Electrochemical noise analysis of chalcopyrite carbon paste electrodes by Acidithiobacillus ferrooxidans

The electrochemical response of chalcopyrite was studied using electrochemical noise analysis (ENA). The assay was carried out under constant aeration using 30 mL in two electrochemical cells containing iron-free mineral salts solution. These cells were initially monitored for 56 hours, After 72 hours, 7.25x 10(10) cells mL(-1) of A, ferrooxidans strain LR were added in both cells and monitored until 128 h. Subsequent to this period, 0.927 mmol L-1 of silver ions and 400 mmol L-1 of chloride ions were added each one separately. Both conditions were monitored until 168 hours. According to results obtained, it was observed that Cl- ions addition induced an accelerated corrosion process. However, there is a tendency of the system to reach the stationary state due to repassivation of the electrodic surface. In the other side, the Ag+ addition contributed for the maintenance of the oxidant atmosphere, in spite of controversial effect caused by considerable variations in the R-n values, resulting in a instability in the chalcopyrite reactivity.

Screening and confirmation of PAHs in vegetable oil samples by use of supercritical fluid extraction in conjunction with liquid chromatography and fluorimetric detection

A fast, simple, non-destructive method for the direct screening of polycyclic aromatic hydrocarbons (PAHs) in vegetable oil samples is proposed. The method uses a supercritical fluid extraction (SFE) system coupled on-line with a fluorimetric detector to determine PAHs. This special assembly avoids the main problems encountered in the determination of PAHs in complex matrices such as vegetable oils. PAHs are selectively extracted by using silica gel in the thimble and cleaned up by passage through a C18 column. Interferences are preferentially retained by the silica gel during the SFE process while PAHs are adsorbed in the C18 column and the remainder of the matrix is sent to waste. Finally, the C18 column is purged to remove residual CO2 gas and adsorbed PAHs are recovered by desorption with a solvent. The extracts from positive samples are subsequently analyzed by liquid chromatography (LC) with fluorescence detection. The proposed method allows the confirmation of vegetable oil safety and hence provides a new tool for consumer protection. (C) 2004 Elsevier B.V. All rights reserved.

On-line determination of Sb(III) and total Sb using baker's yeast immobilized on polyurethane foam and hydride generation inductively coupled plasma optical emission spectrometry

The yeast Saccharomyces cerevisiae was immobilized in cubes of polyurethane foam and the ability of this immobilized material to separate Sb(III) and Sb(V) was investigated. A method based on sequential determination of total Sb (after on-line reduction of Sb(V) to Sb(III) with thiourea) and Sb(Ill) (after on-line solid-liquid phase extraction) by hydride generation inductively coupled plasma optical emission spectrometry is proposed. A flow system assembled with solenoid valves was used to manage all stages of the process. The effects of pH, sample loading and elution flow rates on solid-liquid phase extraction of Sb(III) were evaluated. Also, the parameters related to online pre-reduction (reaction coil and flow rates) were optimized. Detection limits of 0.8 and 0.15 mu g L-1 were obtained for total Sb and Sb (III), respectively. The proposed method was applied to the analysis of river water and effluent samples. The results obtained for the determination of total Sb were in agreement with expected values, including the river water Standard Reference Material 1640 certified by the National Institute of Standards and Technology (NIST). Recoveries of Sb(III) and Sb(V) in spiked samples were between 81 19 and I I I 15% when 120 s of sample loading were used. (c) 2006 Elsevier B.V. All rights reserved.

