Determinação da estrutura molecular do ciclooctano por métodos ab initio e difração de elétrons na fase gasosa

The determination of the molecular structure of molecules is of fundamental importance in chemistry. X-rays and electron diffraction methods constitute in important tools for the elucidation of the molecular structure of systems in the solid state and gas phase, respectively. The use of quantum mechanical molecular orbital ab initio methods offer an alternative for conformational analysis studies. Comparison between theoretical results and those obtained experimentally in the gas phase can make a significant contribution for an unambiguous determination of the geometrical parameters. In this article the determination of the molecular structure of the cyclooctane molecule by electron diffraction in the gas phase and ab initio calculations will be addressed, providing an example of a comparative analysis of theoretical and experimental predictions.

Utilização de filtro de transformada de fourier para a minimização de ruídos em sinais analíticos

Instrumental data always present some noise. The analytical data information and instrumental noise generally has different frequencies. Thus is possible to remove the noise using a digital filter based on Fourier transform and inverse Fourier transform. This procedure enhance the signal/noise ratio and consecutively increase the detection limits on instrumental analysis. The basic principle of Fourier transform filter with modifications implemented to improve its performance is presented. A numerical example, as well as a real voltammetric example are showed to demonstrate the Fourier transform filter implementation. The programs to perform the Fourier transform filter, in Matlab and Visual Basic languages, are included as appendices

Métodos de extração e/ou concentração de compostos encontrados em fluidos biológicos para posterior determinação cromatográfica

When organic compounds present in biological fluids are analysed by chromatographic methods, it is generally necessary to carry out a prior sample preparation due the high complexity of this type of sample, especially when the compounds to be determinated are found in very low concentrations. This article describes some of the principal methods for sample preparation in analyses of substances present in biological fluids. The methods include liquid-liquid extraction, solid phase extraction, supercritical fluid extraction and extraction using solid and liquid membranes. The advantages and disadvantages of these methods are discussed.

Sistema antioxidante envolvendo o ciclo metabólico da glutationa associado a métodos eletroanalíticos na avaliação do estresse oxidativo

The most relevant advances on the analytical applications of glutathione determination based on glutathione redox cycle and the antioxidant system are given. The main enzymes that participate of the glutathione metabolism are the glutathione peroxidase and glutathione reductase. The glutathione peroxidase has a major role in the removal of hydrogen peroxide and lipid peroxides from the cells. These enzymes, operating in tandem with catalase and superoxide dismutase promote a scavenging of oxyradical products in tissues minimizing damages caused by these species. Reduced glutathione is the major intracellular thiol found in mammals and changes in the glutathione concentration in biological fluids or tissues may provide a useful marker in certain disorders like hemolytic anemia, myocardial oxidative stress and in the investigation of some kinds of cancers. Particular attention is devoted to the main advantages supplied by biosensors in which there is an incorporation of bioactive materials for the glutathione determination. The correlation between stability and sensitivity of some reduced glutathione electrochemical sensors is discussed.

Aplicação de métodos quimiométricos na especiação de Cr(VI) em solução aquosa

The equilibria, the spectra and the identities of the species of Cr(VI) that are present in aqueous solution have long been an active subject of discussion in the literature. In this paper, three different chemometric methodologies are applied to sets of UV/Visible spectra of aqueous Cr(VI) solutions, in order to solve a chemical system where there is no available information concerning the composition of the samples nor spectral information about the pure species. Imbrie Q-mode factor analysis, followed by varimax rotation and Imbrie oblique projection, were used to estimate the composition of Cr(VI) equilibrium solutions and, by combining these results with the k-matrix method, to obtain the pure spectra of the species. Evolving factor analysis and self modeling curve resolution were used to confirm the number of the species and the resolution of the system, respectively. Sets of 3.3×10-4 and 3.3×10-5 mol L-1 Cr(VI) solutions, respectively, were analyzed in the pH range from 1 to 12. Two factors were identified, which were related to the chromate ion (CrO4(2-)) and bichromate ion (HCrO4-). The pK of the equilibrium was estimated as 5.8.

Métodos matemáticos para correção de interferências espectrais e efeitos interelementos na análise quantitativa por fluorescência de raios-X

One of the main problems in quantitative analysis of complex samples by x-ray fluorescence is related to interelemental (or matrix) effects. These effects appear as a result of interactions among sample elements, affecting the x-ray emission intensity in a non-linear manner. Basically, two main effects occur; intensity absorption and enhancement. The combination of these effects can lead to serious problems. Many studies have been carried out proposing mathematical methods to correct for these effects. Basic concepts and the main correction methods are discussed here.

Principais métodos de caracterização da porosidade de resinas à base de divinilbenzeno

This paper reviews the most important methods used to characterize the porosity of styrene-divinylbenzene resins. Methods such as adsorption of nitrogen for determination of surface area and mercury intrusion porosimetry for characterization of pore size distribution are related.

