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- Author or Editor: A. Alvárez x
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Abstract
The aim of the present work was the establishment of a procedure for the determination of several metals in rainwater, which includes a concentration step by precipitation with APDC, preceding the EDXRF determination. The influence of different factors (the repose time of the precipitate, the concentrations of APDC, the metal carrier and the oxidant agent, and others) upon the metal precipitation and filtration was investigated. An absolute EDXRF method was used for the quantification of the Fe, Co, Ni, Cu, Zn and Pb contents.
Abstract
The aim of this work consisted on the implementation of sufficiently accurate and sensitive analytical procedure for the analysis of metal concentration in rainwater. Different sample preparation procedures were tested to achieve the required concentration prior to direct total reflection X-ray fluorescence (TXRF) analysis. TXRF and anodic stripping voltammetry (ASV) were compared in regard to achieved detection limits, precision and accuracy.
Abstract
Thermal analysis of jute fibre reinforced vinylester resin with 30 vol% of fibre were performed by TG/DTG under dynamic conditions. The fibres were treated with alkaline solution at different temperatures and the final composition (cellulose, hemicellulose and lignin) of the fibre was determined by chemical analysis. Apparent activation energies were determined using a variety of conventional thermogravimetric methods. Two peaks were found in the composite differential curves: the first peak close to 327 and the second peak at 408°C. The apparent activation energy values for the second peak decreased when fibre were treated. The addition of the jute fibres produced a slightly decrease in the thermal stability of the composites.
Abstract
The isothermal crystallization behavior of layered silicate/starch-polycaprolactone blend nanocomposites was studied by means of differential scanning calorimetry (DSC) measurements. The theoretical melting point was higher for the matrix than for nanocomposites. At low clay concentration, the induction time decreased and the overall crystallization rate increased acting as nucleating agent whereas at higher concentrations became retardants. Classical Avrami equation was used to analyze the crystallization kinetic of these materials. n values suggested that clay not only affected the crystallization rate but also influenced the mechanism of crystals growth. An Arrhenius type equation was used for the rate constant (k). Models correctly reproduced the experimental data.
Abstract
A method has been developed for measuring90Sr activity in liquid samples. After concentrating strontium from the sample by coprecipitation with calcium phosphate, the residue is dissolved in 8N HNO3 and passed through an extraction chromatographic column (Sr.Spec) containing a new material that selectively retains strontium. This is eluted from the column with 0.05N HNO3 and counted by liquid scintillation. Measurement is performed using a double window method, that allows a rapid and single determination of90Sr.
A new species of Guettarda is described, at this time it has been collected only in the type locality. It is similar to the Mesoamerican species G. tikalana Lundell.
Abstract
Tetronic® comprises X-shaped copolymers formed by four poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO) block chains bonded to an ethylene diamine central group. Micellization behaviour of three representative Tetronics (T304, T904 and T1307) was characterized to gain an insight into the interactions between the copolymer unimers and the state of water in their solutions. The enthalpy of demicellization, recorded at 37°C in an isoperibol microcalorimeter, indicated that the process was in all cases exothermic and the enthalpy ranked in the order T1307≥T904>>T304. Micellization is entropy-driven owing to hydrophobic interactions between the PPO chains. DSC analysis showed that the crystallization and melting peaks of the free water remaining in T304 and T904 solutions were progressively shifted toward lower temperatures as the surfactant proportion increased, owing to a colligative effect. Bound water corresponded to 3 water molecules per EO repeating unit. In the case of T1307, which has longer PEO chains, a splitting of the melting peak was observed, one peak appearing around 0°C due to free water and another at –15°C due to interfacial water. As T1307 proportion raised, the enthalpy of the former decreased, whilst the enthalpy of the latter increased. In 40% T1307 solutions, interfacial water overcame the proportion of free water; there being 1 interfacial and 3 bound water molecules per EO repeating unit. Gaussian deconvolution of FTIR spectra also enabled to characterize the evolution of free water as a function of Tetronic proportion. The dependence of micellization and water interaction behaviour on Tetronics structure should be taken into account to use these copolymers as drug solubilizers and micellar carriers.
Hordeum chilense Roem. et Schult. is a South American wild barley that occurs exclusively in Chile and Argentina, where it is a component of natural pastures. This species has been crossed with durum and bread wheats to obtain a new amphiploid, called tritordeum, which presents agronomic traits of a new crop. Two groups represented by H1 and H7 lines have shown differences for spike length and density, number of spikelets per spike and various other characters which are interesting for tritordeum improvement. This study was conducted to investigate inheritance of these three characters in a cross between H1 and H7 lines, including parents, F 1 and F 2 generations. The three characters studied fitted the additive model. The dominance effects were less strong than the additive effects, indicating partial dominance of positive sign for number of spikelets per spike; whereas spike length displayed small dominance effects of negative sign and the spike density showed intermediate inheritance, with the inheritance being almost exclusively additive.