全文获取类型
收费全文 | 2443篇 |
免费 | 164篇 |
国内免费 | 705篇 |
专业分类
化学 | 2843篇 |
晶体学 | 40篇 |
力学 | 23篇 |
综合类 | 24篇 |
数学 | 21篇 |
物理学 | 361篇 |
出版年
2023年 | 30篇 |
2022年 | 84篇 |
2021年 | 105篇 |
2020年 | 101篇 |
2019年 | 55篇 |
2018年 | 69篇 |
2017年 | 82篇 |
2016年 | 118篇 |
2015年 | 98篇 |
2014年 | 102篇 |
2013年 | 205篇 |
2012年 | 152篇 |
2011年 | 142篇 |
2010年 | 138篇 |
2009年 | 140篇 |
2008年 | 113篇 |
2007年 | 132篇 |
2006年 | 111篇 |
2005年 | 102篇 |
2004年 | 123篇 |
2003年 | 111篇 |
2002年 | 84篇 |
2001年 | 66篇 |
2000年 | 66篇 |
1999年 | 70篇 |
1998年 | 76篇 |
1997年 | 54篇 |
1996年 | 67篇 |
1995年 | 65篇 |
1994年 | 83篇 |
1993年 | 98篇 |
1992年 | 91篇 |
1991年 | 54篇 |
1990年 | 20篇 |
1989年 | 13篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 13篇 |
1985年 | 4篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1973年 | 1篇 |
排序方式: 共有3312条查询结果,搜索用时 15 毫秒
61.
A. Ceccato P. Chiap Ph. Hubert B. Toussaint J. Crommen 《Journal of chromatography. A》1996,750(1-2):351-360
A fully automated method for the simultaneous determination of verapamil and its main metabolite norverapamil in human plasma is described. This method is based on on-line sample preparation using dialysis followed by clean-up and enrichment of the dialysate on a precolumn and subsequent HPLC analysis with fluorometric detection. All sample handling operations were performed automatically by a sample processor equipped with a robotic arm (ASTED system). The plasma samples were dialysed on a cellulose acetate membrane (cut-off: 15 kD) and the dialysate was purified and enriched on a short pre-column filled with cyanopropyl silica. Before starting dialysis, this trace enrichment column (TEC) was first conditioned with the HPLC mobile phase and then with pH 3.0 acetate buffer. 370 μl of plasma sample spiked with the internal standard (gallopamil) were dialysed in the static-pulsed mode. The solution at the donor side was pH 3.0 acetate buffer containing Triton X-100 while the acceptor solution was made of the same acetate buffer. When dialysis was discontinued, the analytes were desorbed from the TEC by the HPLC mobile phase and transferred to the C18 analytical column by means of a switching valve. This mobile phase consisted of a mixture of acetonitrile, pH 3.0 acetate buffer and 2-aminoheptane. The influence of different parameters of the dialysis process on the recovery of verapamil and norverapamil has been studied. The effect of the volume, the aspirating and dispensing flow-rates of the dialysis solution has been investigated. The recoveries of verapamil and norverapamil in plasma were close to 75% and the limits of quantification were 5 ng/ml for both analytes. The method was found to be linear in the concentration range from 5 to 500 ng/ml (r2: 0.9996 for both analytes). The intra-day and inter-day reproducibilities at a concentration of 100 ng/ml were 2.3% and 5.6% for verapamil and 1.7% and 5.1% for norverapamil, respectively. 相似文献
62.
Reaction of powdered Zr with ZrCl4, BaCl2 and Be in suitable proportions in a Ta container at 800°C produces the title compound. Suitable monocrystals for X-ray diffraction were obtained from reactions to which a comparable amount of Hg2Cl2 had been added. The structure of Ba3Zr6Cl18Be is a superstructure of the K2ZrCl6 · Zr6Cl18H type (R3 c, Z = 6; a = 9.6852 (9) Å, c = 52.52 (1) Å; R, Rw = 2.7, 3.2% for 826 independent reflections, 2θ ≤ 50°). Trigonally compressed [Zr6(Be)Cl12i]Cl6a clusters are interconnected by six-coordinate barium atoms that lie in Cla antiprisms (a twisted version of the ZrIV site) while (9 + 3)-coordinate barium substitutes for potassium within chlorine layers. Distortions associated with the size and field of barium are responsible for the superstructure and for differences from other analogues. 相似文献
63.
