全文获取类型
收费全文 | 792篇 |
免费 | 37篇 |
国内免费 | 4篇 |
专业分类
化学 | 525篇 |
晶体学 | 16篇 |
力学 | 17篇 |
数学 | 75篇 |
物理学 | 200篇 |
出版年
2024年 | 3篇 |
2023年 | 9篇 |
2022年 | 32篇 |
2021年 | 27篇 |
2020年 | 26篇 |
2019年 | 30篇 |
2018年 | 16篇 |
2017年 | 16篇 |
2016年 | 27篇 |
2015年 | 17篇 |
2014年 | 40篇 |
2013年 | 54篇 |
2012年 | 58篇 |
2011年 | 78篇 |
2010年 | 35篇 |
2009年 | 32篇 |
2008年 | 29篇 |
2007年 | 48篇 |
2006年 | 41篇 |
2005年 | 27篇 |
2004年 | 18篇 |
2003年 | 9篇 |
2002年 | 13篇 |
2001年 | 3篇 |
2000年 | 12篇 |
1999年 | 4篇 |
1998年 | 5篇 |
1997年 | 8篇 |
1996年 | 13篇 |
1995年 | 13篇 |
1994年 | 10篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1973年 | 6篇 |
1961年 | 2篇 |
1960年 | 1篇 |
1959年 | 1篇 |
1941年 | 2篇 |
排序方式: 共有833条查询结果,搜索用时 31 毫秒
771.
Jain DS Subbaiah G Sanyal M Shrivastav PS Pal U Ghataliya S Kakad A Patel H Shah S 《Rapid communications in mass spectrometry : RCM》2006,20(23):3509-3521
A high-throughput and sensitive bioanalytical method using liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) has been developed for the estimation of sibutramine and its two metabolites (M1 and M2). The extraction of sibutramine, its metabolites and imipramine (internal standard (IS)) from the plasma involved treatment with phosphoric acid followed by solid-phase extraction (SPE) using a hydrophilic-lipophilic balanced HLB cartridge. The SPE eluate without drying and reconstitution was analyzed by LC/MS/MS, equipped with a with turbo ion spray (TIS) source, operating in the positive and multiple reaction monitoring (MRM) acquisition mode. Sample preparation by this method yielded extremely clean extracts with quantitative and consistent mean recoveries; 95.12% for sibutramine, 92.74% for M1, 95.97% for M2 and 96.60% for the IS. The total chromatographic run time was 3.0 min with retention times of 2.51, 2.13, 2.09 min for sibutramine, M1, M2 and imipramine, respectively. The developed method was validated in human plasma matrix, with a sensitivity of 0.1 ng/mL (coefficient of variance (CV), 2.07%) for sibutramine, 0.1 ng/mL (CV, 3.59%) for M1 and 0.2 ng/mL (CV, 4.93%) for M2. Validation of the method for its accuracy, precision, recovery, matrix effect and stability was carried out especially with regard to real subject sample analysis. The response was linear over the dynamic range 0.1 to 8.0 ng/mL for sibutramine and M1, and 0.2 to 16.0 ng/mL for M2 with correlation coefficients of r > or = 0.9959 (sibutramine), 0.9935 (M1) and 0.9943 (M2). The method was successfully applied for bioequivalence studies in 40 human subjects with 15 mg capsule formulations. 相似文献
772.
Chopra D Cameron TS Ferrara JD Guru Row TN 《The journal of physical chemistry. A》2006,110(35):10465-10477
A charge density study of crystalline 1-(4-fluorophenyl)-3,6,6-trimethyl-2-phenyl-1,5,6,7-tetrahydro-4H-indol-4-one (A) and 1-(4-fluorophenyl)-6-methoxy-2-phenyl-1,2,3,4-tetrahydroisoquinoline (B) has been carried out using high-resolution X-ray diffraction data collected at 113(2) K. Weak intermolecular interactions of the type C-H...O, C-H...pi, and pi...pi hold the molecules together in the crystal lattice along with interactions of the type C-H...F and unusual C-F...F-C examined via charge density analysis. The topological features are evaluated in terms of Bader's theory of atoms in molecules through the first four criteria of Koch and Popelier. The C-F...F-C contact is observed to be across the center of symmetry in B and not in A, and further, this interaction appears to possess a certain correlation with the electron density properties at the critical point which suggests that such an interaction fits into the hierarchy of weak interactions. 相似文献
773.
Kanaujia PK Tak V Pardasani D Gupta AK Jain R Dubey DK 《European journal of mass spectrometry (Chichester, England)》2006,12(4):261-269
This communication describes GC-MS analysis of bis(trimethylsilyl) (bis-TMS) derivatives of bis(2-hydroxyethylthio)alkanes (BHETAs) and bis(2-hydroxyethylsulfonyl) alkanes (BHESAs) which are important markers of sulfur mustard class of chemical warfare agents. The study was undertaken with a view to develop spectral database of these compounds for verification analysis of Chemical Weapons Convention (CWC). Based on the obtained mass spectra of bis-TMS derivatives of BHETAs and BHESAs, the fragmentation routes are proposed, which explain most of the characteristic ions. 相似文献
774.
