A. Costa García

ADSORPTIVE STRIPPING VOLTAMMETRIC BEHAVIOUR OF COLLOIDAL GOLD AND IMMUNOGOLD ON CARBON PASTE ELECTRODE

Abstract A new adsorptive stripping voltammetric method (ASV) for the determination of colloidal gold is investigated. The method also allows the measurement of the average diameter of the particle size and it is based on the previous metal accumulation onto a carbon paste surface electrode followed by an oxidation process of the adsorved colloid to an Au(III)-chloro complex which is then electrochemically reduced to Au(0). Using differential pulse voltammetry (DPV), a linear calibration range of concentrations between 5.08 × 10 −8 and 4.06 × 10 −6 M can be covered and a detection limit of 1.78 × 10 −8 M ( S / N = 3) was found. When a goat anti-human IgG is labelled with colloidal gold, the differential pulse adsorptive stripping voltammetric method (DPASV) allows the determination of antibodies in a range of concentrations from 2.77 × 10 −8 to 1.86 × 10 −8 M (relative standard deviation = 5%). The proposed method seems to be an excellent analytical approach for the determination of any protein or macromolecule which can be labelled with microparticles of colloidal gold and offers a potential new path to immunoelectroanalytical assay.

Adsorptive Stripping Voltammetric Behaviour of Folic Acid

The effective adsorption of folic acid at mercury electrodes is demonsrated. The adsorptive stripping response is evaluated with respect to experimental parameters such as the preconcentration time and potential, bulk concentration, stirring rate and others. The accumulation curves at different concentrations of folic acid show slopes proportional to the bulk concentration. At a certain stage (40% of the equilibrium surface coverage), the change of the morphology of the adsorption peak indicates that interactions between adsorbed molecules are substantial. The Effect of some surfactants present in the medium on the adsorbed folic acid stripping signal is studied. In the presence of a 700-fold excess of gelatin, the folic acid signal is 40% of its initial value (gelatin-free solution). The ac voltammetric mode applied to the stripping of adsorbed folic acid reveals the most sensitive response, allowing 1×10−11M folic acid to be detected by using 10 min of accumulation time.

Simultaneous adsorptive stripping voltammetric determination of riboflavin and folic acid in multivitamin preparations

Abstract The adsorptive stripping voltammetric behaviour of folic acid and riboflavin was studied at a hanging mercury drop electrode by phase-selective a.c. voltammetry. In 0.1 M sodium acetate buffer (pH 5.0) a cathodic scan gave peaks at −0.29 and −0.55 V vs. Ag/AgCl for riboflavin and folic acid, respectively. The adsorptive stripping response was evaluated with respect to concentration dependence and preconcentration time. Both compounds can be simultaneous determined with a relative standard deviation of 1.4% at 5 × 10 −8 M riboflavin and 0.74% at 10 −8 M folic acid. The method compared favourably with liquid chromatography with UV detection and was succesfully applied to the simultaneous determination of both compounds in multivitamin preparations. The average contents of riboflavin and folic acid were found to be 15.19 mg ± 2.1% and 1.8 mg ± 2.6%, respectively.

An electroanalytical study of the anticancer drug dacarbazine

The electrochemical behaviour of dacarbazine [5-(3,3-dimethyl-1-triazenyl) imidazole-4-carboxamide; DTIC] was investigated by Tast and differential pulse polarography (d.p.p.) at the dropping mercury electrode, by cyclic and differential pulse voltammetry at the hanging mercury drop electrode and by anodic voltammetry at the glassy carbon electrode. Calibration graphs were obtained for 2×10−8−2×10−5 M DTIC by d.p.p., for 5×10−9−1×10−5 M by adsorptive stripping voltammetry ar a hanging mercury drop electrode, and for 1−10×10−5 M by high-performance liquid chromatography with oxidative amperometric detection at a glassy carbon electrode. The methods are compared and applied to determine DTIC added to blood serum after a simple clean-up procedure.

