Irma Lavagnini

Steady state voltammetry at microelectrodes for the hydrogen evolution from strong and weak acids under pseudo-first and second order kinetic conditions

Abstract The reaction of hydrogen evolution involving discharge of protons arising from strong and weak acids, namely perchloric, nitric, acetic, ethylenediaminetetra-acetic and dihydrogen phosphoric acids, was investigated in the concentration range 0.1–10 mM by steady state voltammetry at inlaid platinum micro-disc electrodes (of 10–12.5 μm radius). The second-order CE mechanism for the weak acids was rationalised by digital simulation, demonstrating the effects of the various kinetic, thermodynamic and diffusion parameters on the reduction process. In particular, it was found that proton reduction from acetic acid is associated with a completely mobile equilibrium over the whole concentration range studied, but in buffer solutions containing a large excess of the conjugate base the CE process becomes affected kinetically. For the other two weak acids the chemical step controls the kinetics at any concentration employed.

Amperometric Mediated Carbon Nanotube Paste Biosensor for Fructose Determination

Abstract A new mediated carbon nanotube paste (CNTP) amperometric biosensor for fructose is described. The biosensor is formed by a CNTP electrode modified with an electropolymerized film of 3,4‐dihydroxybenzaldehyde (3,4‐DHB) and is based on the activity of a commercial available D‐fructose dehydrogenase (FDH) immobilized on an immobilon membrane placed on the top of the electrode surface. Analytical parameters such as enzyme immobilization, pH, temperature, and probe lifetime were studied and optimized. The biosensor response current was directly proportional to D‐fructose concentration from 5 × 10−6 to 2 × 10−3 mol/L with a detection limit of 1 × 10−6 mol/L and a good reproducibility (RSD = 1.8%, n = 5). The biosensor was used for the determination of fructose content in three honey samples and validated with a commercial spectrophotometric enzymatic kit.

Performance of a numerical method based on the hopscotch algorithm and on an oblate spheroidal space coordinate-expanding time grid for simulation of v

Abstract The application of a simulation procedure which combines the Hopscoth alogrithm with a hyperbolic space grid in solving the initial value problems at an inlaid disk electrode for electrochemical processes of any reversibility degree is discussed. The importance of the resolution of the space and time grid in connection with the use of the Hopscotch algorithm and of the Butler-Volmer equation to gain good accuracy, is stressed. The improvements introduced to define analytically the diffusion layer at different mass transfer conditions and to save computing time with the use of an exponentially expanding time grid and a cubic-spline interpolation make the simulation algorithm suitable to regression procedures for parameter estimation.

Overall calibration procedure via a statistically based matrix-comprehensive approach in the stir bar sorptive extraction–thermal desorption–gas chromatography–mass spectrometry analysis of pesticide residues in fruit-based soft drinks

Stir bar sorptive extraction (SBSE)-thermal desorption (TD) procedure combined with gas chromatography mass spectrometry (GC-MS) and the statistical variance component model (VCM) is applied to the determination of semi-volatile compounds including organochlorine and organophosphorus pesticides in various synthetic and real fruit-based soft drink matrices. When the matrix effects are corrected using isotopically labelled or non labelled internal standard, but matrix/calibration run-induced deviations are still present in the measurements, the adoption of a variance component model (VCM) in the quantitative analysis of various matrices via an overall calibration curve is successful. The method produces an overall calibration straight line for any analyte accounting for the uncertainty due to all the sources of uncertainty, namely matrix-induced deviations, calibration runs performed at different times, measurement errors. Small increases in the detection limits and in uncertainty in the concentration values obtained in the inverse regression face favourably the decrease in times and costs for routine analyses.

Ion Chromatographic Determination of Nitrite in the Presence of a Large Amount of Chloride.

Abstract Different procedures for the quantitation of nitrite in the presence of a large excess of chloride are described. The results obtained show that, in general, solutions characterized by N0 2 - /Cl - ratios of about 1/10 000 can very easily be analyzed. The best results were obtained with amperometric detection in conjunction with NaCI as the eluent which allows ratios down to about 1/700 000 to be employed. One of the procedures investigated was successfully applied to the analysis of samples of sea-water from fish breeding pools.

