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Searching the world wide Web 总被引:8,自引:0,他引:8
The coverage and recency of the major World Wide Web search engines was analyzed, yielding some surprising results. The coverage of any one engine is significantly limited: No single engine indexes more than about one-third of the "indexable Web," the coverage of the six engines investigated varies by an order of magnitude, and combining the results of the six engines yields about 3.5 times as many documents on average as compared with the results from only one engine. Analysis of the overlap between pairs of engines gives an estimated lower bound on the size of the indexable Web of 320 million pages. 相似文献
104.
Monteverde M Núñez-Regueiro M Rogado N Regan KA Hayward MA He T Loureiro SM Cava RJ 《Science (New York, N.Y.)》2001,292(5514):75-77
We studied the pressure and temperature dependence of the electrical resistivity of the superconducting compound magnesium diboride (MgB(2)). The superconducting transition temperature decreases monotonically with pressure, being parabolic or linear, depending on samples. The rate of decrease under pressure is higher than in conventional superconductors. We discuss our results in terms of the semimetallic character of the electronic band structure of MgB(2). 相似文献
105.
Saucedo-Hernández Y Lerma-García MJ Herrero-Martínez JM Ramis-Ramos G Jorge-Rodríguez E Simí-Alfonso EF 《Journal of agricultural and food chemistry》2011,59(8):4125-4129
Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR), followed by multivariate treatment of the spectral data, was used to classify seed oils of the genus Cucurbita (pumpkins) according to their species as C. maxima, C. pepo, and C. moschata. Also, C. moschata seed oils were classified according to their genetic variety as RG, Inivit C-88, and Inivit C-2000. Up to 23 wavelength regions were selected on the spectra, each region corresponding to a peak or shoulder. The normalized absorbance peak areas within these regions were used as predictors. Using linear discriminant analysis (LDA), an excellent resolution among all categories concerning both Cucurbita species and C. moschata varieties was achieved. The proposed method was straightforward and quick and can be easily implemented. Quality control of pumpkin seed oils is important because Cucurbita species and genetic variety are both related to the pharmaceutical properties of the oils. 相似文献
106.
Jónsdóttir R Sveinsdóttir K Magnússon H Arason S Lauritzsen K Thorarinsdottir KA 《Journal of agricultural and food chemistry》2011,59(8):3893-3904
Flavor characterization and quality of salt-cured and desalted cod (Gadus morhua) products was studied using sensory analysis and gas chromatography techniques. The products were produced in Iceland using two different processing methods (filleting and splitting) and three different salting procedures, i.e., the old single-step kench salting or a multistep procedure, and presalting (injection and brine salting or only brine salting), which was followed by kench salting. The main difference observed was between fillets and split fish, where the split fish was darker and had stronger flavor characteristics. Comparison of different salting procedures showed that the use of presalting improved the appearance of the salted products, which can be described as increased lightness and reduced yellowness of the products. In the same products, the intensity of curing flavors was milder, as described by sensory analysis and key aroma compounds. Derivatives from lipid and protein degradation contribute to the characteristic flavor of the salted products. 相似文献
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Blanco PG Tórtora M Rodríguez R Arias DO Gobello C 《Veterinary journal (London, England : 1997)》2011,190(1):154-159
The aim of this study was to describe changes in cardiac morphology, systolic function and some peripheral hemodynamic parameters during normal pregnancy in dogs. Twenty healthy bitches, 10 pregnant (PG) and 10 non-pregnant controls (CG), were evaluated every 10 days using echocardiography from day 0 of the estrus cycle to parturition or to day 65 for the PG and CG groups, respectively. Systolic blood pressure (SBP) and uterine artery resistance index (RI) were also assessed. Throughout the study, the shortening fraction and cardiac output increased up to 30% vs. 5% (P<0.01) and 45% vs. 2% (P<0.01) in the PG and CG groups, respectively. In contrast, SBP and RI diminished up to 20% vs. 1% (P<0.01) and 29% vs. 0% (P<0.01) in the PG and CG groups, respectively. In conclusion, a decrease in afterload, an increase in cardiac output and cardiac hypertrophy appear to be the result of the hemodynamic modifications occurring during pregnancy in dogs. 相似文献
109.
Mar Cerdán Antonio Sánchez‐Sánchez Margarita Juárez Juan J. Sánchez‐Andreu Juana D. Jordá Dolores Bermúdez 《植物养料与土壤学杂志》2007,170(4):474-478
The most widely used Iron (Fe) fertilizer in calcareous soils is the synthetic chelate Fe(o,o‐EDDHA). However, humic substances are occasionally combined with Fe chelates in drip irrigation systems in order to lower costs. We investigated the effect of various mixtures of Fe(o,o‐EDDHA) and a commercially available humic substance on Fe availability in a calcareous soil from Murcia, Spain (in vitro experiment) and on leaf Fe content and fruit‐quality attributes of Citrus macrophylla (field experiment). In the in vitro experiment, a calcareous soil was incubated for 15 d with solutions of sole Fe(o,o‐EDDHA) and humic substance and of a mixture of humic substance and Fe(o,o‐EDDHA) to determine the dynamics of available Fe. While the mixture did not significantly increase the available soil Fe, it did decrease the rate of Fe retention in the surface soil compared to sole Fe(o,o‐EDDHA). In the field experiment, the substitution in the application solution of 67% of Fe(o,o‐EDDHA) by commercial humic substance increased leaf P in lemon trees from 0.19% with sole Fe(o,o‐EDDHA) to 0.30% and leaf Fe from 94 mg kg–1 to 115 mg kg–1. Some quality parameters like vitamin C content and peel thickness were also improved with a partial substitution of Fe(o,o‐EDDHA) by humic substances. We conclude that a partial substitution of commercial Fe chelates by humic substance can improve crop Fe uptake and may thus be economically attractive. The underlying physiological mechanisms and ecological implications require further studies. 相似文献
110.
Sanchez R Vázquez A Riquelme D Villén J 《Journal of agricultural and food chemistry》2003,51(21):6098-6102
A fully automated on-line reversed phase liquid chromatography-gas chromatography system is described. The system uses a prototype of the automated through oven transfer adsorption desorption interface. The system is demonstrated by presenting a new rapid method for the determination of pesticide residue in olive oil, which is injected directly with no sample pretreatment step other than filtration. Methanol:water is used as the eluent in the LC preseparation step, while the LC fraction containing the pesticide is automatically transferred to the gas chromatograph. Detection limits of pesticides varied from 0.18 to 0.44 mg/L when a flame ionization detector was used. As an example, relative standard deviation and linear calibration are presented for terbutryne. 相似文献