全文获取类型
收费全文 | 41305篇 |
免费 | 1464篇 |
国内免费 | 144篇 |
专业分类
化学 | 22103篇 |
晶体学 | 204篇 |
力学 | 1092篇 |
综合类 | 5篇 |
数学 | 6920篇 |
物理学 | 12589篇 |
出版年
2023年 | 266篇 |
2021年 | 346篇 |
2020年 | 553篇 |
2019年 | 415篇 |
2018年 | 543篇 |
2017年 | 526篇 |
2016年 | 1101篇 |
2015年 | 863篇 |
2014年 | 971篇 |
2013年 | 1990篇 |
2012年 | 1858篇 |
2011年 | 2021篇 |
2010年 | 1333篇 |
2009年 | 1145篇 |
2008年 | 1890篇 |
2007年 | 1768篇 |
2006年 | 1685篇 |
2005年 | 1544篇 |
2004年 | 1353篇 |
2003年 | 1085篇 |
2002年 | 966篇 |
2001年 | 901篇 |
2000年 | 730篇 |
1999年 | 515篇 |
1998年 | 456篇 |
1997年 | 444篇 |
1996年 | 550篇 |
1995年 | 507篇 |
1994年 | 519篇 |
1993年 | 542篇 |
1992年 | 587篇 |
1991年 | 408篇 |
1990年 | 422篇 |
1989年 | 375篇 |
1988年 | 348篇 |
1987年 | 365篇 |
1986年 | 328篇 |
1985年 | 528篇 |
1984年 | 503篇 |
1983年 | 420篇 |
1982年 | 445篇 |
1981年 | 442篇 |
1980年 | 438篇 |
1979年 | 425篇 |
1978年 | 413篇 |
1977年 | 418篇 |
1976年 | 388篇 |
1975年 | 371篇 |
1974年 | 363篇 |
1973年 | 310篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A high-performance liquid chromatography-mass spectrometry method has been developed for the simultaneous determination of native carnitine and eight acylcarnitines in urine. The procedure uses a solid-phase extraction on a cation-exchange column and the separation is performed without derivatization within 17 min on a reversed-phase C8 column in the presence of a volatile ion-pairing reagent. The detector was an ion trap mass spectrometer and quantification was carried out in the MS-MS mode. Validation was done for aqueous standards at ranges between 0.75 and 200 micromol/l, depending on the compound. Carnitine was quantified in urine and comparison with a radioenzymatic assay gave a satisfactory correlation (R2 = 0.981). The assay could be successfully applied to the diagnostic of pathological acylcarnitines profile of metabolic disorders in urines of patients suffering from different organic acidurias. 相似文献
992.
In the case of the formation of thiospinels (MnCr2S4, ZnCr2S4 and CdCr2S4) a simple method for the determination of the activation energy of solid state reactions in polycrystalline samples by means of conductivity measurements is shown. The maxima and minima of conductivity (glow curve maxima and -minima) in the σ vs T-curves got by annealing with linearly increasing temperature are shifted towards higher temperatures with increasing rate of heating(q). By plotting ln (q/T) vs 1/Tσmax a straight line results. The activation energy of the reaction may be deduced from the slope of this straight line in analogy to the determination of the trap depth of electrons from glow curves observed in thermoluminescence. 相似文献
993.
Müller C Ackerman LJ Reek JN Kamer PC van Leeuwen PW 《Journal of the American Chemical Society》2004,126(45):14960-14963
Dendrimers, specifically suited to construct site-isolated groups due to their well-defined hyperbranched structure, have been used as a ligand design element for the construction of nickel catalysts for ethylene oligomerization. The dendritic P,O ligand indeed suppresses the formation of inactive bis(P,O)Ni complexes in toluene, as is evident from NMR studies, and, as a consequence, outperforms the parent ligand in catalysis in this solvent. The dendritic effect observed in methanol is more subtle because both the dendritic ligand 1 and the parent 2 form bis(P,O)nickel complexes in solution according to NMR spectroscopy. Unlike the parent complex 8, the dendritic bis(P,O)Ni complex 7 derived from dendrimer ligand 1 is able to dissociate to a mono-ligated species under catalytic conditions, that is, 40 bar ethylene and 80 degrees C, which can enter the catalytic cycle. Indeed, dendritic ligand 1 gives much more active nickel catalysts for the oligomerization in methanol than does 2. 相似文献
994.
Bracher F Eisenreich WJ Mühlbacher J Dreyer M Bringmann G 《The Journal of organic chemistry》2004,69(25):8602-8608
The first biarylic bis-morphinanedienone alkaloids, saludimerines A (3a) and B (3b), isolated from a tree of Croton flavens (Euphorbiaceae) are described. These naturally occurring dimers of the known alkaloid salutaridine are joined together via a rotationally hindered biaryl axis, giving rise to atropo-diastereomers that are configurationally stable at room temperature but slowly interconvert in methanolic solution within several days. Their structures were established by spectroscopic methods and by partial synthesis, which was achieved by a highly atropo-diastereoselective biomimetic oxidative coupling of the monomeric precursor, salutaridine. Their axial configurations were elucidated by circular dichroism (CD) investigations, which succeeded despite the fact that the two atropo-diastereomers exhibit near-identical CD spectra. This remarkable phenomenon was rationalized by quantum chemical CD calculations. The configurational assignment of saludimerines A (3a) as P-axial and B (3b) as M was corroborated by atropisomer-specific NOE interactions between protons of the one molecular half with nuclei in the other. 相似文献
995.
