D. R. M. de Souza

Electrochemical detection of the herbicide paraquat in natural water and citric fruit juices using microelectrodes

A novel electroanalytical procedure for detecting the paraquat herbicide in natural water and citric fruit juice samples using gold microelectrodes and square wave voltammetry at high frequencies is proposed. The results obtained showed two reversible peaks for the reduction of paraquat, the first peak associated with the reduction of the paraquat molecule in solution, with subsequent adsorption of the intermediate on the electrode surface. This adsorbed species was shown to undergo electroreduction in a reaction associated to the second voltammetric peak. The variation in pH and square wave parameters indicated that the best conditions under which paraquat could be reduced were a pH of 5.0, a frequency of 1000s-1, a scan increment of 2mV and a square wave amplitude of 50mV. Under these conditions, the variation of the concentrations of paraquat from 1.00×10-6 to 1.66×10-4molL-1 presented, for peak 1, detection and quantification limits of 4.51 and 15.05μgL-1 respectively in pure electrolyte with a recovery factor of 99.50%. The proposed analytical procedure was also applied to natural water samples giving recovery factors of 95.00, 89.50 and 92.50% in three water samples collected from an urban stream. The recovery factor was observed to depend on the content of organic matter which was determined by the biochemical and chemical oxygen demand. In lemon and orange juice samples that were spiked with 5.70×10-5molL-1 of paraquat, the recovery factors obtained were 94.30 and 92.70% respectively.

Electrochemical Behavior of Nicotine Studied by Voltammetric Techniques at Boron‐Doped Diamond Electrodes

Abstract The electrochemical behavior of nicotine in alkaline media was studied using a boron doped diamond (BDD) surface as the working electrode. In order to establish the pH dependence and to gain information about the mass transport of the species, cyclic voltammetry studies were carried out in a 0.1 mol L−1 BR (Britton‐Robinson) buffer in the presence of 1.0×10−3 mol L−1 nicotine. The optimum pH value was 8 and the mass transport was controlled by diffusion of the species. The square wave voltammetry technique was used to determine the electroanalytical parameters such as frequency, amplitude, and scan increment. After optimization, an analytical curve was constructed. The limits of detection and quantification were 0.50 and 1.66 mg L−1, respectively. Theoretical calculations indicate that the probable oxidation site on the nicotine molecule was the nitrogen atom denoted “11 N” and a speculation about the reaction mechanism was proposed. Finally, an experiment using a real sample (cigarette tobacco) was carried out and a recovery study revealed a value of about 4.3 mg L−1 in 0.1 g of tobacco.

Stochastic lattice gas model describing the dynamics of the SIRS epidemic process

We study a stochastic process describing the onset of spreading dynamics of an epidemic in a population composed of individuals of three classes: susceptible (S), infected (I), and recovered (R). The stochastic process is defined by local rules and involves the following cyclic process: S → I → R → S (SIRS). The open process S → I → R (SIR) is studied as a particular case of the SIRS process. The epidemic process is analyzed at different levels of description: by a stochastic lattice gas model and by a birth and death process. By means of Monte Carlo simulations and dynamical mean-field approximations we show that the SIRS stochastic lattice gas model exhibit a line of critical points separating the two phases: an absorbing phase where the lattice is completely full of S individuals and an active phase where S, I and R individuals coexist, which may or may not present population cycles. The critical line, that corresponds to the onset of epidemic spreading, is shown to belong in the directed percolation universality class. By considering the birth and death process we analyze the role of noise in stabilizing the oscillations.

Electroanalytical Determination of the Herbicide Paraquat in Natural Water and Commercial Tea Samples with Gold Electrodes Obtained from Recordable Compact Disc

Abstract This work describes the application of gold electrodes constructed from recordable compact discs for the analytical determination of the herbicide paraquat in natural water samples using square wave voltammetry. The detection limit for pure water (laboratory samples) was 21 µg L−1, lower than the EPA limit for drinking water (100 µg L−1). The experimental quantification limit was determined as 73 µg L−1. In polluted creek water samples the detection limit rose to 76.4 µg L−1 and is shown to be dependent on BOD and COD values. Recovery measurements in tea and natural water samples were approximately 95%, which indicates that the methodology can be employed to analyze paraquat in such matrices. The authors acknowledge the financial support of FAPESP and CNPq.

A new scale-invariant ratio and finite-size scaling for the stochastic susceptible?infected?recovered model

The critical behavior of the stochastic susceptible?infected?recovered model on a square lattice is obtained by numerical simulations and finite-size scaling. The order parameter as well as the distribution in the number of recovered individuals is determined as a function of the infection rate for several values of the system size. The analysis around criticality is obtained by exploring the close relationship between the present model and standard percolation theory. The quantity UP, equal to the ratio U between the second moment and the squared first moment of the size distribution multiplied by the order parameter P, is shown to have, for a square system, a universal value 1.0167(1) that is the same for site and bond percolation, confirming further that the SIR model is also in the percolation class.The critical behavior of the stochastic susceptible-infected-recovered model on a square lattice is obtained by numerical simulations and finite-size scaling. The order parameter as well as the distribution in the number of recovered individuals is determined as a function of the infection rate for several values of the system size. The analysis around criticality is obtained by exploring the close relationship between the present model and standard percolation theory. The quantity UP , equal to the ratio U between the second moment and the squared first moment of the size distribution multiplied by the order parameter P , is shown to have, for a square system, a universal value 1.0167(1) that is the same as for site and bond percolation, confirming further that the SIR model is also in the percolation class. PACS numbers: 05.70.Ln, 05.50.+q, 05.65.+b Finite-size scaling of the stochastic susceptible-infected-recovered model 2