Noemi Nagata

Electrocatalytical properties presented by Cu/Ni alloy modified electrodes toward the oxidation of glucose

This paper concerns the deposition of metal alloys formed by nickel and copper on electrode surface aiming at the development of electrocatalytic systems. Such alloys were formed on platinum electrodes by direct reduction of Ni2+ and Cu2+ sulfate salts in different proportions in a simple and straightforward electrochemical treatment. After the deposition, the conversion to the electrocatalytic oxide form was done in alkaline solutions by cyclic voltammograms. The experimental parameters, such as deposition time and the proportion of copper and nickel in the synthetic solution, were investigated toward the catalytic oxidation of glucose. The materials were characterized by electrochemical experiments, infrared and Raman spectroscopies, and X-ray diffraction, showing that the material is not a simple mixture of nickel and copper oxides. The modified electrodes showed remarkable electrocatalytic properties, indicating an interesting application in the sensor and fuel cell development.

Simultaneous voltammetric determination of dopamine and ascorbic acid using multivariate calibration methodology performed on a carbon paste electrode modified by a mer-[RuCl3(dppb)(4-pic)] complex

The preparation and electrochemical characterization of a carbon paste electrode (CPE) modified with mer-[Ru0Cl3(dppb)(4-pic)] (dppb=Ph2P(CH2)4PPh2, 4-pic=CH3C5H4N), referred to as Rupic, were investigated. The CPE/Rupic system displayed only one pair of redox peaks, with a midpoint potential at 0.28 V vs. Ag/AgCl, which were ascribed to RuIII/RuII charge transfer. This modified electrode presented the property of electrocatalysing the oxidation of dopamine (DA) and ascorbic acid (AA) at 0.35 V and 0.30 V vs. Ag/AgCl, respectively. Because the oxidation for both AA and DA practically occurred at the same potential, distinguishing between them was difficult with cyclic voltammetry. This limitation was overcome using Partial Least Square Regression (PLSR), which allowed us, with the optimised models, to determine four synthetic samples with prediction errors (RMSEP) of 5.55´10-5 mol L-1 and 7.48´10-6 mol L-1 for DA and AA, respectively.

Determinação simultânea de amarelo tartrazina e amarelo crepúsculo em alimentos via espectrofotometria UV-VIS e métodos de calibração multivariada

Synthetic food dyes are usually added to processed foods to give and restore their desired esthetic quality. This aspect and the toxicological evidence justify quality control and the development of methodologies to quantify these food additives. In this work, the mathematical methodologies such as the additivity principle, derivative spectrophotometry and multivariate technique (Partial Least Square Regression - PLSR) were studied in the simultaneous determination of two food colorants: Tartrazine (TY) and Sunset Yellow (SY), extracted from natural wool. These methodologies were evaluated and compared according to their prevision capacities. The best PLSR model (spectral range of 305 to 645 nm with data transformation by first derivative and two principal components) presented the lower Root Mean Square Error Prediction value, RMSEP (TY = 0.191 and SY = 0.102). The concentration range was from 1.0 to 16.0 mg L-1 for SY and 2.0 to 22.0 mg L-1 for TY. The validation set was used and showed an average error of 1.97% for SY and 1.39% for TY. The application of this methodology in real samples showed concentration values in agreement with the limits established by the Brazilian law and regulations

Otimizaçâo das condições de preparação de eletrodos à base de carbono cerâmico utilizando-se planejamento fatorial

Different parameters of carbon ceramic electrodes (CCE) preparation, such as type of precursor, carbon material, catalyst amount, among others, significantly influence the morphological properties and consequently their electrochemical responses. This paper describes a 23 factorial design (2 factors and 3 levels with central point replicates), which the factors analyzed were catalyst amount (HCl 12 mol L-1), graphite/precursor ratio, and precursor type (TEOS - tetraethoxysilane and MTMOS - methyltrimetoxysilane). The design resulted in a significant third order interaction for peak current values (Ipa) and a second order interaction for potential difference (ΔE), between thefactors studied, which could not be observed when using an univariated study.