Comparison of accuracy between compression- and injection-molded complete dentures

Statement of problem. A clinically significant incisal pin opening may occur after processing complete dentures if a compression molding technique is used. To recover the proper vertical dimension of occlusion, a time-consuming occlusal adjustment is necessary that often destroys the anatomy of the artificial teeth. A new injection molding process claims to produce dentures that require few, if any, occlusal adjustments in the laboratory after processing.Purpose. This laboratory study compared incisal pin opening, dimensional accuracy, and laboratory working time for dentures fabricated by this new injection system with dentures constructed by the conventional compression molding technique.Material and methods. Two groups of 6 maxillary and 6 mandibular dentures were evaluated as follows: group 1 (control), Lucitone 199, compression molded with a long cure cycle; and group 2, Lucitone 199, injection molded with a long cure. Incisal pin opening was measured with a micrometer immediately after deflasking. A computerized coordinate measuring machine was used to measure dimensional accuracy of 3-dimensional variations in selected positions of artificial teeth in 4 stages of denture fabrication. Analysis of variance (ANOVA) and t tests were performed to compare the groups.Results. A significant difference was found in pin opening between groups (t test). Horizontal dimensional changes evaluated with repeated measures ANOVA revealed no significant differences between groups. However, analysis of vertical dimensional changes disclosed significant differences between the groups. There was no appreciable difference in laboratory working time for flasking and molding denture bases between the injection and compression molding techniques when polymethyl methacrylate resin was used.Conclusion. The injection molding method produced a significantly smaller incisal pin opening over the standard compression molding technique. The injection molding technique, using polymethyl methacrylate, was a more accurate method for processing dentures. There were no appreciable differences in laboratory working time between the injection and compression molding techniques.

Label-free DNA detection based on modified conducting polypyrrole films at microelectrodes

A label-free electrochemical detection method for DNA hybridization based on electrostatic modulation of the ion-exchange kinetics of a polypyrrole film deposited at microelectrodes is reported. Synthetic single-stranded 27-mer oligonucleotides (probe) have been immobilized at 2,5-bis(2-thienyl)-N-(3-phosphorylpropyl)pyrrole film formed by electropolymerization on the previously formed polypyrrole layer. The 27- or 18-mer target oligonucleotides were monitored via the electrochemically driven anion exchange of the inner polypyrrole film. The performance of the miniaturized DNA biosensor system was studied in respect to selectivity, sensitivity, reproducibility, and regeneration of the sensor. Control experiments were performed with a noncomplementary target of 27-mer DNA and 12 base-pair mismatched 18-mer sequences, respectively, and did not show any unspecific binding. Under optimized experimental conditions, the label-free electrochemical biosensor enabled the detection limits of 0.16 and 3.5 fmol for the 18- and 2 7-mer DNA strand, respectively. Furthermore, we demonstrate reusability of the electrochemical DNA biosensor after successful recovery of up to 100% of the original signal by regenerating the DNA label-free electrode with 50 mM HCl at room temperature.

Non-linear boundary element analysis of floor slabs reinforced with rectangular beams

In this work, a numerical model to perform non-linear analysis of building floor structures is proposed. The presented model is derived from the Kirchhoff-s plate bending formulation of the boundary element method (BENI) for zoned domains, in which the plate stiffness is modified by the presence of membrane effects. In this model, no approximation of the generalized forces along the interface is required and the compatibility and equilibrium conditions along interfaces are imposed at the integral equation level. In order to reduce the number of degrees of freedom, the Navier Bernoulli hypothesis is assumed to simplify the strain field for the thin sub-regions (rectangular beams). The non-linear formulation is obtained from the linear formulation by incorporating initial internal force fields, which are approximated by using the well-known cell sub-division. Then, the non-linear solution of algebraic equations is obtained by using the concept of the consistent tangent operator. The Von Mises criterion is adopted to govern the elasto-plastic material behaviour checked at points along the plate thickness and along the rectangular beam element axes. The numerical representations are accurately obtained by either computing analytically the element integrals or performing the numerical integration accurately using an appropriate sub-elementation scheme. (C) 2007 Elsevier Ltd. All rights reserved.

