全文获取类型
收费全文 | 10966篇 |
免费 | 304篇 |
国内免费 | 32篇 |
专业分类
化学 | 8101篇 |
晶体学 | 31篇 |
力学 | 170篇 |
数学 | 1838篇 |
物理学 | 1162篇 |
出版年
2020年 | 141篇 |
2019年 | 112篇 |
2018年 | 78篇 |
2017年 | 83篇 |
2016年 | 196篇 |
2015年 | 203篇 |
2014年 | 193篇 |
2013年 | 363篇 |
2012年 | 405篇 |
2011年 | 464篇 |
2010年 | 344篇 |
2009年 | 287篇 |
2008年 | 476篇 |
2007年 | 477篇 |
2006年 | 488篇 |
2005年 | 459篇 |
2004年 | 397篇 |
2003年 | 329篇 |
2002年 | 338篇 |
2001年 | 227篇 |
2000年 | 171篇 |
1999年 | 145篇 |
1998年 | 183篇 |
1997年 | 152篇 |
1996年 | 158篇 |
1995年 | 158篇 |
1994年 | 158篇 |
1993年 | 198篇 |
1992年 | 145篇 |
1991年 | 133篇 |
1990年 | 123篇 |
1989年 | 147篇 |
1988年 | 140篇 |
1987年 | 176篇 |
1986年 | 166篇 |
1985年 | 178篇 |
1984年 | 192篇 |
1983年 | 159篇 |
1982年 | 147篇 |
1981年 | 154篇 |
1980年 | 165篇 |
1979年 | 152篇 |
1978年 | 146篇 |
1977年 | 129篇 |
1976年 | 137篇 |
1975年 | 97篇 |
1974年 | 93篇 |
1973年 | 88篇 |
1971年 | 84篇 |
1970年 | 72篇 |
排序方式: 共有10000条查询结果,搜索用时 421 毫秒
991.
Thorsten Perl Bertram Bödeker Melanie Jünger Jürgen Nolte Wolfgang Vautz 《Analytical and bioanalytical chemistry》2010,397(6):2385-2394
Multicapillary column (MCC) ion mobility spectrometers (IMS) are increasingly in demand for medical diagnosis, biological
applications and process control. In a MCC-IMS, volatile compounds are differentiated by specific retention time and ion mobility
when rapid preseparation techniques are applied, e.g. for the analysis of complex and humid samples. Therefore, high accuracy
in the determination of both parameters is required for reliable identification of the signals. The retention time in the
MCC is the subject of the present investigation because, for such columns, small deviations in temperature and flow velocity
may cause significant changes in retention time. Therefore, a universal correction procedure would be a helpful tool to increase
the accuracy of the data obtained from a gas-chromatographic preseparation. Although the effect of the carrier gas flow velocity
and temperature on retention time is not linear, it could be demonstrated that a linear alignment can compensate for the changes
in retention time due to common minor deviations of both the carrier gas flow velocity and the column temperature around the
MCC-IMS standard operation conditions. Therefore, an effective linear alignment procedure for the correction of those deviations
has been developed from the analyses of defined gas mixtures under various experimental conditions. This procedure was then
applied to data sets generated from real breath analyses obtained in clinical studies using different instruments at different
measuring sites for validation. The variation in the retention time of known signals, especially for compounds with higher
retention times, was significantly improved. The alignment of the retention time—an indispensable procedure to achieve a more
precise identification of analytes—using the proposed method reduces the random error caused by small accidental deviations
in column temperature and flow velocity significantly. 相似文献
992.
