全文获取类型
收费全文 | 1944篇 |
免费 | 49篇 |
国内免费 | 15篇 |
专业分类
化学 | 1327篇 |
晶体学 | 8篇 |
力学 | 42篇 |
数学 | 338篇 |
物理学 | 293篇 |
出版年
2021年 | 16篇 |
2020年 | 23篇 |
2019年 | 10篇 |
2018年 | 18篇 |
2017年 | 23篇 |
2016年 | 31篇 |
2015年 | 37篇 |
2014年 | 44篇 |
2013年 | 72篇 |
2012年 | 110篇 |
2011年 | 142篇 |
2010年 | 53篇 |
2009年 | 49篇 |
2008年 | 113篇 |
2007年 | 91篇 |
2006年 | 104篇 |
2005年 | 113篇 |
2004年 | 97篇 |
2003年 | 80篇 |
2002年 | 76篇 |
2001年 | 27篇 |
2000年 | 27篇 |
1999年 | 30篇 |
1998年 | 33篇 |
1997年 | 22篇 |
1996年 | 27篇 |
1995年 | 22篇 |
1994年 | 31篇 |
1993年 | 31篇 |
1992年 | 24篇 |
1991年 | 18篇 |
1990年 | 21篇 |
1989年 | 16篇 |
1988年 | 17篇 |
1987年 | 16篇 |
1986年 | 15篇 |
1985年 | 28篇 |
1984年 | 27篇 |
1983年 | 17篇 |
1982年 | 21篇 |
1981年 | 31篇 |
1980年 | 21篇 |
1979年 | 25篇 |
1978年 | 12篇 |
1977年 | 14篇 |
1976年 | 20篇 |
1975年 | 13篇 |
1974年 | 9篇 |
1973年 | 7篇 |
1972年 | 7篇 |
排序方式: 共有2008条查询结果,搜索用时 31 毫秒
21.
22.
Determination of fenpyroximate in apples by supercritical fluid extraction and packed capillary liquid chromatography with UV detection 总被引:6,自引:0,他引:6
A method using off-line supercritical fluid extraction (SFE) and micro liquid chromatography (μLC) with UV detection at 260 nm, was developed for selective determination of fenpyroximate in apple samples. The packed capillary liquid chromatography method utilises 20 μl injection volumes with on-column focusing. A 350×0.32 mm capillary column packed with Kromasil 100-C18 of 5 μm particle size was used with a mobile phase of acetonitrile–10 mM ammonium acetate (85:15, v/v) at a flow of 5 μl/min. A two-step SFE procedure was used to extract fenpyroximate selectively in 2 g apple samples, with Hydromatrix (HMX) added as a water absorbent at a 1:1 (w:w) ratio. Fenpyroximate was extracted at 200 bar and 90°C for 15 min using carbon dioxide at a flow of 2 ml/min, and solvent trapping collection in 10 ml acetonitrile. The volume of the acetonitrile extract was reduced by evaporation and water was added to a final composition of acetonitrile–water (40:60, v/v). The resulting 2.0 ml solution was filtered using a 0.45 μm poly(vinylidene difluoride) syringe filter before μLC analysis. Validation of the method was accomplished with apple samples spiked with fenpyroximate, covering the range of 0.1 to 1.0 μg/kg. The within-day and between-day repeatabilities were in the range 4–18% relative standard deviation. Accuracy, measured as recovery, was found to be approximately 60%. Apple samples from a field treated with fenpyroximate were analysed. None of the samples contained fenpyroximate above the quantification level. 相似文献
23.
The application of Ag(I) catalysis of S2O82? to the oxidation of organic carbon to CO2 for determination of dissolved organic carbon in aqueous samples is described. The resulting method combines to a significant degree the speed of high-temperature combustion methods and the sensitivity of wet chemical oxidation. For some samples, a higher oxidation efficiency has been observed than with an uncatalyzed wet chemical oxidation method. 相似文献
24.
