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排序方式: 共有4627条查询结果,搜索用时 15 毫秒
981.
982.
983.
In the Federal Republic of Germany, benzene is one of the most important basic materials for the chemical industry. Only a relatively small proportion of the pure benzene processed in the chemical industry is emitted into the environment (ca. 40t in 1991). But the substance is also a natural component of the crude oil in gasoline and is released during incomplete combustion or is formed out of other aromatic substances. The primary source of emissions, with more than 10,000 t/yr (approximately 85% of this from motor vehicles with Otto engines), is commercial motor vehicle transportation. Benzene concentrations in the environment are approximately <1 μg/m3 in rural areas, 20–30 μg/m3 near main roads (peak levels in highly urbanized regions with much traffic as high as approximately 100 μg/m3) and 7–15 μg/m3 in the vicinity of industrial polluters. It has not been possible to detect a specific trend over time during the last ten years. An increased exposure (approx. 350–27,000 μg/m3) is likely while filling the tank and within the motor vehicle (approx. 10–200 μg/m3) due to gasoline volatilization from pipes, etc. Compared to outdoor air, higher concentrations of benzene (approx, 2–11 μg/m3) are measured in the indoor air which contains additionally benzene from tobacco smoke, equipment, renovating work and heating. The primary exposure pathway of benzene in humans is inhalation. Apart from individuals with occupational exposure, smokers have the highest internal benzene burden. Measures undertaken during recent years to reduce the amount of emissions have been counteracted at least in part by the increase in motor vehicle traffic. Further measures to reduce the emissions or to change the transporttation policies must still be or have already been initiated. 相似文献
984.
R. Väinölä 《Marine Biology》1992,114(4):539-550
Inter-and intraspecific allozyme differentiation in the mysid crustacean genus Mysis in the North Atlantic region was studied in order to evaluate earlier concepts of evolutionary and systematic relationships and to assess patterns of subdivision within widespread taxa. The results support a relatively ancient divergence of the marine and non-marine species of the genus, and are generally in line with the current subgeneric tridivision into Mysis s.str., Michteimysis and Auricomysis. However, the North American littoral species M. gaspensis should be returned to subgenus Mysis s.str. from its present position in Michteimysis with M. mixta. The closest observed affinities within Mysis s.str. were between M. gaspensis and the freshwater M. relicta group, and between M. oculata and M. litoralis. Intraspecific differentiation among North European coastal populations of M. oculata and M. litoralis was moderately strong (F
ST0.1), suggesting population bottlenecks and limited dispersal in the post-glacial time. On the other hand, ransoceanic differences were not essentially greater, indicating the systematic homogeneity and long-term dispersal capacity in the marine species. This contrasts with the strong genetic and systematic fragmentation earlier found within the circumboreal M. relicta species group. 相似文献
985.
Walter Tötsch 《Environmental management》1990,14(3):333-338
Because of its toxicity, cadmium creates an environmental problem as well as a health hazard for exposed workers. Most cadmium
emissions arise from the intentional use of the element. It is therefore mandatory to reduce cadmium consumption to the lowest
possible level. Cadmium pigments, mainly used in plastic processing, can be replaced in all applications where the processing
temperature does not exceed 300°C. Newly developed polyvinyl chloride stabilizers promise to be an excellent substitute for
cadmium stabilizers in even the most demanding applications. Cadmium plating, still extensively used in the United States
and the West Germany, has been virtually abandoned in Japan. Improved lead acid batteries are replacing vented nickel cadmium
batteries because of their cost effectiveness. While in these applications cadmium use is declining, more and more cadmium
is needed for the manufacturing of sealed nickel cadmium batteries. These relatively small electrochemical cells are used
mainly by individual consumers. Here cadmium can only be replaced in some marginal applications. The high cadmium content
of these batteries (up to 22%) makes them a good candidate for recycling of the heavy metal. 相似文献
986.
987.
988.
Gerhard Pfleiderer 《Die Naturwissenschaften》1978,65(8):397-406
Lactate dehydrogenase is the prototype of an oligomeric, intracellular enzyme. Its primary, secondary tertiary, and quaternary structures are now well known. This fact, especially with regard to the occurrence of isoenzymes, has effected a great progress in the understanding of the structure-function relationship of enzymes. The historical development of modern enzymology is described, together with discussions of the various methods and problems of enzymology. 相似文献
989.
The ant Eutetramorium mocquerysi (Myrmicinae) is endemic to the island of Madagascar. During foraging and nest emigration the ants lay recruitment trails
with secretions from the poison gland. We identified three pyrazine compounds in the poison gland secretion: 2,3-dimethyl-5-(2-methylpropyl)pyrazine
1, 2,3-dimethyl-5-(3-methylbutyl)pyrazine 3, 2,3-dimethyl-5-(2-methylbutyl)pyrazine 4. Only the first component elicited trail-following behavior in the ants. We were unable to investigate whether the other
pyrazine components have a synergistic function.
Received: 21 February 2000 / Accepted in revised form: 27 June 2000 相似文献
990.