Cathodic stripping voltammetry of 5-fluorouracil

An investigation has been carried out into the anodic differential-pulse polarographic and cathodic stripping voltammetric behaviour of the anticancer drug 5-fluorouracil at mercury indicator electrodes. The anodic peak, corresponding to the formation of a mercury salt, has been used for the determination of the drug in formulations with a relative standard deviation of 0.5%. The cathodic stripping voltammetric behaviour of this mercury salt can be used to monitor 10–7–10–8M concentrations of 5-fluorouracil in a borax-HClO4 buffer, pH 7.8. The interference of Cl– with the cathodic stripping signals is only evident at concentrations of Cl– four orders of magnitude higher than the concentration of the drug and uric acid was found not to interfere at up to equal concentrations of 5-fluorouracil. The cathodic stripping method has been applied to the determination of 5-fluorouracil in human serum.

Adsorptive stripping voltammetry on mercury-coated carbon fibre ultramicroelectrodes

Abstract The conditions for the electrodeposition of mercury on the surface of carbon fibres to permit the sensitive and reproducible voltammetry of folic acid and mitoxantrone (MXT) were investigated. The accumulation behaviour of both molecules on a mercury film ultramicroelectrode was studied and the possibility of carrying out a.c. adsorptive stripping analysis using mercury-coated carbon ultramicroelectrodes was demonstrated. Optimum conditions for both the deposition of mercury and a.c. monitoring are described. This procedure gave rise to a linear calibration graph from 5.0 x 10−10 to 2.0 x 10−8 M, a detection limit of 5.0 x 10−10 M and a relative standard deviation (R.S.D.) of 5.05% at 5.0 x 10−9 M (n = 10 ) for the determination of MXT. For folic acid a linear calibration graph from 1.0 x 10−9 to 5.0 x 10−8 M, a detection limit of 9.0 x 10−10 M and an R.S.D. of 1.44% at 4.0 x 10−8 M (n = 10) were found. Both compounds can be determined directly in biological samples at physiological levels without any separation or clean-up procedure.

Electrooxidation of methadone on carbon paste electrodes

The oxidation of methadone on carbon paste electrodes has been studied using voltammetric techniques under semi-infinite linear diffusion and hydrodynamic conditions. A simple “in situ” electrode pretreatment is proposed to obtain a good reproducibility of the current signal. The diffusion coefficient (0.19 × 10−5 cm2 s−1), the charge transfer coefficient (β = 0.30) and the charge transfer rate constant (K = 1.2 × 10−11 cm s−1) are reported. A mechanism involving two electrons and two protons is proposed for methadone oxidation, which is in agreement with reported data concerning other aliphatic amines.

Voltammetric assay of methadone in urine

A simple, rapid and sensitive voltammetric method for the determination of methadone in human urine is proposed, which permits the detection of concentrations of methadone as low as 0.3–0.4 µg ml–1. The proposed method was compared with methods normally used in clinical trials, such as the enzyme multiplied immunoassay technique, and was found to be superior with regard to cost and to possess high sensitivity.

Adsorptive stripping voltammetric assay of folic acid in human serum

Abstract Folic acid was voltammetrically measured after preconcentration by adsorption at the static mercury drop electrode. Phase selective AC voltammetry provided the most sensitive stripping signal. After previous analyte extraction, using liquid—solid extraction with a C-18 reversed phase cartridge, the method was suitable for folic acid measurement in human serum. The detection limit was 5.9 × 10−9 M with an overall precision of 9.9% (R.S.D.; n = 7) at the concentration level of 1.0 × 10−7 M with a mean recovery of 57%.

A voltammetric study of thiazolidine-4-carboxylic acid on mercury electrodes

Abstract In this paper, the polarographic behaviour of the thiazolidine-4-carboxylic add is studied. The compound gives rise to anodic waves which are assumed to be due to the formation of mercury salts on the HMDE. Cyclic Voltammetry (CV) of the amino acid shows that the salts can be stripped and cathodic stripping voltammetry (CSV) can be used to determine traces of the compound. The species responsible for the stripping signal are proposed and the best conditions for CSV are elucidated. Interferences occurring in biological matrices on the stripping current are studied. The proposed method enables the detection at concentrations as low as 1.0×10 −9 M .