Large-Volume Direct Injection for Determining Naphthalene Derivative Pesticides in Water Using a Restricted-Access Medium Column in RPLC-LC with Fluorescence Detection

A simple multidimensional system for direct injection of large volumes has been developed for the determination of three naphthalene-derivative pesticides [2-(1-naphthyl) acetamide (NAD), 1-naphthyl methylcarbamate (carbaryl) and 1-naphthol] in water using fluorescence detection. The capability of restricted-access medium (RAM), pre-columns for eliminating large interfering molecules, combined with an optimised, coupled-column separation procedure, large volume injection (LVI) and fluorescence detection, gave excellent sensitivity for target analytes. The maximum admissible concentration levels established by the European Union (EU) for individual pesticides and their related compounds in drinking water (0.1 μg L^?1) were easily reached. Detection limits were lower than, or equal to, 0.003 μg L^?1. Average recoveries ranged: 81–104% (n=3) with relative standard deviation (RSD) values: 2–7%. Application of the coupled-column liquid chromatography (LC-LC) method was by determining target analytes in ground water samples from five agricultural zones of Almería, (Spain) over three months.     

Dynamics of a single ion in a perturbed Penning trap. Sextupolar perturbation

In the frame work of classical mechanics, we study the nonlinear dynamics of a single ion trapped in a Penning trap perturbed by an electrostatic sextupolar perturbation. The perturbation is caused by a deformation in the configuration of the electrodes. By using a Hamiltonian formulation, we obtain that the system is governed by three parameters: the z-component of the canonical angular momentum P _φ - which is a constant of the motion because the perturbation we assume is axial-symmetric -, the parameter δ that determines the ratio between the axial and the cyclotron frequencies, and the parameter a which indicates how far from the ideal design the electrodes are. We study the case P _φ = 0. By means of surfaces of section, we show that the phase space structure is made of three fundamental families of orbits: arch, loop and box orbits. The coexistence of these kinds of orbits depends on the parameter δ. The escape is also explained on the basis of the shape of the potential energy surface as well as of the phase space structure. Received 6 September 2001 / Received in final form 19 March 2002 Published online 28 June 2002     

Determination of prazosin in pharmaceutical samples by flow injection analysis with multiple-pulse amperometric detection using boron-doped diamond electrode

This work describes the development of a simple, fast and low-cost method for determining prazosin (PRA) in pharmaceutical samples by flow injection analysis with multiple-pulse amperometric (FIA-MPA) detection using a boron-doped diamond film electrode. Electrochemical detection of PRA was optimized in phosphate buffer pH 4.0 by cyclic voltammetry, in which PRA presented two oxidation processes around at 0.97 and 1.40 V versus Ag/AgCl (3.0 mol L^?1 KCl). In these conditions, PRA also showed one reduction process at ?0.75 V that is dependent on the oxidation processes. Thus, the determination of PRA by FIA-MPA detection consisted on the application of a two-potential waveform, E ₁ (generator potential)?=?1.6 V/400 ms and E ₂ (collector potential)?=??1.0 V/30 ms, with sample loop of 150 μL and flow rate of 3.0 mL min^?1. The method showed good repeatability (RSD?<?3.0 %) and high analytical frequency (70 injections per h). The working linear range was obtained from 2 to 200 μmol L^?1 with a limit of detection of 0.5 μmol L^?1. The recovery tests in all samples were approximately 100 %, and the results were compared with chromatographic methods.     

Hydrotalcite catalysis for the synthesis of new chiral building blocks

The use of hydrotalcites for the synthesis of two chiral building blocks in a simple way is described as a new and green methodology. The synthesis of these compounds implies a regioselective Baeyer–Villiger reaction in a very selective way with ulterior opening and lactonisation. This methodology should be considered green for the use of hydrogen peroxide as the only oxidant and hydrotalcites as the catalyst, and because no residues are produced apart from water. The procedure is very adequate for using in gram scale, in order to increase the value of the obtained compounds. The conditions are excellent and can be applied for nonstable compounds, as they are very mild. The synthesised compounds are magnific starting materials for the synthesis of biologically active or natural compounds. The use of a cheap, commercial and chiral compound as carvone disposable in both enantiomeric forms adds an extra value to this methodology.     

