Optical Fiber Sensing System Based on Long-Period Gratings for Remote Refractive Index Measurement in Aqueous Environments

Abstract

In the field of aqueous environment studies, long-period fiber gratings are very attractive for the real-time monitoring of physical parameters, such as temperature and salinity. However, due to the fiber jacket removal, these are fragile devices when applied in real conditions, where mechanical loads and contamination with algae and other organic materials must be taken into account. This work describes a refractive index sensing head that has been developed and characterized for in situ measurement of water salinity and resistance to hard conditions.     

Beam evolution equation with variable coefficients in non‐cylindrical domains

In this article, we present results concerning with the existence of global solutions and a rate decay estimate for energy associated with an initial and boundary value problem for a beam evolution equation with variable coefficients in non‐cylindrical domains. Copyright © 2007 John Wiley & Sons, Ltd.     

On Λ-generalized closed sets in topological spaces

We introduce new classes of sets called Λ _g -closed sets and Λ _g -open sets in topological spaces. We also investigate several properties of such sets. It turns out that Λ _g -closed sets and Λ _g -open sets are weaker forms of closed sets and open sets, respectively and stronger forms of g-closed sets and g-open sets, respectively. Dedicated to Professor Maximilian Ganster on the occasion of his 50th birthday     

Recurrence time statistics for finite size intervals

We investigate the statistics of recurrences to finite size intervals for chaotic dynamical systems. We find that the typical distribution presents an exponential decay for almost all recurrence times except for a few short times affected by a kind of memory effect. We interpret this effect as being related to the unstable periodic orbits inside the interval. Although it is restricted to a few short times it changes the whole distribution of recurrences. We show that for systems with strong mixing properties the exponential decay converges to the Poissonian statistics when the width of the interval goes to zero. However, we alert that special attention to the size of the interval is required in order to guarantee that the short time memory effect is negligible when one is interested in numerically or experimentally calculated Poincare recurrence time statistics.     

Electronic properties of polymer crystals: the effect of interchain interactions

We present a theoretical study of the transport parameters in a prototype conjugated-polymer, poly-para-phenylenevinylene, in two different possible crystalline packings. Our analysis is performed through density-functional electronic structure calculations, and allows one to obtain the fundamental parameters describing charge transport. The transfer integrals are found to be a crucial quantity to appreciate the effects of crystalline aggregation on conduction properties: our results indicate that interchain interactions can be viewed as a tunable parameter for the design of efficient electronic devices based on organic materials.     

Transport in the plasma edge of a tokamak with high MHD activity

Summary Electrostatic turbulence and various aspects of magnetic fluctuations have been investigated, in the edge region of the TBR-1 tokamak, by using a set of Langmuir and magnetic probes, and a triple probe. Measurements of plasma parameters such as plasma potential, density, temperature and magnetic field were taken in order to elucidate, the effect on transport of the electrostatic and magnetic fluctuations in the edge. The fluctuations levels are found to be higher than in most tokamaks. The particle flux is outward and slightly higher than that calculated from Bohm diffusion, and occurs in the frequency region typical of the macroscopic MHD oscillations. The Mirnov-oscillation frequencies in TBR-1 are higher than those observed in other tokamaks and, consequently, there is an uncommon superposition between the Mirnov and turbulent density fluctuations spectra. This fact and the presence of high MHD activity may contribute to elucidate the possible influence of the magnetic oscillations on the electrostatic transport observed in the plasma edge. Work partially supported by FAPESP and CNPq.     

Influence of resonant helical windings on the mirnov oscillations in a small tokamak

Summary The influence of resonant helical perturbations on the plasma confined by the small tokamak TBR-1 was experimentally investigated. Strong attenuations of the Mirnov oscillations could be easily obtained by activating different resonant helical windings. The amplitudes of these oscillations were restored as the helical perturbation ended. Partially supported by CNPq.     

Evaluation of alternative sorbents for dispersive solid‐phase extraction clean‐up in the QuEChERS method for the determination of pesticide residues in rice by liquid chromatography with tandem mass spectrometry

Many compounds are used for pest control during the production and storage of rice, making it necessary to employ multiclass methods for pesticide residues determination. For this purpose, QuEChERS‐based methods are very efficient, fast and accurate, and improvements in the clean‐up step are important, especially for complex matrices, like cereals. In this work, different sorbents such as chitosan, florisil^®, alumina, diatomaceous earth, graphitized carbon black, besides the commonly used primary secondary amine and octadecylsilane, were evaluated for dispersive solid‐phase extraction clean‐up in acetate‐buffered QuEChERS method for the determination of residues of 20 representative pesticides and one metabolite in rice by liquid chromatography coupled to tandem mass spectrometry. The sorbent C18 presented the best results, however, chitosan showed similar results, and the best performance among the unconventional sorbents evaluated. The method limit of quantification, attending accuracy (70–120% recovery) and precision (RSD ≤20%) criteria, ranged from 5 to 20 μg/kg. Results showed that chitosan is an effective alternative to reduce analysis costs, maintaining the method reliability and accuracy.     

Applicability of MNDO techniques AM1 and PM3 to ring-structured polymers

Semiempirical Hartree‐Fock techniques are widely used to study properties of long ring‐structured chains, although these types of systems were not included in the original parametrization ensembles. These techniques are very useful for an ample class of studies, and their predictive power should be tested. We present here a study of the applicability of some techniques from the NDDO family (MNDO, AM1, and PM3) to the calculation of the ground state geometries of a specific set of molecules with the ring‐structure characteristic. For this we have chosen to compare results against ab initio Restricted Hartree‐Fock 6‐31G(d,p) calculations, extended to Møller‐Plesset 2 perturbation theory for special cases. The systems investigated comprise the orthobenzoquinone (O₂C₆H₄) molecule and dimers (O₂C₆H₄)₂, as well as trimers of polyaniline, which present characteristics that extend to several systems of interest in the field of conducting polymers, such as ring structure and heterosubstitution. We focus on the torsion between rings, because this angle is known to affect strongly the electronic and optical properties of conjugated polymers. We find that AM1 is always in qualitative agreement with the ab initio results, and is thus indicated for further studies of longer, more complicated chains. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1135–1142, 2002     

Exploiting Sulphide Generation and Gas Diffusion Separation in a Flow System for Indirect Sulphite Determination in Wines and Fruit Juices

A flow system for sulphite determination in wines and fruit juice samples has been developed exploiting H₂S generation and gas diffusion separation. The composed sandwich gas diffusion cell was coupled in the flow manifold to permit sulphite reduction to H₂S and H₂S diffusion through a microporous PTFE membrane to produce methylene blue resulting from the reaction with N,N-dimethylphenylene diamine (DMPD) and Fe³⁺. The flow parameters were optimized, and the best results were obtained by using NaOH solution for H₂S collection. Interferences of H₂S, CO₂ and ascorbic acid were evaluated. Limits of detection (3σ) for the free and total sulfite were calculated at 0.12 and 0.25 mg L⁻¹, respectively. The results obtained by the proposed process were in agreement with the reference methods for a 95% confidence level.