Single-step microwave digestion of food and biological samples for the quantitative conversion of Se into the +4 oxidation state.

An improved single step microwave digestion procedure is described for providing the fast and easy exhaustive mineralisation of biological samples concomitantly with the quantitative conversion of any type of selenium compounds into Se(IV). In such a way, digested samples are directly suitable for the subsequent Se analysis at trace and ultratrace levels by both spectrometric methods such as HG-ICP-MS or HG-ICP-OES and differential pulse cathodic stripping voltammetry (DPCSV). It is based on the use, under suitably optimised microwave irradiation conditions, of a digestion mixture with a carefully tailored composition such that its redox potential is made lower than that allowing Se(IV) to be oxidized to Se(VI), but high enough to permit total destruction of biological or, in general, organic matrices. It consists of a nitric acid (65%, w/w) and hydrogen peroxide (30%, w/w) mixture in a volume ratio 5:1, frequently adopted for the mineralisation of organic and biological samples, but added simply with 0.25 g mL(-1) of NaCl. Successful application of the procedure, in terms of both repeatability and accuracy, to the quantification of selenium by the instrumental methods above in standard compounds and in a certified biological sample proved its good performance. The application to the Se determination in human blood plasma and in a wide variety of foods is also reported.

An electroanalytical investigation on the redox properties of lacidipine supporting its anti-oxidant effect.

The redox properties of lacidipine (PyH2), one of the most pharmacologically active N-unsubstituted 1,4-dihydropyridines, have been studied by cyclic voltammetry and controlled potential electrolysis in acetonitrile, an aprotic solvent that is, at best, a mimic of the lipofilic layer of biological membranes. PyH2 undergoes a two-electron oxidation process involving two consecutive one-electron releases, the latter requiring potentials much less positive than the former. The overall process occurs through a primary one-electron step accompanied by a fast proton release, with the formation of a neutral radical (PyH*), which undergoes a further and quite easier one-electron step, thus providing the main ultimate product (PyH+) consisting in the protonated form of the parent pyridine derivative. This appears relevant for the anti-oxidant effect since the radical intermediate is much more prone to be oxidized than to be reduced, thus preventing the propagation of the oxidative chain reaction. The mentioned release of protons in the primary electrode step causes the overall process to be complicated by a parassite side reaction involving the coupling between one of the electrode products (H+) and the starting species. The protonation of PyH2 subtracts part of the original species from the electrode process because the parent cationic species (PyH3+) is no longer electroactive. This parassite reaction occurs rather slowly in the timescale of electroanalytical measurements (the relevant kinetic constant has been estimated to be 6.4 l mol(-1) s(-1)), thus markedly affecting the process only in the presence of relatively high PyH2 concentrations and progressively decreasing with the starting PyH2 concentration. All the products formed in the oxidation process (PyH+, H+ and PyH3+) have been identified by voltammetric evidences based on deep investigations on their cathodic behaviour. The advantageous anti-oxidant properties displayed by PyH2 with respect to those exhibited by phenolic anti-oxidants such as vitamin E are also discussed.

Potential shifts at electrodes coated with ion-exchange polymeric films.

Voltammetry at electrodes modified with ion-exchange polymers, named "ion exchange voltammetry", has been recently developed for characterizing and determining quantitatively ionic electroactive analytes preconcentrated at the electrode surface. Like for other voltammetric techniques, characterization is based on the position of the response on the potential scale, but an appreciable difference is frequently observed between the formal half-wave potential for redox couples incorporated within ion-exchange polymeric films and those for the same redox couples in solution as measured at bare electrodes. Such a difference has been rationalized here by a generalized equation, inferred from a suitable elaboration of the Nernst equation, whose validity has been tested by a thorough investigation performed at glassy carbon electrodes modified with either cationic (Nafion) or anionic (Tosflex) polymeric films. With this purpose, the effect of both charge and concentration of the analyte and of the loading counterion, this last introduced as the cation or anion of the supporting electrolyte, of the ion-exchange selectivity coefficients of the redox partners and of their stoichiometric coefficients, as well as of the number of electrons involved in the charge transfer has been evaluated. The results obtained agree quite well with theoretical expectations and indicate that the potential shifts found are mainly conditioned by both charge and concentration of the counterion from the supporting electrolyte and by the ratio of the ion-exchange equilibrium constants for the two redox partners involved. Other parameters considered have no influence on the potential shift or lead to negligible effects, provided that the quantities of the redox partners incorporated within the ion-exchange coating represents less than 5% of the film capacity. Again in agreement with theoretical expectations, positive shifts are found for increasing supporting electrolyte concentrations when cationic redox species incorporated within cationic films are involved, while the opposite effect is found for anionic redox species incorporated within anionic films.

