全文获取类型
收费全文 | 2266篇 |
免费 | 81篇 |
国内免费 | 16篇 |
专业分类
化学 | 1592篇 |
晶体学 | 9篇 |
力学 | 58篇 |
数学 | 410篇 |
物理学 | 294篇 |
出版年
2023年 | 20篇 |
2022年 | 27篇 |
2021年 | 37篇 |
2020年 | 58篇 |
2019年 | 51篇 |
2018年 | 15篇 |
2017年 | 29篇 |
2016年 | 72篇 |
2015年 | 69篇 |
2014年 | 77篇 |
2013年 | 106篇 |
2012年 | 128篇 |
2011年 | 169篇 |
2010年 | 89篇 |
2009年 | 111篇 |
2008年 | 157篇 |
2007年 | 135篇 |
2006年 | 137篇 |
2005年 | 103篇 |
2004年 | 113篇 |
2003年 | 75篇 |
2002年 | 80篇 |
2001年 | 32篇 |
2000年 | 32篇 |
1999年 | 27篇 |
1998年 | 26篇 |
1997年 | 20篇 |
1996年 | 19篇 |
1995年 | 16篇 |
1994年 | 9篇 |
1993年 | 11篇 |
1992年 | 12篇 |
1991年 | 13篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 11篇 |
1987年 | 21篇 |
1985年 | 11篇 |
1984年 | 18篇 |
1983年 | 20篇 |
1982年 | 11篇 |
1981年 | 17篇 |
1980年 | 18篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1977年 | 11篇 |
1974年 | 8篇 |
1973年 | 8篇 |
1972年 | 9篇 |
1964年 | 7篇 |
排序方式: 共有2363条查询结果,搜索用时 78 毫秒
971.
A sensitive and selective liquid chromatography-mass spectrometry method has been developed for the simultaneous identification and quantitation of drug substances and metabolites in rat plasma. The method combines on-line turbulent-flow chromatography, high-performance liquid chromatography and mass spectrometry. This combination is considered to be a new approach suitable for fast bio-analysis in drug discovery. Dextromethorphan, and its two metabolites, dextrorphan and 3-methoxymorphinan served as model substances. The analytes present in plasma were collected on a Cyclone column using turbulent-flow chromatography and were subsequently transferred on-line to and focused on an X-Terra MS C8 column. The analytes were eluted by a linear gradient and detected by a fast scanning mass spectrometer. The detector response was quadratic and the dynamic range was estimated to be 0.5-100 ng/ml plasma or 12.5 pg to 2.50 ng injected into the system. 相似文献
972.
Lars I. Andersson Johan Henriksson Lars-Inge Olsson Mohamed Abdel-Rehim 《Analytica chimica acta》2004,526(2):147-154
Molecular imprints selective for a homologous series of local anaesthetics, including bupivacaine, ropivacaine and mepivacaine, were prepared and the resultant polymers were used for solid-phase extraction of human plasma. The template was a structural analogue, pentycaine, which was imprinted in methacrylic acid-ethylene glycol dimethacrylate copolymers. Equilibrium ligand binding experiments using radiolabelled bupivacaine were performed to characterize the imprinted polymers, as well as to identify optimal conditions for selective extraction of plasma samples. Dilution of the plasma prior to extraction with citrate buffer pH 5.0 containing ethanol and Tween 20 was found optimal for selective imprint-analyte binding, and for reduction of non-specific adsorption of lipophilic contaminants to the hydrophobic MIP surface. Wash steps using 20% methanol in water followed by a solvent switch to 10% ethanol in acetonitrile removed contaminants and strengthened the selective imprint-analyte binding. Elution under basic conditions using triethylamine-water-acetonitrile mixtures recovered bupivacaine in 89% yield with superior selectivity over elution under acidic conditions. The final protocol extracted trace levels of ropivacaine and bupivacaine from human plasma and allowed determination of bupivacaine in the range of 3.9-500 nmol L−1 and ropivacaine in the range of 7.8-500 nmol L−1 with inter-assay accuracies of 94-99 and 95-104%, respectively. This present investigation provides an improved understanding of approaches available for optimization of protocols for molecular-imprint based solid-phase extraction of plasma samples. 相似文献
973.
The design of immobilized-enzyme reactors for use in flow injection analysis is discussed. The reactors should be optimized for a short residence time and a very high (> 99.9%) conversion of substrate to products. Selection of carrier and immobilization method is important in order to increase the amount of active enzyme per unit volume. The effeciency of the reactor can be increased by decreasing the particle size in packed-bed reactors and the radius of open tubular reactors. The maximum inherent rate constant that can be obtained under optimal conditions is estimated for a number of enzymes of analytical interest; it is shown that with high rate constants and small particle diamters, residence times less than seconds can be obtained. Some applications of immobilized-enzyme reactors in flow systems are reviewed. 相似文献
974.
The crystal structures of the room and the high temperature modifications of cesium trifluoromethyl sulfonate were solved from high resolution X‐ray powder diffraction data. At room temperature, α‐CsSO3CF3 crystallizes in the monoclinic space group P21 with lattice parameters a = 9.7406(2) Å, b = 6.1640(1) Å, c = 5.4798(1) Å, and β = 104.998(1)°; Z = 2. At temperatures above T = 380 K, a second order phase transformation towards a disordered C‐centered orthorhombic phase in space group Cmcm occurs with lattice parameters at T = 492 K of a = 5.5074(3) Å, b = 19.4346(14) Å, and c = 6.2978(4) Å; Z = 4. Within the crystal structures, the triflate anions are arranged in double layers with the apolar CF3‐groups pointing towards each other. The cesium ions are located between the SO3‐groups. CsSO3CF3 shows a specific ion conductivity ranging from σ = 1.06·10?8 Scm?1 at T = 393 K to σ = 5.18·10?4 Scm?1 at T = 519 K. 相似文献
975.