Voltammetric determination of herbicide molinate in river water and rice samples using solid silver amalgam electrode fabricated with nanoparticles

The evaluation of the voltammetric behaviour and the determination of herbicide molinate were performed for the first time over the surface of solid amalgam electrode fabricated with silver nanoparticles using cyclic voltammetry and square-wave voltammetry techniques. The experimental and instrumental parameters were evaluated to reach the maximum analytical response for molinate. It was achieved when a medium composed of 0.04 mol L^?1 Britton–Robinson buffer at the pH value of 4.0 was used. Under these conditions, molinate showed one pronounced reduction peak at E_p = ?0.37 V (vs. Ag/AgCl 3 mol L^?1) that was characterised as an irreversible system. An analytical curve was constructed at the concentration range from 9.36 to 243.49 µg L^?1 and a limit of detection of 2.34 µg L^?1 was obtained. The amalgam electrode presented good stability during the measurements with relative standard deviation (RSD) values of 2.9% for the repeatability and 5.4% for the reproducibility. The voltammetric method developed here could be conveniently applied for the determination of molinate in river water and rice spiked samples at levels below those established on the legislations of European Union and Brazil with good accuracy (RSD of less than 5% for all samples). Comparison with HPLC technique was carried out and the results indicated satisfactory concordance. According to the results depicted here, the silver nanoparticles solid amalgam electrode showed itself highly sensitive and an interesting alternative for the routine analysis of molinate in water and food samples. Furthermore, it introduces an environmentally acceptable alternative to the mercury electrodes, most commonly used for determination of reducible pesticides.     

Optimization of β-1,4-Endoxylanase Production by an Aspergillus niger Strain Growing on Wheat Straw and Application in Xylooligosaccharides Production

Plant biomass constitutes the main source of renewable carbon on the planet. Its valorization has traditionally been focused on the use of cellulose, although hemicellulose is the second most abundant group of polysaccharides on Earth. The main enzymes involved in plant biomass degradation are glycosyl hydrolases, and filamentous fungi are good producers of these enzymes. In this study, a new strain of Aspergillus niger was used for hemicellulase production under solid-state fermentation using wheat straw as single-carbon source. Physicochemical parameters for the production of an endoxylanase were optimized by using a One-Factor-at-a-Time (OFAT) approach and response surface methodology (RSM). Maximum xylanase yield after RSM optimization was increased 3-fold, and 1.41- fold purification was achieved after ultrafiltration and ion-exchange chromatography, with about 6.2% yield. The highest activity of the purified xylanase was observed at 50 °C and pH 6. The enzyme displayed high thermal and pH stability, with more than 90% residual activity between pH 3.0–9.0 and between 30–40 °C, after 24 h of incubation, with half-lives of 30 min at 50 and 60 °C. The enzyme was mostly active against wheat arabinoxylan, and its kinetic parameters were analyzed (K_m = 26.06 mg·mL⁻¹ and V_max = 5.647 U·mg⁻¹). Wheat straw xylan hydrolysis with the purified β-1,4 endoxylanase showed that it was able to release xylooligosaccharides, making it suitable for different applications in food technology.     

A numerical study of planar arrays of correlated spin islands

We analyze a system of interacting islands of XY spins on a triangular lattice. This model has been introduced a few years ago by Eley et al. [Nat. Phys. 8, 59 (2012)] to account for the phenomenology in experiments on tunable arrays of proximity coupled long superconductor-normal metal-superconductor junctions. The main features of the model are the separation of a local and a global interaction energy scale and the mesoscopic character of the spin islands. Upon lowering the temperature the model undergoes two crossovers corresponding to an increasing phase coherence on a single island and to the onset of global coherence across the array; the latter is a thermodynamical phase transition in the Ising universality class. The dependence of the second transition on the island edge-to-edge spacing is related to the proximity-effect of the coupling constant.     

On the Stability of a Class of Permanent Rotations of a Heavy Asymmetric Gyrostat

We consider the motion of an asymmetric gyrostat under the attraction of a uniform Newtonian field. It is supposed that the center of mass lies along one of the principal axes of inertia, while a rotor spins around a different axis of inertia. For this problem, we obtain the possible permanent rotations, that is, the equilibria of the system. The Lyapunov stability of these permanent rotations is analyzed by means of the Energy–Casimir method and necessary and sufficient conditions are derived, proving that there exist permanent stable rotations when the gyrostat is oriented in any direction of the space. The geometry of the gyrostat and the value of the gyrostatic momentum are relevant in order to get stable permanent rotations. Moreover, it seems that the necessary conditions are also sufficient, but this fact can only be proved partially.     

Improving solution of discrete competitive facility location problems

We consider discrete competitive facility location problems in this paper. Such problems could be viewed as a search of nodes in a network, composed of candidate and customer demand nodes, which connections correspond to attractiveness between customers and facilities located at the candidate nodes. The number of customers is usually very large. For some models of customer behavior exact solution approaches could be used. However, for other models and/or when the size of problem is too high to solve exactly, heuristic algorithms may be used. The solution of discrete competitive facility location problems using genetic algorithms is considered in this paper. The new strategies for dynamic adjustment of some parameters of genetic algorithm, such as probabilities for the crossover and mutation operations are proposed and applied to improve the canonical genetic algorithm. The algorithm is also specially adopted to solve discrete competitive facility location problems by proposing a strategy for selection of the most promising values of the variables in the mutation procedure. The developed genetic algorithm is demonstrated by solving instances of competitive facility location problems for an entering firm.