Ernst Lücker Wolfgang Biedermann Thomas Alter Andreas Hensel 《Analytical and bioanalytical chemistry》2010,398(2):963-972
Methods for the detection of central nervous tissue (CNT) are urgently needed in food control as a means for controlling strict
adherence to both food labeling and banning of specified BSE risk material. Here, we report data on heat stability of the
CNT markers neuron-specific enolase (NSE) in western blotting, glial fibrillary acidic protein (GFAP) in an enzyme linked
immunoassay, mRNAGFAP in a real-time PCR assay, and several fatty acids (C22:6, C24:0-OH, C24:1ω9/ω7, C24:1ω9-OH/ω7-OH, and C24:0) in gas chromatography
mass spectrometry (GC/MS). The sample matrix, a standard material of emulsion-type sausage with varied contents of CNT (brain),
was heat-treated in three studies: (1) routine meat technological heat treatment with low (85 °C, 30 min), medium (115 °C,
30 min), and high (133 °C, 30 min, 3 bar) heating of 72 anonymous samples from a blind trial; (2) heat treatment under experimental
conditions (100, 110, …, 200 °C, 45 min); and (3) fractionized heating of central nervous system (up to three times) under
moderate routine technological conditions (85, 100, and 115 °C, 30 min). The markers of the immunochemical methods showed
a low GFAP or very low NSE temperature stability at medium and high temperature conditions. The real-time PCR assay gave inconsistent,
non-quantitative results, which indicated an uncontrollable matrix effect. The relevant GC/MS markers (C24:0-OH, C24:1ω9/ω7,
and C24:1ω9-OH/ω7-OH) proved to be extremely stable. Neither meat and bone meal conditions (133 °C) nor experimental heating
(up to and above 140 °C) showed any reduction of GC/MS CNT quantification. On the contrary, a slight but significant increase
was noted over a certain temperature range (120–140 °C) for most fatty acids, possibly due to an improved extractability of
the fatty acids. We conclude that a quantitative approach is highly unreliable when using immunochemical methods; moreover,
these methods might be basically prone to false-negative results depending on heat treatment and matrix composition. Therefore,
antibodies with higher affinity to heat-treated CNT marker epitopes are needed. Relevant amounts of CNT (≥0.5%) in low- and
medium-heated products would still be reliably detectable by the GFAP ELISA, which justifies its use as a screening method
in official food control. The results obtained by the real-time PCR assay were contradictory to recently published data, indicating
a need for further protocol optimization and collaborative trials. Up to date, the analytical approach using GC/MS is the
only valid procedure as pertaining to heat stability and quantitative analysis; consequently, it should be recommended as
the reference procedure in official food control for CNT detection in heat-treated meat products. 相似文献
993.
Simone Portmann Hiu Ying Kwan Regula Theurillat Andrea Schmitz Meike Mevissen Wolfgang Thormann 《Journal of chromatography. A》2010,1217(51):7942-7948
Ketamine, a phencyclidine derivative, is used for induction of anesthesia, as an anesthetic drug for short term surgical interventions and in subanesthetic doses for postoperative pain relief. Ketamine undergoes extensive hepatic first-pass metabolism. Enantioselective capillary electrophoresis with multiple isomer sulfated β-cyclodextrin as chiral selector was used to identify cytochrome P450 enzymes involved in hepatic ketamine and norketamine biotransformation in vitro. The N-demethylation of ketamine to norketamine and subsequently the biotransformation of norketamine to other metabolites were studied via analysis of alkaline extracts of in vitro incubations of racemic ketamine and racemic norketamine with nine recombinantly expressed human cytochrome P450 enzymes and human liver microsomes. Norketamine was formed by CYP3A4, CYP2C19, CYP2B6, CYP2A6, CYP2D6 and CYP2C9, whereas CYP2B6 and CYP2A6 were identified to be the only enzymes which enable the hydroxylation of norketamine. The latter two enzymes produced metabolic patterns similar to those found in incubations with human liver microsomes. The kinetic data of ketamine N-demethylation with CYP3A4 and CYP2B6 were best described with the Michaelis–Menten model and the Hill equation, respectively. This is the first study elucidating the individual enzymes responsible for hydroxylation of norketamine. The obtained data suggest that in vitro biotransformation of ketamine and norketamine is stereoselective. 相似文献
994.
Wolfgang Holzer Angelika Ebner Karin Schalle Gyselle Batezila Gernot A. Eller 《Journal of fluorine chemistry》2010,131(10):1013-1024
A straightforward, two-step synthesis of fluoro substituted chromeno[2,3-c]pyrazol- and [1]benzothieno[2′,3′:5,6]pyrano[2,3-c]pyrazol-4(1H)-ones, respectively, is presented. Hence, treatment of 1-substituted or 1,3-disubstituted 2-pyrazolin-5-ones with fluoro substituted 2-fluorobenzoyl chlorides or 3-chloro-6-fluoro-1-benzothiophene-2-carbonyl chloride using calcium hydroxide in refluxing 1,4-dioxane gave the corresponding 4-aroylpyrazol-5-ols, which were cyclized into the fused ring systems. 5-Fluorochromeno[2,3-c]pyrazol-4(1H)-one was obtained upon treatment of the 1-(4-methoxybenzyl) protected congener with trifluoroacetic acid. Treatment of 5-fluorochromeno[2,3-c]pyrazol-4(1H)-ones with methylhydrazine afforded novel tetracyclic ring systems such as 2-methyl-7-phenyl-2,7-dihydropyrazolo[4′,3′:5,6]pyrano[4,3,2-cd]indazole. Detailed NMR spectroscopic investigations (1H, 13C, 15N, 19F) with the obtained compounds were undertaken. 相似文献
995.
