M. Carmo V. F. Vaz
Instituto Superior de Engenharia do Porto
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Featured researches published by M. Carmo V. F. Vaz.
Analytica Chimica Acta | 1999
José F.C.C Lima; Cristina Delerue-Matos; M. Carmo V. F. Vaz
Abstract A flow-injection analysis (FIA) system with a potentiometric tubular detector and a gas diffusion unit was developed for the determination of Kjeldahl nitrogen in milk and dairy products. The samples were digested by the Kjeldahl method and afterwards injected into the FIA system without prior treatment. NaOH solution was introduced into the system to promote the conversion of ammonium ions to ammonia gas. The gas produced diffused through a gas-permeable membrane towards an acceptor channel (Tris–HCl, pH=7.5). When in contact with this solution, ammonia gas was reconverted to amonium cation and carried to a tubular electrode sensitive to this species. Therefore, a PVC tubular ammonium ion-selective electrode was prepared without inner reference solution and with the sensor system based on nonactine and monactine dissoved in Tris–(2-ethylhexyl) phosphate. The results obtained with the developed FIA system were in good agreement with those of the reference method the relative deviations being less than 4%. Reproducibility of the method was assessed by the relative standard deviation, which was always less than 0.5% for 12 consecutive injections of sample digests. A sampling rate of 100 samples h−1 was obtained.
Food Chemistry | 1998
José L. F. C. Lima; Cristina Delerue-Matos; M. Carmo V. F. Vaz
Abstract This paper describes a flow injection manifold with a colourimetric detection system that enables the determination of iron and copper in different types of milks, namely cow milk and infant formula powdered milks. The methodology used is based on the formation of coloured complexes produced by the reaction of iron (II) with 1,10-phenanthroline and copper (II) with 1,5-diphenylcarbazide. The samples were digested with nitric and sulphuric concentrated acids and inserted in the FIA system without additional treatment. The pH adjustment was carried out inside the manifold including the addition of the reagents needed to form the absorbing species. The sampling rate of both species was never lower than 120 determinations h −1 . The results obtained were compared with those given by the reference methods, and the relative deviation was less than 5 and 4% for the determinations of iron and copper respectively. The precision of the results evaluated by the relative standard deviation (RSD%) was less than 0.5% for iron and 2% for copper determinations.
Analytical Letters | 2006
M. Fátima Barroso; M. Carmo V. F. Vaz; M. Goreti F. Sales; Paula Paíga; Cristina Delerue-Matos
Abstract A detailed study of voltammetric behavior of ethiofencarb (ETF) is reported using glassy carbon electrode (GCE) and hanging mercury drop electrode (HMDE). With GCE, it is possible to verify that the oxidative mechanism is irreversible, independent of pH, and the maximum intensity current was observed at +1.20 V vs. AgCl/Ag at pH 1.9. A linear calibration line was obtained from 1.0×10−4 to 8.0×10−4 mol L−1 with SWV method. To complete the electrochemical knowledge of ETF pesticide, the reduction was also explored with HMDE. A well‐defined peak was observed at –1.00 V vs. AgCl/Ag in a large range of pH with higher signal at pH 7.0. Linearity was obtained in 4.2×10−6 and 9.4×10−6 mol L−1 ETF concentration range. An immediate alkaline hydrolysis of ETF was executed, producing a phenolic compound (2‐ethylthiomethylphenol) (EMP), and the electrochemical activity of the product was examined. It was deduced that it is oxidized on GCE at +0.75 V vs. AgCl/Ag with a maximum peak intensity current at pH 3.2, but the compound had no reduction activity on HMDE. Using the decrease of potential peak, a flow injection analysis (FIA) system was developed connected to an amperometric detector, enabling the determination of EMP over concentration range of 1.0×10−7 and 1.0×10−5 mol L−1 at a sampling rate of 60 h−1. The results provided by FIA methodology were performed by comparison with results from high‐performance liquid chromatography (HPLC) technique and demonstrated good agreement with relative deviations lower than 4%. Recovery trials were performed and the obtained values were between 98 and 104%.
International Journal of Environmental Analytical Chemistry | 2008
M. Goreti F. Sales; M. Carmo V. F. Vaz; Cristina Delerue-Matos; Sofia A. A. Almeida; M. Fátima Barroso; Helder A. O. Ferreira
A simple, rapid, and precise amperometric method for quantification of N-methylcarbamate pesticides in water samples and phytopharmaceuticals is presented. Carbofuran and fenobucarb are the target analytes. The method is developed in flow conditions providing the anodic oxidation of phenolic-based compounds formed after alkaline hydrolysis. Optimization of instrumental and chemical variables is presented. Under the optimal conditions, the amperometric signal is linear for carbofuran and fenobucarb concentrations over the range of 1.0 × 10−7 to 1.0 × 10−5 mol L−1, with a detection limit of about 2 ng mL−1. The amperometric method is successfully applied to the analysis of spiked environmental waters and commercial formulations. The proposed method allows 90 samples to be analysed per hour, using 500 µL of sample, and producing wastewaters of low toxicity. The proposed method permits determinations at the µg L−1 level and offers advantages of simplicity, accuracy, precision, and applicability to coloured and turbid samples, and automation feasibility.
International Journal of Environmental Analytical Chemistry | 2008
M. Fátima Barroso; M. João Ramalhosa; Cristina Delerue-Matos; M. Carmo V. F. Vaz; M. Goreti F. Sales
The electroactivity of butylate (BTL) is studied by cyclic voltammetry (CV) and square wave voltammetry (SWV) at a glassy carbon electrode (GCE) and a hanging mercury drop electrode (HMDE). Britton–Robinson buffer solutions of pH 1.9–11.5 are used as supporting electrolyte. CV voltammograms using GCE show a single anodic peak regarding the oxidation of BTL at +1.7 V versus AgCl/Ag, an irreversible process controlled by diffusion. Using a HMDE, a single cathodic peak is observed, at −1.0 V versus AgCl/Ag. The reduction of BTL is irreversible and controlled by adsorption. Mechanism proposals are presented for these redox transformations. Optimisation is carried out univaryingly. Linearity ranges were 0.10–0.50 mmol L−1 and 2.0–9.0 µmol L−1 for anodic and cathodic peaks, respectively. The proposed method is applied to the determination of BTL in waters. Analytical results compare well with those obtained by an HPLC method.
Journal of Agricultural and Food Chemistry | 1998
José L. F. C. Lima; and Cristina Delerue-Matos; M. Carmo V. F. Vaz
Analytica Chimica Acta | 2002
Cristina M.F Ribeiro; Cristina Delerue Matos; M. Goreti F. Sales; M. Carmo V. F. Vaz
Analytical and Bioanalytical Chemistry | 2003
Dionísia C. Portela; Isabel M. F. Pereira; Paula Paíga; Cristina Delerue-Matos; M. Carmo V. F. Vaz
Analyst | 2000
José L. F. C. Lima; Cristina Delerue-Matos; M. Carmo V. F. Vaz
Analytical and Bioanalytical Chemistry | 2005
M. Fátima Barroso; Olga C. Nunes; M. Carmo V. F. Vaz; Cristina Delerue-Matos