Digital simulation via the hopscotch algorithm of a microelectrode-based channel flow-through amperometric detector

Abstract The current flowing at a band ultramicroelectrode in a rectangular channel under mass transfer-limited conditions is computed numerically. The procedure is based on the “hopscotch” algorithm and on an appropriate space transformation, δ = tanh( x /α). The edge effect is found to influence the response depending on the flow rate. The polarization times necessary to reach the steady-state current are also computed under different operating conditions.

Alcohol biosensor based on the immobilization of meldola blue and alcohol dehydrogenase into a carbon nanotube paste electrode

Abstract The preparation and electrochemical characterization of a new carbon nanotube paste (CNTP) amperometric biosensor for ethanol is described. The biosensor is formed by a CNTP electrode modified with the phenoxazine compound Meldola Blue (MB) as mediator. It shows a good electrocatalytic activity towards NADH oxidation at potentials around 0.0 V vs. Ag/AgCl, which represents a strong diminution in the overpotential. The biosensor is based on the activity of a commercially available alcohol dehydrogenase enzyme (ADH) and is realized by co‐immobilizing the enzyme ADH, the coenzyme NAD+, and the mediator into the paste. Analytical parameters such as enzyme, co‐enzyme and mediator immobilization concentration, pH, temperature, and probe lifetime are studied and optimized. The biosensor yields a linear response to 1×10−7 –4×10−6 mol/L ethanol at an operating potential of +50 mV (vs. Ag/AgCl), where interfering reactions do not occur, with a detection limit of 1×10−7 mol/L and a good reproducibility (RSD of 2.8%, n=6). Application of the biosensor to the determination of ethanol in alcoholic beverages was achieved successfully.

Hyphenated gas chromatography–mass spectrometry analysis of 3-mercaptohexan-1-ol and 3-mercaptohexyl acetate in wine: Comparison with results of other sampling procedures via a robust regression

This work describes a new purge and trap gas chromatography electron impact mass spectrometry (PT-GC-EIMS) method for quantifying 3-mercaptohexan-1-ol (3-MH) and 3-mercaptohexyl acetate (3-MHA), two molecules able to characterize some wines with their tropical scents. Firstly the experimental conditions of the purge and trap extraction (sample temperature, extraction time, trap temperature, flow rate) following a multivariate approach were optimized. Then the method through the construction of the calibration curves and the establishment of the detection limits was validated. The purge and trap procedure appears faster and more sensitive than both the headspace solid phase microextraction (HS-SPME) and the solid phase extraction (SPE) procedures, reaching detection limits for the two thiols closer to their sensory thresholds. Evidence of similar performances of the three sampling methods considered was gained comparing the results relevant to same wine samples. The Theils regression method was used for purpose of comparison.

Microelectrode voltammetry for studying host-guest complexation equilibria. An analysis of the possibilities of the method

Abstract The use of microelectrode voltammetry in the study of host-guest complexation is described. A digital simulation approach to describe the complexation as a second-order CE mechanism for any kinetic condition is proposed. For a completely mobile equilibrium, the constant can be determined by the very simple relationship (I1/I1,0) = (1 + DrK[L]b)/(1 + K[L]b), where I1/I1,0 is the ratio of the limiting current in the presence and absence of the host species, Dr is the ratio of the diffusion coefficients of the complex and the guest species, K is the equilibrium constant and [L]b is the bulk equilibrium concentration of the host species. Advantage is gained from the linear dependence of the limiting current on the diffusion coefficient of the reactant which allows an enlargement of the measurement range. The potential applicability of the method to electroinactive guest species was also explored by using it in a competitive procedure. Agreement was found between the theoretical results and those relative to the ferrocenecarboxylic acid + β-cyclodextrin and ferrocenecarboxylic acid + p-nitrophenol + β-cyclodextrin systems in aqueous solution at pH 9.2.

Comparison of the simplex, marquardt, and extended and iterated extended kalman filter procedures in the estimation of parameters from voltammetric curves

Abstract The use of the simplex, Marquardt, and extended and iterated extended Kalman filter nonlinear least-squares curve-fitting methods is examined for evaluation of parameters in linear-sweep and cyclic voltammetric curves. The EE mechanism (two-electron transfer) is considered as a reference process and the relevant results are compared on the basis of accuracy, computing time and robustness. The E° values for l -β-3,4-dihydroxyphenylalanine are calculated.