Gas chromatographic system for the identification of halogenated pesticides by retention indices using n-alkanes as standards.

A gas chromatographic system for the evaluation of linear temperature-programmed retention indices allowing n-alkanes to be adopted as the reference retention markers for any type of analyte, irrespective of the atoms present in their molecules, is described. It is based on the simultaneous use of two different detectors (a flame ionization detector and a specific detector suitable for the sample components), both connected (in parallel) to the same column outlet. The performance of this system has been tested by measuring the retention indices of fifteen chlorinated pesticides under conditions of linear programming temperature, by adopting an electron-capture detector as the specific detector. The reliability of the retention indices thus determined has been proven by verifying that they can be reproduced under different chromatographic conditions.

Improved high-performance liquid chromatographic determination with amperometric detection of alpha-amanitin in human plasma based on its voltammetric study.

A sensitive and simple method is described for the selective determination in human plasma of alpha-amanitin, the most poisonous and prevalent toxin in the lethal fungi of species Amanita, using high-performance liquid chromatography with amperometric detection. After an extraction of plasma with disposable C18 silica cartridges, the extracts were separated by isocratic reversed-phase chromatography using a macroporous poly(styrene-divinylbenzene) column and a mobile phase of 0.05 M phosphate buffer-acetonitrile (91:9) at the apparent pH of 9.5. Amperometric detection was performed by applying an oxidation potential as low as +350 mV (vs. Ag/AgCl) to a glassy carbon electrode, in a thin-layer flow-cell. The linear range for alpha-amanitin was 3-200 ng/ml, and the relative limit of detection in plasma was 2 ng/ml at a signal-to-noise ratio of 2. The intra-assay precision was evaluated at levels of 10 and 200 ng/ml; the coefficients of variation were 4.5 and 2.6% (n = 5), respectively. Inter-assay coefficients of variation were 6.5 and 4.2% (n = 5) for the same concentrations of toxin. These analytical conditions have been chosen on the basis of a preliminary in batch cyclic voltammetric investigation of alpha-, beta- and gamma-amanitins, which has allowed their oxidation process to be clarified and the pH dependence of their oxidation potentials to be determined. All three amanitins are oxidized at the same potential values, and adsorption onto the electrode surface of both reactant and products was found in all cases. This adsorption did not affect the signal recorded for alpha- and gamma-amanitins at the amperometric detector, and for beta-amanitin a stronger adsorption for the anodic product was found, which leads to a marked positive shift of the potential required for the oxidation of this isomer in the amperometric detector cell.

Determination of residual aromatic amine hydrochlorides in process samples of di-isocyanate used in the manufacture of polyurethane A comparison of electroanalytical methods.

Aromatic diamine hydrochlorides have been determined in the presence of different amounts of hydrochloric acid by acid-base titration followed potentiometrically, by linear sweep voltammetry, by differential pulse voltammetry and also by direct DPV measurement. Tests on simulated and production samples have shown that the best results are obtained by using DPV at a mercury electrode, because of its good resolution, which makes it possible to achieve good accuracy (error < - 1 %) and good precision (rsd 1.5%) at 0.1M levels. Moreover, it has the advantage that neither preliminary treatment of the sample nor a trial titration is required.

A survey of the information on co-ordination compound features which can be gained from electroanalytical methods.

The information provided by modern electroanalytical techniques on co-ordination compounds is surveyed. The problem of the interaction between the electrode and intermediates or products is also briefly considered; it is pointed out that the electroanalytical approach can be successfully employed to provide new insight into chemical properties of metal complexes only when weakly interacting species are involved. The information obtainable is considered under the following headings: (i) mechanistic studies on metal complexes and electroanalytical evidence for their reactivity and stability; (ii) structural features of co-ordination compounds in solution; (iii) feasibility of electrochemical syntheses; and (iv) stability of intermediate oxidation states with reference to the nature of the ligands co-ordinated to the metal.