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排序方式: 共有161条查询结果,搜索用时 15 毫秒
71.
Conte M Budroni G Bartley JK Taylor SH Carley AF Schmidt A Murphy DM Girgsdies F Ressler T Schlögl R Hutchings GJ 《Science (New York, N.Y.)》2006,313(5791):1270-1273
Vanadium phosphates are important catalysts for the oxidation of alkanes, and commercial catalysts comprise a complex range of V4+ and V5+ phosphates. We used three complementary in situ characterization methodologies-powder x-ray diffraction and laser Raman and electron paramagnetic resonance spectroscopies-to show that the metastable phase omega-VOPO4 is very sensitive to many of the reactants and products of butane oxidation. A rapid transformation from omega-VOPO4 to delta-VOPO4 occurs on exposure to butane at the reaction temperature, and hence the metastable omega-VOPO4 may play a role in the formation of commercial catalysts. 相似文献
72.
de Rijke E Ruisch B Bakker J Visser J Leenen J Haiber S de Klerk A Winkel C König T 《Journal of agricultural and food chemistry》2007,55(16):6417-6423
In this study a specific taste modulating flavor ingredient, N-lactoylguanosine 5'-monophosphate (N-lactoyl GMP), was determined in bonito (Japanese, Katsuobushi, dried fermented skipjack) and in powdered bonito using liquid chromatography-electrospray ionization (+) mass spectrometry-mass spectrometry (LC-ESI(+)-MS/MS) with a methanol/ammonium acetate or formate gradient. Furthermore, the influence of ion suppression due to sample matrix effect was investigated and was found to substantially influence the total MS response of N-lactoyl GMP; by adjusting the LC conditions the response could be approximately 5-fold-enhanced. The N-lactoyl GMP concentrations in different types of bonito products were between 0.2 and 2.4 microg/g. 相似文献
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74.
Eurasian lynx are slowly recovering in Germany after an absence of about 100 years, and additional reintroduction programs have been launched. However, suitable habitat is patchily distributed in Germany, and whether patches could host a viable population or contribute to the potential spread of lynx is uncertain. We combined demographic scenarios with a spatially explicit population simulation model to evaluate the viability and colonization success of lynx in the different patches, the aim being to conclude guidelines for reintroductions. The spatial basis of our model is a validated habitat model for the lynx in Germany. The dispersal module stems from a calibrated dispersal model, while the demographic module uses plausible published information on the lynx’ life history. The results indicate that (1) a viable population is possible, but that (2) source patches are not interconnected except along the German-Czech border, and that (3) from a demographic viewpoint at least 10 females and 5 males are required for a start that will develop into a viable population with an extinction probability of less than 5% in 50 years. The survival rate of resident adults was the most sensitive parameter, and the best management strategy for the success of reintroduction would be to reduce the mortality of residents in the source patches. Nevertheless, the extremely low probability of connectivity between suitable patches makes most of the reintroduction plans isolated efforts, and they are therefore questionable in the long run. With such a model, the suitability of the single habitat patches can be assessed and the most appropriate management scheme applied. This study shows that simulation models are useful tools for establishing the comparative effectiveness of reintroduction plans aimed at increasing the viability of the species. 相似文献
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77.
Richling E Preston C Kavvadias D Kahle K Heppel C Hummel S König T Schreier P 《Journal of agricultural and food chemistry》2005,53(20):7925-7930
The delta15N(AIR) and delta2H(VSMOW) data for several alkylpyrazines formed during the roasting process of coffee are reported. Samples of commercially available roasted (n = 9) as well as self-roasted (n = 8) coffee beans (Coffea arabica L. and Coffea canephora var. robusta) of different origins were investigated. By use of extracts prepared by simultaneous distillation extraction (SDE) and subsequently fractionated by liquid chromatography on silica gel, on-line capillary gas chromatography-isotope ratio mass spectrometry was employed in the combustion (C) and pyrolysis (P) modes (HRGC-C/P-IRMS) to determine the delta15N(AIR) and delta2H(VSMOW) values, respectively. In addition to the constituents of coffee beans, data for commercial synthetic alkylpyrazines and substances declared to be "natural" were determined. The delta15N(AIR) data for coffee alkylpyrazines under study-2-ethyl-5-methylpyrazine (1) and 2-ethyl-6-methylpyrazine (2) (measured as sum 1/2), 2-ethyl-3-methylpyrazine (3), 2-methylpyrazine (4), 2,5-dimethylpyrazine (5) and 2,6-dimethylpyrazine (6) (measured as sum 5/6), and 2,3-dimethylpyrazine (7), as well as 2,3,5-trimethylpyrazine (8)-varied in the range from +8.3 to -10.2 per thousand, thus revealing their biogeneration from amino acids (delta15N(AIR) ranging from +8 per thousand to -10 per thousand). The delta2H(VSMOW) values were determined in the range from -5 per thousand to -127 per thousand. Owing to the analytical differentiation observed between coffee alkylpyrazines and synthetic/"natural" samples of 3, 4, and 7, authenticity assessment of coffee-flavored products seems to be promising, provided that extended data will be available in the future. In the literature, there were no IRMS data available for the alkylpyrazines (1-8) under study. 相似文献
78.
Fluctuations of carabid populations inhabiting an ancient woodland (Coleoptera, Carabidae) 总被引:1,自引:0,他引:1
The changes in population size of carabid beetles inhabiting an ancient woodland were analysed in a long-term investigation covering a period of nine years (1994–2002). The amplitude of fluctuations of the relative density of these species was estimated and compared with those of carabid beetles inhabiting recent woodlands and more unstable habitats in open landscapes. To analyse this, 8 pitfall traps were set in an ancient beech and oak woodland in the nature reserve “Lüneburger Heide” in northern Germany and were emptied every fortnight the whole year round.Catching rates for some species (e.g. Carabus problematicus and Abax parallelepipedus) fluctuated only slightly, whereas those of other species (e.g. C. violaceus and C. auronitens) varied as much as ten-fold. Comparison of these results with those of other long-term investigations of recent woodlands and also of more open habitats showed that in each of the three habitat types some species varied very little in abundance and others, very widely. So it seems that the amplitude of fluctuations in abundance is a feature of each single species rather than a special attribute of their habitats. 相似文献
79.
80.
Hugel T Holland NB Cattani A Moroder L Seitz M Gaub HE 《Science (New York, N.Y.)》2002,296(5570):1103-1106
Light-powered molecular machines are conjectured to be essential constituents of future nanoscale devices. As a model for such systems, we have synthesized a polymer of bistable photosensitive azobenzenes. Individual polymers were investigated by single-molecule force spectroscopy in combination with optical excitation in total internal reflection. We were able to optically lengthen and contract individual polymers by switching the azo groups between their trans and cis configurations. The polymer was found to contract against an external force acting along the polymer backbone, thus delivering mechanical work. As a proof of principle, the polymer was operated in a periodic mode, demonstrating for the first time optomechanical energy conversion in a single-molecule device. 相似文献