Solutions of nitronium tetrafluoroborate in acetonitrile prepared by anodic oxidation of nitrogen dioxide, efficiently nitrate aromatics, enolsilyethers, alkenes and conjugated dienes to afford respectively nitroaromatics, α-nitroketones, vicinal nitroamides and mixtures of nitroacetamides by 1,2- and 1,4-addition. 相似文献
25.
S Bayne C M Hoogerbrugge J Thomsen L Skriver S C van Buul-Offers J L van den Brande 《Journal of chromatography. A》1991,562(1-2):391-402
Insulin-like growth factors 1 and 2 were purified from porcine plasma. In addition to the determination of their isoelectric points, the primary structures of both proteins were determined, using low microgram quantities of protein, by the versatile combination of time-of-flight plasma desorption mass spectrometry and automated Edman degradation. Porcine insulin-like growth factor 1 was shown to be homologous to both human and bovine proteins; the type 2 growth factor showed one mutation to both human and bovine type 2 proteins. 相似文献
26.
Bruce M. Howard William Fenical Edward V. Arnold Jon Clardy 《Tetrahedron letters》1979,20(31):2841-2844
The structure of a unique bromine-containing ketal, obtusin (), has been determined by spectral and X-ray crystallographic techniques. Obtusin is a natural component of the Mediteranean red alga . 相似文献
27.
Sven J. Cyvin Bjørg N. Cyvin Jon Brunvoll Rong-Si Chen 《Monatshefte für Chemie / Chemical Monthly》1989,120(10):833-847
Summary Classes of coronoids (or degenerate coronoids), r and q are obtained by providing the rectangle-shaped benzenoids Rj (m, 3) with a naphthalenic or pyrenic hole, respectively. The numbers of Kekulé structures (K) are studied. It was found for theK numbers of the classes in question:r=4/5R andq=1/5R. The classes r and q are similar to r and q, respectively, but the naphthalenic or pyrenic hole is oriented in a different way. For these classes it was found:r=3/5R,q=2/5R.
Die Anzahl von Kekulé-Strukturen für hochkondensierte Benzenoide mit rechteckigem Umriß, 8. Mitt.: Einige perforierte benzenoide Rechteck-Strukturen
Zusammenfassung Es werden Klassen r und q von Coronoiden (oder degenerierten Coronoiden) erhalten, wobei Benzenoide mit rechteckigem Umriß, Rj(m, 3), mit einem Naphthalin- oder Pyren-Loch versehen werden. Die Anzahl von Kekulé-Strukturen (K) wird untersucht; dabei wurde für die untersuchten Klassenr=4/5R undq=1/5R gefunden. Die Klassen r und q verhalten sich ähnlich, das Naphthalin- oder Pyren-Loch ist jedoch anders orientiert; dafür wurder=3/5R undq=2/5R gefunden.相似文献
28.
Sonnenschein RN Farias JJ Tenney K Mooberry SL Lobkovsky E Clardy J Crews P 《Organic letters》2004,6(5):779-782
A reinvestigation of Auletta sp. yielded the novel compound milnamide C (3) plus the known compounds milnamide A (1), milnamide B (hemiasterlin) (2), jasplakinolide (5), and geodiamolides A (6), D (7), E (8), and G (9). The isolation work was guided by cytoskeletal bioactivity data. Compounds 2 and 3 were shown to cause microtubule depolymerization, and 6-9 were shown to cause microfilament disruption. This biological activity and the structural elucidation of 3, including X-ray analysis, are reported here. [structure: see text] 相似文献
29.
Ashok G. Bajaj Sukh Dev Edward Arnold Bruce Tagle Jon Clardy 《Tetrahedron letters》1981,22(46):4623-4626
The steroechemistry of guggulsterol-1, a component of the exudate of , is shown to be 20(R), 22(R)(). 相似文献
30.
Three new Δ3-norcycloartene triterpenoids, with differing side chain functionalities, have been isolated from the tropical green alga collected in Guam. The structures of these new compounds were secured by x-ray crystallography and by chemical interconversion. 相似文献