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A project-scheduling problem (PSP) is addressed to coordinate and program supply in the chain of a company based on projects (PBO - Project Based Organizations). The model firstly considered the cost attributable to the use of resources needed for the implementation of such activities, and subsequently programming of the provision of resources and cash fl ow bound to pay suppliers, considering among other limitations the capacity and diversity of resources offered by suppliers of the project. The results show that after the implementation of the strategy of coordination of shared information, the cost of procurement management could be optimized within projects. The methodology developed can be applied by PBO organizations or companies that eventually carry out projects. [ABSTRACT FROM AUTHOR]

Las dibenzo-p-dioxinas policloradas y los dibenzofuranos policlorados (PCDDs/Fs), conocidos comúnmente como dioxinas, han sido reconocidos como contaminantes ambientales ubicuos. De entre todos los congéneres, los 2,3,7,8-sustituidos, los compuestos más tóxicos, fueron medidos en tejido de peces (Caranx caninus, Caranx hippos y Chirostoma estor estor) de tres regiones marinas y una laguna dulceacuícola de México. Las muestras estuvieron disponibles a nivel comercial y se adquirieron entre septiembre y diciembre de 2005 en Ensenada (Océano Pacífico), El Barril (Golfo de California), Ciudad del Carmen (Golfo de México) y el Lago de Pátzcuaro (centro del país). Las muestras de peces fueron analizadas usando cromatografía de gases/espectrometría de masas de alta resolución. Las concentraciones medias del total de PCDDs/Fs variaron de 0.3 a 1.6 pg g-1 de peso húmedo en el siguiente orden: Pátzcuaro > Ciudad del Carmen > El Barril > Ensenada. Se encontró un perfil de congéneres particular para cada región estudiada, pero los congéneres OCDD y OCDF fueron relativamente importantes en todos los sitios de muestreo. En base al peso húmedo, las concentraciones totales de PCDDs/Fs y sus equivalentes tóxicos, encontradas en este estudio, fueron más bajas o estuvieron en el mismo intervalo que las de otros sitios alrededor del mundo, pero en base al contenido de lípidos, los equivalentes tóxicos de los peces de Pátzcuaro fueron mayores que algunos sitios. La ingesta de dioxinas fue calculada considerando porciones de filete de 200 g usando nuestros resultados, y ninguno de los peces muestreados en este estudio alcanzó la ingesta diaria tolerable establecida por la Organización Mundial de la Salud. Hasta donde tenemos conocimiento, éste es el primer reporte sobre niveles de PCDDs/Fs en peces de México. [ABSTRACT FROM AUTHOR]

Some relevant results are presented of research leading to an alternative redox electrode to platinum. Several different electrodes were subjected to experiments under controlled conditions, mainly in artificial seawater. Among those tested were platinum, graphite, glassy carbon, gold and iridium. Our results, when combined with those estimated by using a model based on the MINEQL+ program, suggest that the O[sub2]/H[sub2]O[sub2] redox couple is the one that controls the redox potential under these experimental conditions. In well-oxygenated waters (P[sub 02] = 0.21 atm) and pH = 8, the value calculated for EH = 0.748 V and the pE at the surface should read 12.67 (pE = (F/2.3RT) EH = 12.67). However, in practice, the redox potential experimentally measured was only between +0.45 and +0.50 V. Additionally, the presence of H[sub2]O[sub2] was determined from our own measurements. Its concentration was estimated at about 10[sub-8] M. This value is in good agreement with values measured and reported by other authors for surface waters under natural conditions. The four sets of measurements that were carried out were: measurements where the O[sub2] concentration of the solution was changed and the pH was kept constant (and vice versa); measurements of several redox couples (including Fe²+/Fe[sup3+], NO[sub3][sup-]/NO[sub2][sup-] and I[sub3][sup-]/I[sup-]); measurements in mud flats at San Quintín, Baja California; and measurement of the redox potential in natural seawater and in filtered and UV seawater. Most measurements indicate that the electrode with a response closest to that predicted by thermodynamics is the Ir electrode. We also found that the reaction of Pt, when compared to Ir, appeared to be slow. [ABSTRACT FROM AUTHOR]

In January 1998, four sediment cores were collected to reconstruct the history of contamination by persistent organic compounds in the southern region of the Bight of the Californias. Two groups of compounds were determined using chromatographic methods: petroleum hydrocarbons (aliphatic hydrocarbons, AHs, and polynuclear aromatic hydrocarbons, PAHs) and chlorinated hydrocarbons (pesticides and polychlorinated biphenyls, PCBs). The AHs had low concentrations, from 0.30 to 3.38 µg g[sup-1], and presented a biogenic allocthonous origin. The PAH concentrations ranged from 0.99 to 673 ng g[sup-1]. The distribution of the alkyl homologues and the compound indices showed a clear predominance of PAHs of petrogenic origin. The chlorinated hydrocarbons presented a surface maximum, decreasing with depth to non-detectable levels. The PCB Concentrations ranged from 0.18 to 7.15 ng g[sup-1] and pesticides from 0.09 to 19.71 ng g[sup-1]. The pesticides that occurred most frequently, were p,p'-DDT and its metabolites (p,p'-DDE and p,p'-DDD). Other pesticides detected were γ-chlordane and BHC isomers (α and γ). The statigraphy of the cores was carried out with molecular markers (chlorinated hydrocarbons), which were Used to calculate a rate of sedimentation of 0.17 to 0.55 cm yr[sup-1]. [ABSTRACT FROM AUTHOR]

An evaluation was made of the distribution and enrichment of Hg in surface sediments collected in August 1998 at 37 stations along the northwest coast of Baja California (NCBC). Mercury was analyzed by cold-vapour atomic absorption spectrophotometry. The percentages of mud (<63 μm) and organic matter were also measured. Mercury concentrations showed a homogeneous spatial distribution in the study area. The mean concentration of Hg (58.2 ng g-1) was similar to that found in the Southern California Bight. Mercury enrichment was observed at four stations located in the northern and central zones of the study area. In general, the Hg concentrations (30.4-96.5 ng g-1) were relatively low and pose no environmental concerns according to North American marine sediment quality guidelines. No correlations were found between Hg and mud or organic matter, suggesting that these variables do not influence the concentration and spatial distribution of this element in sediments from NCBC. [ABSTRACT FROM AUTHOR]