Determination of 2,4-dichlorophenoxyacetic acid and its major transformation product in soil samples by liquid chromatographic analysis

The 2,4-dichlorophenoxyacetic acid (2,4-D) is one of the most applied herbicides around the world to control broad leave herbs in many crops: In this study, a method was developed for simultaneous extraction and determination of 2,4D and its major transformation product, i.e., the 2,4-dichlorophenol (2,4-DCP), in soil samples. The herbicide and its degradation product were extracted twice from soil samples, after acidification, by dichloromethane on ultrasound system for 1 h. Both extracts were combined and filtrated in qualitative filter paper and Celitee. The total extract was concentrated in rotatory evaporator, dried under N-2 and finally dissolved in 1 ml of methanol. High Performance Liquid Chromatography with UV detection at 230 nm was used for analysis. Recoveries were obtained from soil samples fortified at 0.1, 1.0, 2.0, 3.0 and 4.0 mg kg(-1) levels and the results varied from 85 to 111% (for 2,4-D) and from 95 to 98% (for 2,4-DCP). For both compounds, the limits of quantification were 0.1 mg kg(-1), which were the loss level at which the accuracy and the precision were studied. Nevertheless, the limits of detection, calculated by considering the blank standard deviation and the minimum concentration level, were 0.03 and 0.02 mg kg for 2,4-D and 2,4-DCP, respectively. This proposed method was applied to soil samples of eucalyptus crops, which was previously treated with the herbicide. Despite that, neither 2,4-D nor its degradation product were detected 30 days after the herbicide application. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Gold electrodes from recordable CDs

Gold electrodes are widely used in electrochemistry and electroanalytical chemistry. The notable performance when used in stripping analysis of many ionic species and the extraordinary affinity of thio compounds for its surface make these electrodes very suitable for many applications. This paper reports a simple and novel way to construct gold electrodes (CDtrodes) using recordable CDs as the gold source. The nanometer thickness of the gold layer of recordable disks (50-100 mm) favors the construction of band nanoelectrodes with areas as small as 10(-6) cm(2). The plane surface can be easily used for the construction of conventional-sized gold electrodes for batch or now injection analysis or even to obtain electrodes as large as 100 cm(2). The low price of commercial recordable CDs allows a one way use. The evaluation and applicability of these electrodes in the form of nanoelectrodes, in batch and associated with flow cells, are illustrated in this paper.

Electrocatalytic study of an electrode modified with Reactive Blue 4 dye covalently immobilized on amine-functionalized silica

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

A New Indirect Electrochemical Method for Determination of Ozone in Water Using Multiwalled Carbon Nanotubes

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

HPLC-EICD: An Useful Tool for the Pursuit of Novel Analytical Strategies for the Detection of Antioxidant Secondary Metabolites

Living cells are continuously exposed to a variety of challenges that exert oxidative stress and are directly related with senescence and the onset of various pathological conditions such as coronary heart disease, rheumatoid arthritis and cancer. Nevertheless, living organisms have developed a complex antioxidant network to counteract reactive species that are detrimental to life. With the aim of bio-prospecting plant species from the Brazilian Cerrado and Atlantic Forest, we have established a methodology to detect secondary antioxidant metabolites in crude extracts and fractions obtained from plant species. Combining HPLC with an electrochemical detector allowed us to detect micromolecules that showed antioxidant activities in Chimarrhis turbinata (DC) leaf extracts. Comparison with purified flavonoid standards led us to identify the compounds in their natural matrices giving valuable information on their antioxidant capacity.

In vitro propagation of Maytenus ilicifolia (Celastraceae) as potential source for antitumoral and antioxidant quinomethide triterpenes production. A rapid quantitative method for their analysis by reverse-phase high-performance liquid chromatography

Cell culture of Maytenus ilicifolia were established in order to produce and to quantify the antitumoral and antioxidant quinonemethide triterpenes. In vitro calli were induced from leaf explants of native plants and cultured in semi-solid medium under controlled conditions of humidity, temperature and photoperiod. The quinonemethide triterpenes showed maximum accumulation in the logarithmic phase growth of the cell culture. A rapid, sensitive and reliable reverse-phase HPLC method was used for quantitative determination of the antitumoral and antioxidant quinonemethide triterpenes, 22β-hydroxymaytenin and maytenin in callus of Maytenus ilicifolia. Well resolved peaks with good detection response and linearity in the range 1.0 - 100 μg/mL were obtained. This quantitative work was performed by an external standard method.

