Arturo J. Miranda Ordieres

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.

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.

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.

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.