Simultaneous voltammetric determination of dypirone and paracetamol with carbon paste electrode and multivariate calibration methodology

Esse artigo descreve a determinacao simultânea de dipirona (DIP) e de paracetamol (PAR) utilizando a tecnica de Voltametria de Pulso Diferencial (VPD) e eletrodo de pasta de carbono nao modificado. Devido a sobreposicao dos picos voltametricos de DIP e PAR, a metodologia de calibracao multivariada baseada na Regressao de Minimos Quadrados Parciais (PLSR) foi proposta. O conjunto de voltamogramas obtidos na presenca de ambos os analitos em diferentes concentracoes foi pre-processado pelos dados centrados na media. Para escolha do numero de componentes principais, um procedimento de validacao cruzada foi empregado, sendo que quatro componentes principais foram necessarias para obtencao dos menores valores de PRESS (Prediction Residual Error Sum of Squares). Esse modelo explicou aproximadamente 95,5% da variância do conjunto de dados. Os dados obtidos utilizando-se esse modelo mostraram uma alta correlacao entre as concentracoes reais e previstas. Entretanto, para baixas concentracoes do PAR, os erros relativos aumentaram para 25%. Comparando-se os valores de RMSEP (Root Mean Square of Error Prediction), entre PAR e DIP, foi observado que este foi menor para DIP, provavelmente devido a maior quantidade de informacao analitica apresentada pelos voltamogramas desse analito quando comparado ao processo redox de PAR, o qual devido a sua oxidacao irreversivel apresenta apenas um unico pico.

Simultaneous Determination of Rifampicin and Isoniazid in Urine and Pharmaceutical Formulations by Multivariate Visible Spectrophotometry

Partial least squares regression (PLSR) was used for the simultaneous quantification of rifampicin (RIF)and isoniazid (INH) by visible spectrophotometry using a simple derivatization reaction. In the presence of neocuproine, copper(II) is easily reduced by INH to a Cu(I)-neocuproine complex that shows an absorption maximum at 455 nm. Under these conditions, RIF shows an absorption maximum at 449 nm. The calibration set was established between 8and 57 mg L-1 for RIFand 1.5and 7 mg L-1 for INH. The method was applied to the determination of the drugs in urine samples (recoveries between 92and 119%)and in pharmaceutical formulations (relative error lower than 5%).

Differentiation of five pine species cultivated in Brazil based on chemometric analysis of their volatiles identified by gas chromatography-mass spectrometry

The composition of the volatiles emitted by different trees has been successfully used as a marker in a wide variety of studies, such as chemotaxonomy. Characterization of the volatiles emitted by different species of Pinus has proven to be an important tool to understand the process of host-tree selection by herbivorous insects. The volatiles present in samples of the branches of five species of Pinus (P. caribaea, P. elliottii, P. maximinoi, P. patula and P. taeda) were collected by aeration, and the contents analyzed by gas chromatography using a flame ionization detector (GC-FID), applying non-polar and chiral column phases, and mass spectrometry (GC-MS) using a quadrupole mass analyzer. The relative composition of the different volatiles was used to perform a discriminant analysis among the five pine species, by means of cluster (HCA) and principal component (PCA) analyses.

Análise exploratória de adoçantes de mesa via espectroscopia no infravermelho (FTIR) e análise por componentes principais (ACP)

In the last twenty years, the consumption of diet and light foods has grown steadily, leading to the constant development of such products. Much emphasis has been placed on products that replace sucrose with sweeteners of low or zero calorie content. The development of new commercial sweeteners illustrates this tendency. In this work, principal component analysis and infrared spectroscopy were used to successfully differentiate the vehicles (mediums) employed in the production of sweeteners. This chemometric methodology reduced the dimensional space to two factors, accounting for 82% of the total variance of the data. The variables responsible for this discrimination were localized in the fingerprint region of the infrared spectrum (752.2 to 1284.5 cm-1). The exploratory analysis proved effective for visualizing these data, generating semi-quantitative information for the sweeteners composed of lactose/aspartame, which would have been impossible to obtain without the use of the chemometric tool.

Determinação simultânea de ácido kójico e hidroquinona por espectrofotometria visível e calibração multivariada

A multivariate spectrophotometric method was developed for analysis of kojic acid/hydroquinone associations in skin whitening cosmetics. The method is based on the reaction between kojic acid and Fe3+ and on the reduction of Fe3+ by hydroquinone and further complexation of Fe2+ with 1,10-phenanthroline. The multivariate model was developed by Partial Least Squares Regression (PLSR), using 25 synthetic mixtures and mean-centered spectral data (350-380 nm). The use of 3 (kojic acid) and 2 (hydroquinone) latent variables permits the observation of mean errors of about 5% in the external validation phase.

Multivariate spectroscopic determination of the lamivudine-zidovudine association

The multivariate spectroscopic determination of lamivudine-zidovudine associations was carried out by partial least square regression (PLS). This model was developed from 20 synthetic mixtures using mean-centered spectral data acquired from 190 to 350 nm and with 3 latent variables. External validation was performed with 6 synthetic mixtures providing prediction errors close to 1%. Moreover, the analysis of commercial drugs showed good results with prediction errors lower than 10%. The multivariate methodology was validated according to International Conference on Harmonization (ICH) criteria, demonstrating precision, accuracy and robustness within legal requirements.