Use of the internal standardization for difficult sampling by graphite furnace atomic absorption spectrometry

This work shows the potentiality of As as internal standard to compensate errors from sampling of sparkling drinking water samples in the determination of selenium by graphite furnace atomic absorption spectrometry. The mixture Pd(NO 3) 2/Mg(NO 3) 2 was used as chemical modifier. All samples and reference solutions were automatically spiked with 500 μg l -1 As and 0.2% (v/v) HNO 3 by the autosampler, eliminating the need for manual dilutions. For 10 μl dispensed sample into the graphite tube, a good correlation (r=0.9996) was obtained between the ratio of analyte absorbance by the internal standard absorbance and the analyte concentrations. The relative standard deviations (R.S.D.) of measurements varied from 0.05 to 2% and from 1.9 to 5% (n=12) with and without internal standardization, respectively. The limit of detection (LD) based on integrated absorbance was 3.0 μg l -1 Se. Recoveries in the 94-109% range for Se spiked samples were obtained. Internal standardization (IS) improved the repeatability of measurements and increased the lifetime of the graphite tube in ca. 15%. © 2004 Elsevier B.V. All rights reserved.

Voltammetric determination of dipyrone using a N,N′- ethylenebis(salicylideneaminato)oxovanadium(IV) modified carbon-paste electrode

The preparation and electrochemical characterization of a carbon paste electrode modified with N.N′-ethylenebis(salicylideneiminato) oxovanadium(IV) complex ([VO(Salen)]) as well as its behavior as electrocatalyst toward the oxidation of dipyrone were investigated. The electrochemical behavior of the modified electrode and the electrooxidation of dipyrone were explored using cyclic voltammetry. The voltammetric response of the modified electrode is based on two reactions. One electrochemical related to the oxidation of the metallic center of the [VO(Salen)] and the other involving the chemical redox process involving the oxidized form of the complex and the reduced form of dipyrone. The best voltammetric response was observed for a paste composition of 25% (m/m) [VO(Salen)], KCl solution pH from 5.5 to 8.0 as the electrolyte and potential scan rate of 10 mV s-1 in the presence of dipyrone. A linear voltammetric response for dipyrone was obtained in the concentration range from 9.9 × 106 to 2.8 × 10 -3 mol L-1, with a detection limit of 7.2 × 10 -6 mol L-1. Among of several compounds tested as potential interference, only ascorbic acid presented some interference. The proposed electrode is useful for the quality control and routine analysis of dipyrone in pharmaceutical formulations.

Synthesis, characterization, and biological activity of a new palladium(II) complex with deoxyalliin

Synthesis, characterization, and biological activity of a new water-soluble Pd(II)-deoxyalliin (S-allyl-L-cysteine) complex are described in this article. Elemental and thermal analysis for the complex are consistent with the formula [Pd(C6H10NO2S)2]. 13C NMR, 1H NMR, and IR spectroscopy show coordination of the ligand to Pd(II) through S and N atoms in a square planar geometry. Final residue of the thermal treatment was identified as a mixture of PdO and metallic Pd. Antiproliferative assays using aqueous solutions of the complex against HeLa and TM5 tumor cells showed a pronounced activity of the complex even at low concentrations. After incubation for 24 h, the complex induced cytotoxic effect over HeLa cells when used at concentrations higher than 0.40 mmol/L. At lower concentrations, the complex was nontoxic, indicating its action is probably due to cell cycle arrest, rather than cell death. In agreement with these results, the flow cytometric analysis indicated that after incubation for 24 h at low concentrations of the complex cells are arrested in G0/G1. © 2005 NRC Canada.