Estudo da geometria da uréia por métodos ab initio e simulação computacional de líquidos

A study was carried out on the urea geometries using ab initio calculation and Monte Carlo computational simulation of liquids. The ab initio calculated results showed that urea has a non-planar conformation in the gas phase in which the hydrogen atoms are out of the plane formed by the heavy atoms. Free energies associated to the rotation of the amino groups of urea in water were obtained using the Monte Carlo method in which the thermodynamic perturbation theory is implemented. The magnitude of the free energy obtained from this simulation did not permit us to conclude that urea is non-planar in water.

Métodos de extração e determinação do herbicida glifosato: breve revisão

This paper supplies a compact revision about the main extraction, clean-up and pre-concentration methods of the herbicide glyphosate for water and soil samples, as well as methods for its determination and quantification.

Métodos sintéticos para preparação de 2,2'-bipiridinas substituídas

The 2,2'-bipyridine has been entitled as the most widely used ligand. Nowadays there is a large variety of known molecules comprising at least two 2,2'-bipyridine units and the number of applications in many areas such as catalysis, new materials, optoeletronics and electrochemistry have increased very much in the past decades. Nevertheless, there is no article that gives an overview of the main synthetic methods for obtaining the substituted 2,2'-bipyridines, generally non available. This article presents a synthetic discussion about the three different methods (coupling reaction, ciclo-functionalization and functionalization of the heteroaromatic rings of 2,2'-bipyridine) for preparing these heterocyclic compounds and also provides a practical and fundamental guide, for obtaining more than eighty different symmetric and unsymmetrical substituted 2,2'-bipyridines, shown in a table with the corresponding references.

Métodos de baixo custo para purificação de reagentes e controle da contaminação para a determinação de metais traços em águas naturais

Probably one of the most difficult and challenging aspects of measuring trace metals in natural waters is to avoid contamination during sampling, manipulation and analysis. This work discusses how to avoid contamination using simple procedures, and considers alternative methods to purify deionised water and low grade reagents to enable accurate determination of trace metals in natural waters in a common laboratory. Measurements were performed by cathodic stripping voltammetry and copper was used as a model metal to test the procedures. It was possible to evaluate copper speciation in natural waters even when total dissolved copper concentration was as low as 1.5 nmol L-1. The methods' accuracy was confirmed by analysis of certified seawater.

A quimiluminescência como ferramenta analítica: do mecanismo a aplicações da reação do luminol em métodos cinéticos de análise

Relevant aspects of proposed mechanisms of the chemiluminescent reaction of luminol are presented and commented to emphasize its perspectives for kinetic analysis. A careful search for analytical applications of this reaction is discussed in order to point out new trends of the studies. Kinetic analysis using the luminol reaction is proposed to be a very attractive due to the good performance of the reaction in analytical applications and the positive characteristics of kinetic analysis, such as low cost and sensibility. It is pointed out that kinetic analysis using the chemiluminescent reaction of luminol should be encouraged.

Álcoois quirais: métodos químicos e catalíticos de obtenção por redução assimétrica

The importance of chiral alcohols as starting materials for the production of fine chemicals and as useful chirons for the building of several interesting molecules or natural products is reported. The useful and common methods of asymmetric reduction such as the chemical (with organoboron or organoaluminum reagents) and the catalytic ones (with ruthenium or rhodium complexes) for preparation of chiral alcohols are described; even the newer and much more rare electrocatalytic methods are reported.

Breve revisão de métodos de determinação de resíduos do herbicida ácido 2,4-diclorofenoxiacético (2,4-D)

This paper supplies a revision about the main techniques of extraction, clean-up and pre-concentration of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) in water and soil samples, as well as chromatographic methods and immune assays for its identification and quantification.

Avaliação dos métodos de extração sequencial de Tessier, Keller e Miller na determinação de ferro nativo em três tipos de solos: orgânico, brunizem e latossolo

Sequential extraction is not totally effective to dissolve distinct forms of trace elements. The extractive solution, for example, can dissolve less of the target fraction and more than another not wished fraction. The goal of this work is to compare the extraction of native iron with three sequential extraction methods of the heavy metals, using three soils with different physical chemistry characteristics: a histosol, an oxisol, and a mollisol. The results obtained in this work demonstrate that a smaller relation soil/extractor results in a larger extraction in almost all phases. The use of many stages of the sequential extraction, with the purpose of more association of the metal with different components of the soil, can result, among other things, in modification of the substratum by the action of the reagents used, besides reducing the selectivity of the more specific extractors. Readsorption and redistribution of the heavy metals could have happened with larger intensity in the fraction where hidroxilamine was used with higher temperature. Sequential extraction of iron, without enrichment of soil samples, in the respective fractions in each method, it was important to better understand the behavior of the reagents considered specific to each form of the metal in soil.