Lars V. Andreasen Alan Hazell Ole Wernberg 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(7):m385-m387
In the title compound, [Pt(C18H15P)(C28H28P2S)](ClO4)2·C3H6O or [Pt(PPh3)(PSP)](ClO4)2·CH3COCH3, where PSP is the potentially tridentate chelate ligand bis(2‐diphenylphosphinoethyl) sulfide, all three donor groups of the PSP ligand are coordinated to the central Pt atom, with Pt—P = 2.310 (1) Å and Pt—S = 2.343 (1) Å. The fourth coordination site is occupied by the P donor of the triphenylphosphine ligand [Pt—P = 2.289 (1) Å]. The complex cation has exact mirror symmetry, with the S atom, the Pt atom and the P atom of the PPh3 ligand in the mirror plane. The Pt atom has a distorted square‐planar coordination geometry. A π–π interaction is present between the phenyl rings of the PPh3 ligand and the terminal –PPh2 group of the PSP chelate. 相似文献
976.
[reaction: see text] A concise stereoselective approach to both orthogonally protected (2S,4R)- and (2S,4S)-4-hydroxyornithine, key constituents of the biphenomycin- and clavalanine-type antibiotics, respectively, has been developed. The approach is based on bis(oxazoline) copper(II)-complex-catalyzed diastereoselective Henry reactions of nitromethane with the homoserine-derived aldehyde 6. The synthesis of this versatile chiral building block has been markedly improved. 相似文献
977.
Following an initial clean-up step on silica, high-speed counter-current chromatography (HSCCC) was used to purify an aryl ketone, 1-[2',4'-dihydroxy-3',5'-di-(3"-methylbut-2"-enyl)-6'-methoxy] phenylethanone from an extract of the stem bark of the shrub Acronychia pedunculata. The two-phase solvent system used was composed of n-heptane-ethyl acetate-methanol-water at an optimized volume ratio of 4:1:4:1 (v/v/v/v). Target compound (58.1 mg) with a purity of 98.9% was obtained after HSCCC of 183.5 mg sample with a purity of 35.7% recovered after the silica clean-up step. Identification of the target compound was performed by 1H NMR, 13C NMR, two-dimensional NMR and LC-electrospray ionization MS. 相似文献
978.
Vester B Lundberg LB Sørensen MD Babu BR Douthwaite S Wengel J 《Journal of the American Chemical Society》2002,124(46):13682-13683
Incorporation of two alpha-L-LNA/LNA nucleotides into each of the two binding arms of a "10-23" DNAzyme has been accomplished and the RNA cleavage with these novel LNAzymes studied. In comparison with the unmodified DNAzyme, the LNAzymes show significantly improved cleavage of the phosphodiester backbone at the target nucleotide in a small RNA substrate (58n RNA) under single-turnover conditions. The LNAzymes show efficient multiple turnover. With the LNAzymes, efficient cleavage was accomplished also of a naturally occurring ribosomal RNA at a target site within a highly structured region. The reference DNAzyme was ineffective at cleaving the ribosomal RNA target. 相似文献
979.
Ulf Henriksson Tomas Klason Lars Ödberg Jan Christer Eriksson 《Chemical physics letters》1977,52(3):554-558
Quadrupole splittings in the 2H NMR spectrum from C6D6 and C6D12 solubilized in aqueous solutions of hexadecyltrimethylammonium bromide (CTAB) have been observed in the hexagonal liquid crystalline phase (E) as well as in the concentrated micellar phase (L1). The order parameter that is obtained for solubilized C6D6 is more than two orders of magnitude greater than previously reported from measurements of linear dichroism spectra. Various possibilities for the nature of the solubilization sites are discussed. 相似文献
980.
Jeuken LJ Jones AK Chapman SK Cecchini G Armstrong FA 《Journal of the American Chemical Society》2002,124(20):5702-5713
A new approach for studying intramolecular electron transfer in multicenter enzymes is described. Two fumarate reductases, adsorbed on an electrode in a fully active state, have been studied using square-wave voltammetry as a kinetic method to probe the mechanism of the long-range electron transfer to and from the buried active site. Flavocytochrome c(3) (Fcc(3)), the globular fumarate reductase from Shewanella frigidimarina, and the soluble subcomplex of the membrane-bound fumarate reductase of Escherichia coli (FrdAB) each contain an active site FAD that is redox-connected to the surface by a chain of hemes or Fe-S clusters, respectively. Using square-wave voltammetry with large amplitudes, we have measured the electron-transfer kinetics of the FAD cofactor as a function of overpotential. The results were modeled in terms of the FAD group receiving or donating electrons either via a direct mechanism or one involving hopping via the redox chain. The FrdAB kinetics could be described by both models, while the Fcc(3) data could only be fit on the basis of a direct electron-transfer mechanism. This raises the likelihood that electron transfer can occur via a superexchange mechanism utilizing the heme groups to enhance electronic coupling. Finally, the FrdAB data show, in contrast to Fcc(3), that the maximum ET rate at high overpotential is related to the turnover number for FrdAB measured previously so that electron transfer is the limiting step during catalysis. 相似文献