Pardasani D Palit M Gupta AK Kanaujia PK Sekhar K Dubey DK 《Journal of chromatography. A》2006,1108(2):166-175
Detection and identification of environmental signatures of chemical warfare agents is an important aspect of verification program of Chemical Weapons Convention (CWC). Alkylphosphonic acids (APAs) are ultimate and persistent degradation products of nerve agents. Their identification in a sample submitted for off-site analysis infers possible indication of contamination with nerve agents. This paper describes the development of a new sample preparation method which involves 'in situ derivatization and extraction' (INDEX) of acids from water. Derivatization is performed by alkylation of APAs with alkylbromides in surfactant less microemulsion (SLME). The derivatized analytes were analyzed by gas chromatography coupled with mass spectrometry. The developed method involves simultaneous derivatization (alkylation) and extraction of acidic analytes mediated by surfactant less microemulsion. Various derivatization-extraction parameters such as solvent, reaction time and temperature, base and alkyl bromides were optimized. Pentyl bromide in the presence of potassium carbonate and diisopropylamine at 100 degrees C derivatized the selected acids efficiently. Kinetic data for alkylation of methylphosphonic acids and some carboxylic acids were obtained to assess their relative susceptibility for alkylation in microemulsion. Methylphosphonic acid and isopropyl methylphosphonic acid took 140-150 min to reach completion while carboxylic acids took 100 min to complete the reaction. INDEX could be successfully performed even in the presence of interfering Ca(2+) and Mg(2+) ions. 相似文献
775.
A series of benzo[b][1,8]naphthyridines has been synthesized by Friedl?nder condensation of 2-aminoquinoline-3-carbaldehyde 1 (o-aminoaldehyde) with alicyclic ketones in basic medium. Benzonaphthyridines branched with various side-chains and substituents
are prepared with the aim of being investigated as a good fluorescent material. Electronic absorption and fluorescence properties
of some representative benzonaphthyridines in organic solvents, water-dioxane, and SDS, CTAB and Triton-X-100 micelles have
been examined. The linear correlation between solvent polarity and fluorescence properties is observed. This study may provide
new directions for the development of fluorescence probes as reporters of microenvironments of organized assemblies. 相似文献
776.
Pyrazolo[3,4-b]pyridines having 4-chloro-5-chloroethyl side chain are synthesized by the reaction of 5-aminopyrazole and cyclic β-formylester
gave aminopyrazolodihydrofuranone intermediate, which on cyclization in phosphorous oxychloride exclusively converted in to
4-chloro-5-chloroethyl pyrazolo[3,4-b]pyridines 4(a-b) in major amount. The side chain with acetic acid, thiourea and aromatic amines are used to form angular ring leads to formation
of tricyclic Furo[2,3-d]pyrazolo[2,3-b]pyridines 5(a-b), pyrazolo[3,4-b]thieno[2,3-d]pyridines 6(a-b) and pyrazolo[3,4-b]pyrrolo[2,3-d]pyridines 7(a-n) respectively. The substituents effect at C4 position on fluorescence properties of pyrazolopyridines has been studied. Moreover the effect of electron donor and halogen
substituents on fluorescence properties of pyrazolopyrrolopyridines 7(a-n) has been investigated along with their fluorescent quantum yield. 相似文献
777.
778.
Deepak Ajwani 《Discrete Applied Mathematics》2010,158(4):240-250
Many applications like pointer analysis and incremental compilation require maintaining a topological ordering of the nodes of a directed acyclic graph (DAG) under dynamic updates. All known algorithms for this problem are either only analyzed for worst-case insertion sequences or only evaluated experimentally on random DAGs. We present the first average-case analysis of incremental topological ordering algorithms. We prove an expected runtime of under insertion of the edges of a complete DAG in a random order for the algorithms of Alpern et al. (1990) [4], Katriel and Bodlaender (2006) [18], and Pearce and Kelly (2006) [23]. 相似文献
779.
K.L. Chopra K. Solomon Harshvardhan S. Rajagopalan L.K. Malhotra 《Solid State Communications》1981,40(4):387-390
Large photocontraction of up to 26% has been observed in some chalcogenide films. Conditions necessary for the occurrence of large photocontraction are indicated to be: ability to form a glass, high bond ionicity, strong electron-phonon coupling, low density of the bulk material, and a large density difference between the bulk and the thin-film. These studies suggest that contraction (densification) is due to the mechanical collapse of the columns resulting from the large internal stresses caused by the interaction of the photogenerated carriers and the charged dangling bonds. 相似文献
780.
This is the second part of two papers addressing the study of the facial structure of the Steiner tree polyhedron. In this paper we identify several classes of facet defining inequalities and relate them to special classes of graphs on which the Steiner tree problem is known to be NP-hard.Corresponding author.The author appreciates partial support from National Science Foundation Grants Nos. DSM-8606188 and ECS 8800281. 相似文献