Using instrumental neutron activation analysis, the variations in the serum levels of Br, Cs, Na, Rb, Se, and Zn were investigated
in 18 persons in the normal state. By determining the element levels in relation to changes is the serum water content, information
about the binding to the serum protein was obtained. The influence of physiological factors on the serum element levels was
investigated. Changes in the concentrations due to variations in the protein/water ratio and in the zinc level as a result
of different posture were observed. When standardized sampling procedures were used, the variations in the serum element concentrations
during one hour and from day were found to be less than 10%. 相似文献
996.
S. Yariv M. Müller-Vonmoos G. Kahr A. Rub 《Journal of Thermal Analysis and Calorimetry》1989,35(6):1941-1952
The adsorption of crystal violet on laponite was investigated by X-ray diffraction and thermal analysis. DTA, TG and DTG curves were recorded in air. The evolved H2O, CO2, NO2, H2 and C2H6 were simultaneously determined by mass spectrometry. The thermal analysis curves were compared on one hand with the thermal analysis curves of laponite and on the other hand with thermal analysis curves of non-adsorbed crystal violet and of crystal violet adsorbed on montmorillonite. The thermal analysis curves of crystal violet adsorbed on laponite show similarities to the curves of the non-adsorbed crystal violet, but differ from the curves of crystal violet adsorbed on montmorillonite. The differences in the thermal behaviour were attributed toπ interactions which do not occur between crystal violet and laponite but do occur between this dye and montmorillonite. 相似文献
997.
Analysis, purification and properties of a 50,000-dalton membrane-associated phosphoprotein from human platelets 总被引:3,自引:0,他引:3
Recently, the development of a monospecific antiserum against a 46,000/50,000-dalton membrane protein from human platelets which was stoichiometrically and reversibly phosphorylated in intact human platelets in response to vasodilators was reported. Using this antiserum, the subcellular distribution and the purification of this vasodilator-stimulated phosphoprotein (VASP) from human platelets has now been analysed. The VASP of human platelets is primarily a membrane-associated protein and can be purified to apparent homogeneity by salt extraction and sequential ion-exchange and dye-ligand chromatography with a purification factor of 1200 and a yield of 13%. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis under reducing and non-reducing conditions indicated that purified monomers of this VASP are linked by interchain disulphide bonding. 相似文献
998.
A fast stepwise systematic approach for the conversion of conventional reversed-phase high-performance liquid chromatographic (HPLC) assays involving liquid-liquid extraction of biological fluids into fully automated HPLC assays using solid-phase extraction and cartridge exchange is described. The suitability of this procedure is demonstrated for the determination of cebaracetam in human urine. 相似文献
999.
Kiessig S Reissmann J Rascher C Küllertz G Fischer A Thunecke F 《Electrophoresis》2001,22(7):1428-1435
A screening procedure for protein-protein interactions in cellular extracts using a green fluorescent protein (GFP) and affinity capillary electrophoresis (ACE) was established. GFP was fused as a fluorescent indicator to the C-terminus of a cyclophilin (rDmCyp20) from Drosophila melanogaster. Cyclophilins (Cyps) belong to the ubiquitously distributed enzyme family of peptidyl-prolyl cis/trans isomerases (PPlases) and are well known as cellular targets of the immunosuppressive drug cyclosporin A (CsA). The PPlase activity of the GFP fused rDmCyp20 as well as the high affinity to CsA remain intact. Using native gel electrophoresis and ACE mobility-shift assays, it was demonstrated that the known moderate affinity of Cyp20 to the capsid protein p24 of HIV-1 was detectable in the case of rDmCyp20 fused to the fluorescent tag. For the p24 / rDmCyp20-GFP binding an ACE method was established which allowed to determine a dissociation constant of Kd = 20+/-1.5 x 10(-6) M. This result was verified by size-exclusion chromatography and is in good agreement with published data for the nonfused protein. Moreover the fusion protein was utilized to screen rDmCyp20-protein interactions by capillary electrophoresis in biological matrices. A putative ligand of rDmCyp20 in crude extracts of embryonic D. melanogaster was discovered by mobility-shift assays using native gel electrophoresis with fluorescence imaging and ACE with laser-induced fluorescence detection. The approach seems applicable to a wide range of proteins and offers new opportunities to screen for moderate protein-protein interactions in biological samples. 相似文献
1000.
The application of the principle of ligand-exchange capillary electrophoresis to two substance classes is described. As chiral selector N-(2-hydroxyoctyl)-L-4-hydroxyproline-copper(II) complex was used. This principle was applied to the chiral separation of alpha-hydroxy acids and drugs containing amino alcohol structure such as beta-blockers. The enantioselectivity was found to be strongly dependent on pH corresponding to the optimal conditions for complex formation for each structure class. 相似文献