Anina Wöhl Dr. Wolfgang Müller Dr. Stephan Peitz Normen Peulecke Dr. Bhaskar R. Aluri Dr. Bernd H. Müller Dr. Detlef Heller Dr. Uwe Rosenthal Prof. Dr. Mohammed H. Al‐Hazmi Dr. Fuad M. Mosa 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(26):7833-7842
In this paper we report the results of an extensive experimental kinetic study carried out on the novel ethylene trimerization catalyst system, comprising the chromium source [CrCl3(thf)3] (thf=tetrahydrofuran), a Ph2P‐N(iPr)‐P(Ph)‐N(iPr)H (PNPNH) ligand (Ph=phenyl, iPr=isopropyl), and triethylaluminum (AlEt3) as activator. It could be shown that the initial activity shows a first‐order dependency on the ethylene concentration. Also, a first‐order dependency was found for the catalyst concentration. The initial activity follows a typical Arrhenius behavior with an experimentally determined activation energy of 52.6 kJ mol?1. At elevated temperatures (ca. 80 °C), a significant deactivation was observed, which can be tentatively traced back to a ligand rearrangement in the presence of AlEt3. After a fast initial phase, a pronounced ‘kink’ in the ethylene‐uptake curve is observed, followed by a slow, almost linear, further increase of the total ethylene consumption. The catalyst composition, in particular the ligand/chromium and the cocatalyst/chromium molar ratio, has a strong impact on the catalytic performance of the trimerization of ethylene. 相似文献
996.
News and Announcements
VIM3 Corrigendum available online 相似文献997.
Sanner C Su EJ Keshet A Huang W Gillen J Gommers R Ketterle W 《Physical review letters》2011,106(1):010402
Spin fluctuations and density fluctuations are studied for a two-component gas of strongly interacting fermions along the Bose-Einstein condensate-BCS crossover. This is done by in?situ imaging of dispersive speckle patterns. Compressibility and magnetic susceptibility are determined from the measured fluctuations. This new sensitive method easily resolves a tenfold suppression of spin fluctuations below shot noise due to pairing, and can be applied to novel magnetic phases in optical lattices. 相似文献
998.
We study the totally asymmetric simple exclusion process (TASEP) on complex networks, as a paradigmatic model for transport subject to excluded volume interactions. Building on TASEP phenomenology on a single segment and borrowing ideas from random networks we investigate the effect of connectivity on transport. In particular, we argue that the presence of disorder in the topology of vertices crucially modifies the transport features of a network: irregular networks involve homogeneous segments and have a bimodal distribution of edge densities, whereas regular networks are dominated by shocks leading to a unimodal density distribution. The proposed numerical approach of solving for mean-field transport on networks provides a general framework for studying TASEP on large networks, and is expected to generalize to other transport processes. 相似文献
999.
Miyake H Siviloglou GA Puentes G Pritchard DE Ketterle W Weld DM 《Physical review letters》2011,107(17):175302
We have observed Bragg scattering of photons from quantum degenerate ^{87}Rb atoms in a three-dimensional optical lattice. Bragg scattered light directly probes the microscopic crystal structure and atomic wave function whose position and momentum width is Heisenberg limited. The spatial coherence of the wave function leads to revivals in the Bragg scattered light due to the atomic Talbot effect. The decay of revivals across the superfluid to Mott insulator transition indicates the loss of superfluid coherence. 相似文献
1000.
Spiess HW 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,(2):326-328
The development of four-pulse DEER as described, which has been published in the Journal of Magnetic Resonance more than 10 years ago. The corresponding paper is an example where a slight advance, such as adding a refocusing pulse, which in retrospect looks so simple, can have a remarkable impact on an entire field of science. In our case it offered a simple way to exact measurements of distances between defined species in the nanometer range. The current applications are mainly in determining structures of proteins and nucleic acids. 相似文献