Eugenia M. Brazwell Dianela Y. Filos Cary J. Morrow 《Journal of polymer science. Part A, Polymer chemistry》1995,33(1):89-95
Enzyme-catalyzed preparation of polymers offers several potentially valuable advantages over the usual polymerization procedures and has been studied for several years. A significant limitation on the polyesters prepared to date has been the low molecular weights achieved. The present studies have established that, in the polycondensation of bis(2,2,2-trifluoroethyl) glutarate with 1,4-butanediol using porcine pancreatic lipase as the catalyst, this limitation arises from at least two sources: hydrolysis of activated ester end groups by water introduced along with the enzyme and the polymerization's reaching equilibrium despite using the poorly nucleophilic 2,2,2-trifluoroethanol as the leaving group. Evidence is also developed that the presence of trifluoroethanol accelerates the release of the enzyme-bound water which hydrolyzes the activated ester end groups. The hydrolysis could be avoided by choosing a relatively high-boiling solvent, such as bis(2-ethoxyethyl) ether, then removing the trifluoroethanol by placing the reaction mixture under vacuum periodically or by drying the enzyme rigorously. The vacuum method also removed the limitation on molecular weight resulting from the reaction's reaching equilibrium. A further improvement in the molecular weight to nearly 40,000 daltons, well within the range that is technically interesting, was achieved by using 1,2-dimethoxybenzene or 1,3-dimethoxybenzene as the polymerization solvent. © 1995 John Wiley & Sons, Inc. 相似文献
64.
Vera P. Shmachkova Nina S. Kotsarenko Vladislav V. Kanazhevskiy Galina N. Kryukova Dimitri I. Kochubey Jacques Vedrine 《Reaction Kinetics and Catalysis Letters》2007,91(1):177-185
Methods of the preparation of catalysts for alkane skeletal isomerization based on uniform nanoparticles of sulfated zirconia
anchored to different supports were investigated. These catalysts were characterized by using the ICP, HRTEM and BET techniques.
The activities of the catalysts in the reaction of n-butane isomerization were measured and compared with those of bulk catalysts. 相似文献
65.
The state of art of the various preparative planar liquid chromatographic (PLC) methods is summarized, especially for off-line and on-line sample application. The sample purification possibilities for PLC techniques are discussed. Purification and isolation strategies using forced-flow planar chromatographic techniques, such as overpressured layer chromatography and rotation planar chromatography, are suggested in the form of flow charts. 相似文献
66.
67.
可再生甲壳素吸附铬(Ⅵ)的特性研究 总被引:9,自引:0,他引:9
可再生甲壳素吸附铬(Ⅵ)的特性研究陈炳稔*汤又文李国明万春华(华南师范大学化学系广州510631)关键词可再生甲壳素,制备,铬(Ⅵ),吸附1997-08-19收稿,1997-12-23修回广东省科委及高教厅资助课题利用甲壳素作为铬(Ⅵ)的捕集剂已有报... 相似文献
68.
Rosin glycerin ester and its bromide were prepared from natural renewable rosin, glycerin and liquid bromine which were first
subjected to an esterification reaction, followed by an addition reaction. Their structures were characterized by an infrared
(IR) spectrum and their thermal resistance was conducted with thermal gravity (TG) and differential scanning calorimetry (DSC).
It showed that the bromide in the rosin glycerin ester decomposed faster than the ester; hence it may be used as fire-resistant
material. 相似文献
69.
Ammonium Chloro(thio)phosphate Betaines, a Class of Coordination-stabilized Chloro(thio)metaphosphates Ammonium chloro(thio)phosphate betaines behave like systems being in equilibrium with tert. amine and monomeric chloro(thio)metaphosphate. In solution at room temperature the amine component can be displaced by stronger donors. Mass spectra reveal the presence of ClPS2 as monomer and dimer, of ClPO2 by fragment ions of the dimer, trimer, and tetramer before reorganization takes place to form PXCl3 and P4X10 (X = O, S). Detection of ClPOS fails owing to fast equilibration into ClPO2 and ClPS2. 相似文献
70.
Hohyun Kim Moon Sun Jang Jung-Ae Lee Dongjin Pyo Hye-Ran Yoon Hee Joo Lee Kyung Ryul Lee 《Chromatographia》2004,60(5-6):335-339
A sensitive and selective method for the determination of sofalcone in human plasma was established by use of protein precipitation and liquid chromatography-tandem mass spectrometry. Plasma samples were transferred into 96-well plate using an automated sample handling system and spiked with 10 L of 2 g mL–1 internal standard solution (d3-sofalcone). 0.5 mL of acetonitrile was added to the 96-well plate and the plasma samples were then vortexed for 30 sec. After centrifugation, the supernatant was transferred into another 96-well plate and completely evaporated at 40 °C under a stream of nitrogen. The dry residue was reconstituted with mobile phase. All sample transfer and protein precipitation was automated through the application of both the PerkinElmer MultiPROBE II HT and TOMTEC Quadra 96 workstation. The limit of quantitation of sofalcone was 2 ng mL–1 using a sample volume of 0.2 mL for the analysis. The reproducibility of the method was evaluated by analyzing five samples at nine quality control (QC) levels over the nominal concentration range from 2 ng mL–1 to 1000 ng mL–1. Validation of the method showed that the assay has good precision and accuracy. Sofalcone and internal standard produced a protonated precursor ion ([M+H]+) at m/z 451 and 454, and both gave a corresponding product ion at m/z 315. The high sample throughput of the method has been successfully applied to a pharmacokinetic study of sofalcone in human plasma. 相似文献