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  • Author or Editor: C. Gomez x
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Se describe la anatomía de la madera y de la corteza de un nuevo género y especie de Anacardiaceae: Attilaea abalak E. Martínez et Ramos. Las muestras se obtuvieron de la región de Calakmul, Campeche, México, en selva baja caducifolia con suelo calizo. Se prepararon cortes finos de madera y corteza utilizando un micrótomo de deslizamiento, los cuales fueron teńidos con azul alciano-safranina. Las características anatómicas se estudiaron en cortes transversales, tangenciales y radiales. Se encontraron diferencias significativas en las características anatómicas cuando se compararon con las de Spondias purpurea L., que es la especie más parecida morfológicamente que habita en sus cercanías. El análisis de las características anatómicas y el significativo hábito escandente de A. abalak llevan a concluir que se trata de un taxon diferente como lo señalan Martínez y Ramos (2007).

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Abstract  

A procedure is described for neutron activation analysis of chlorine in zirconium and zirconium alloys. Calculation of chlorine concentration is performed relative to zirconium concentration in the matrix in order to minimize effects of differences in irradiation and counting geometry. Principles of the method, and the results obtained are discussed.

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Glass transitions of several non-ionic cellulose ethers differing in molecular mass and nature and amount of substituents were analyzed (as compressed probes) by differential scanning calorimetry (DSC), modulated temperature differential scanning calorimetry (TMDSC@®), and oscillatory rheometry. In general, the low energy transitions accompanying the Tg of methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), and hydroxypropylcelluloses of low (L-HPC) or medium-high (HPC) degree of substitution were difficult to characterize using DSC. Non-reversing heat flow signals obtained in TMDSC experiments were more sensitive. However, the best resolution was obtained using oscillatory rheometry since these cellulose ethers undergo considerable changes in their storage and loss moduli when reaching the Tg. Oscillatory rheometry also appears as a useful technique to characterize the viscoelastic behavior and thermal stability of pharmaceutical tablets. Tg values followed the order HPC (105°C)<HPMC (170-198°C)<MC (184-197°C)<L-HPC (220°C). For HPMCs, the Tg increases as the methoxyl/hydroxypropoxyl content ratio decreases. The results indicate that Tg depends strongly on the structure of the cellulose ethers. In general, increasing the degree of substitution of cellulosic hydroxyls, the hydrogen bonding network of cellulose decreases (especially when the substituents cannot form hydrogen bonds) and, in consequence, Tg also decreases.

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Summary  

Modulated differential scanning calorimetry (MDSC) and dielectric analysis (DEA) have been used to characterize the cure process of the system diglycidyl ether of bisphenol A (DGEBA(n=0)/1,2 diaminocyclohexane (1,2 DCH). The trans isomer and a mixture cis/trans(30-70% respectively) of 1,2 DCH were used to find their different behaviour. The study allowed to check the influence of the cisisomer on the thermoset curing process. Gelation times were obtained through the equation proposed by Johari and vitrification times from the point of inflection of the complex calorific capacity modulus.

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Ubiquitous elements like carbon and oxygen always contaminate surfaces and, therefore, are the soucre of important analytical errors at trace level. Even in the case of radioactivation (with charged particles), where the sample can be etched after irradiation, analytical problems exist. In this work, we show that laser desorption/ablation can efficiently clean surfacaes, in the case of GaAs samples, resulting in better analytical conditions. Under ultra high vacuum, these surfaces remain clean long enough, so that the analysis of carbon and oxygen can be carried out using various nuclear methods, according to the needs of the analyst.

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Abstract  

A method for arsenic, antimony, selenium and molybdenum determination in water, loess and volcanic glasses, by radiochemical neutron activation analysis based on coprecipitation with Bi2S3 in hydrochloric medium, is described. The results of application to a study of natural contamination in the south-eastern region of the province of Córdoba, Republic of Argentina, are presented and discussed.

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