present could be determined. The method compared favourably with instrumental methods that are more commonly used for the quantification of heroin in clinical laboratories such as chromatographic, spectrophotometric or radiometric techniques. Moreover, the proposed method showed good selectivity and was superior with respect to cost and time of analysis, permitting the determination of heroin in seized drug samples over a wide concentration range.

Application of a Nafion-modified carbon paste electrode for the adsorptive stripping voltammetric determination of fenoterol in pharmaceutical preparations and biological fluids

The preconcentration of fenoterol on a Nafion-modified carbon paste electrode and its subsequent determination using differential pulse voltammetry is described. The effect of pH and percentage Nafion concentration on the accumulation behaviour of fenoterol was studied, and accumulation curves, calibration graphs and reproducibility studies at two different Nafion concentrations have been carried out in the range 2.5 x 10(-8)-5.0 x 10(-7) M fenoterol. A limit of detection in aqueous solutions, calculated using a signal-to-noise ratio (S/N) of 3, was 9.0 x 10(-9) M. Application of the electrode to pharmaceutical preparations, without sample pretreatment, resulted in acceptable deviation from the stated concentration (RSD = +/- 3.81%, n = 4). For more complex matrices, a suitable extraction procedure was developed, resulting in recoveries of > 90% (urine) and > 75% (serum).

Adsorptive stripping voltammetric determination of pipemidic acid in human urine

The electrochemical behaviour of pipemidic acid (8-ethyl-5,8-dihydro-5-oxo-2-(1-piperazinyl)-pyrido[2,3-d]pyrimidine-6- carboxylic acid), a well known antimicrobial agent used for urinary infections, was investigated by linear-sweep, differential-pulse and square-wave voltammetry at a hanging mercury drop electrode. Two reduction processes were observed in Britton-Robinson buffers at acid pH, whereas only one or two processes were observed in alkaline solutions, dependent on the pH of the buffer employed. Adsorptive effects were used to accumulate the drug on to the electrode. The adsorbed species were measured voltammetrically by using a cathodic process appearing at -0.76 V in 0.1 M HCIO4. Linear calibration graphs were obtained in the range 2.5 x 10(-9)-2.0 x 10(-7) M. A simple procedure of extraction was employed for the determination of the drug in urine samples.

Adsorptive stripping behaviour of mitoxantrone on carbon paste electrodes.

The electrooxidation of mitoxantrone (MXT) on carbon paste electrodes was studied using voltammetric techniques in adsorption conditions. The analyte was accumulated at the working electrode (a carbon paste electrode) under precisely controlled mass-transport conditions and stripped electrochemically in the same solution. An electrode pretreatment is proposed which shows good reproducibility of the analytical signal (0.81%). The stripping step was studied with alternating current voltammetry providing a linear response in the concentration range 5 x 10(-11) to 7 x 10(-10)M in aqueous samples. Finally, a method using the medium exchange and AC phase-selective adsorptive stripping voltammetry technique was proposed for MXT analysis in urine samples.

Phase-selective alternating current polarographic assay of methotrexate in human serum

Abstract The a.c. polarographic behaviour of methotrexate (MTX) was studied and the nature of the process taking place at the dropping mercury electrode was elucidated. Both the molecule and its reduced product appeared to be absorbed at the surface of the electrode. The observed electrochemical behaviour was in close agreement with theoretical predictions for an absorbed species which is reversibly reduced. This permitted a very sensitive and selective determination of MTX in serum samples by phase-selective a.c. polarography. The method, which involves a very simple and rapid previous clean-up procedure, has a limit of detection of 3.0 × 10−7 M in serum and the results compare well with those obtained by liquid chromatography with oxidative amperometric detection (LC-ED) at +1.1 V. Both methods form an interesting alternative to others previously reported for monitoring MTX in cancer patients under treatment with high doses of the drug.

Phase-selective alternating current adsorptive stripping voltammetry of aminopterin on a mercury thin film carbon fibre ultramicroelectrode

The electrodeposition of mercury thin films onto carbon fibres for the determination of aminopterin and its analogues has been optimized following an investigation of the electrochemical reduction processes of aminopterin obtained at a static mercury drop electrode. The advantageous characteristics of ultramicroelectrodes combined with adsorptive preconcentration and phase-selective a.c. stripping voltammetry were found to yield a very sensitive and reproducible method. By using this electrode, accumulation was performed at five different concentrations of aminopterin ranging from 5 × 10–10 to 5 × 10–8 mol dm–3. The electrode yielded a calibration graph from 2 × 10–10 to 8 × 10–9 mol dm–3(r= 0.994) with a limit of detection [signal-to-noise ratio (S/N)= 3] of 1 × 10–10 mol dm–3 aminopterin in aqueous solutions. The reproducibility of the signal was evaluated at three different concentrations of aminopterin producing relative standard deviations ranging from 3.57% at the 5 × 10–10 mol dm–3 level to 2.49% at the 1 × 10–8 mol dm–3 level (n= 10). The electrode was applied to the determination of aminopterin in urine resulting in a limit of detection (S/N = 3) of 2.5 × 10–7 mol dm–3 without the employment of any pre-treatment of the urine.