全文获取类型
收费全文 | 2410篇 |
免费 | 44篇 |
国内免费 | 12篇 |
专业分类
化学 | 1532篇 |
晶体学 | 16篇 |
力学 | 56篇 |
数学 | 372篇 |
物理学 | 490篇 |
出版年
2021年 | 12篇 |
2020年 | 16篇 |
2019年 | 25篇 |
2017年 | 17篇 |
2016年 | 38篇 |
2015年 | 35篇 |
2014年 | 34篇 |
2013年 | 93篇 |
2012年 | 99篇 |
2011年 | 125篇 |
2010年 | 68篇 |
2009年 | 66篇 |
2008年 | 128篇 |
2007年 | 110篇 |
2006年 | 141篇 |
2005年 | 124篇 |
2004年 | 120篇 |
2003年 | 77篇 |
2002年 | 96篇 |
2001年 | 45篇 |
2000年 | 43篇 |
1999年 | 30篇 |
1998年 | 28篇 |
1997年 | 25篇 |
1996年 | 34篇 |
1995年 | 33篇 |
1994年 | 39篇 |
1993年 | 28篇 |
1992年 | 37篇 |
1991年 | 21篇 |
1990年 | 20篇 |
1989年 | 40篇 |
1988年 | 29篇 |
1987年 | 20篇 |
1986年 | 23篇 |
1985年 | 24篇 |
1984年 | 28篇 |
1983年 | 28篇 |
1982年 | 38篇 |
1981年 | 37篇 |
1980年 | 33篇 |
1979年 | 30篇 |
1978年 | 31篇 |
1977年 | 42篇 |
1976年 | 32篇 |
1975年 | 25篇 |
1974年 | 32篇 |
1973年 | 38篇 |
1972年 | 11篇 |
1966年 | 10篇 |
排序方式: 共有2466条查询结果,搜索用时 239 毫秒
61.
Wiseman G 《Journal of AOAC International》2002,85(3):792-796
Consequential to the implementation of European Commission (EC) Regulation 1139/98, EC Regulation 49/2000, and EC Regulation 50/2000 has been the need to measure accurately the levels of the genetically modified (GM) species Roundup Ready Soya and Bt 176 Maize that are present in food. Analytical methods to detect and quantitate these transgenic species have received much attention particularly with respect to the deminimus threshold of 1% for their presence in materials derived from non-GM identity-preserved (IP) supplies. The relative advantages and limitations of threshold analysis by double-competitive polymerase chain reaction (PCR) and quantitative real-time PCR are discussed in their application to the quantitative analysis of processed foods. Consideration is also given to other factors involved in the analyses that affect the performance of quantitative procedures, and to the many uncertainties involved in the precision of a reported analytical result. 相似文献
62.
Exact rotamer optimization for protein design 总被引:1,自引:0,他引:1
Computational methods play a central role in the rational design of novel proteins. The present work describes a new hybrid exact rotamer optimization (HERO) method that builds on previous dead-end elimination algorithms to yield dramatic performance enhancements. Measured on experimentally validated physical models, these improvements make it possible to perform previously intractable designs of entire protein core, surface, or boundary regions. Computational demonstrations include a full core design of the variable domains of the light and heavy chains of catalytic antibody 48G7 FAB with 74 residues and 10(128) conformations, a full core/boundary design of the beta1 domain of protein G with 25 residues and 10(53) conformations, and a full surface design of the beta1 domain of protein G with 27 residues and 10(60) conformations. In addition, a full sequence design of the beta1 domain of protein G is used to demonstrate the strong dependence of algorithm performance on the exact form of the potential function and the fidelity of the rotamer library. These results emphasize that search algorithm performance for protein design can only be meaningfully evaluated on physical models that have been subjected to experimental scrutiny. The new algorithm greatly facilitates ongoing efforts to engineer increasingly complex protein features. 相似文献
63.
Reaction of 3-nitrobenzaldehyde with diethyl oxalacetate in the presence of piperidine acetate and then ammonium acetate/acetic acid gave a moderate yield of tetraethyl 1,4-dihydro-4-(3-nitrophenyl)-2,3,5,6-pyridinetetracarboxylate. This was oxidized with nitric acid to tetraethyl 4-(3-nitrophenyl)-2,3,5,6-pyridinetetracarhoxylate which was hydrolyzed and decarboxylated to give 4-(3-nitrophenyl)pyridine. 相似文献
64.
Gordon Slade 《Communications in Mathematical Physics》1985,102(3):425-462
We study the loop expansion for the effective potential, defined as the Fenchel transform (convex conjugate) of the pressure in an external field, in theP()2 quantum field theory. For values of the classical fielda for which the classical potentialU
0(a)=P(a)+1/2m
2
a
2 equals its convex hull and has nonvanishing curvature we prove that the 1-PI loop expansion is asymptotic as 0. We also give an example of a double well classical potential for which the 1-PI loop expansion fails to be asymptotic, and find the true asymptotics.This paper is a condensed version of the author's Ph.D. thesis for the Department of Mathematics, University of British Columbia, Vancouver, B.C., Canada V6T 1Y4 相似文献
65.
In the determination of free and combined chlorine, the reaction of permanganate standards with N,N-diethyl-p-phenylenediamine (DPD) exhibits nonlinearity presumably because both the colored semiquinoid product and the colorless quinoid product are both formed. The titrimetric DPD method titrates both of the products while the colorimetric method monitors only the colored semiquinoid products. This results in a nonlinear response for the colorimetric method above 1.0 mg/l. as Cl(2). Under FIA conditions, the nonlinearity of the DPD colorimetric method is eliminated in the 0.1-5.0 mg/l. (as Cl(2)) range and the linear range is expanded to 0.1-8.0 mg/l. as Cl(2). Also, relative standard deviations are improved by 0.5-11% relative to the colorimetric method and 1.5-4.0% relative to the titrimetric method. The FIA method was developed further to sequentially determine both free and combined chlorine, since chloramine was found to have a negligible interference in the free chlorine determination. 相似文献
66.
An improved synthesis of the anti-cancer agent DMU-212 (trans-3,4,5,4′-tetramethoxystilbene) is described. The methodology involves the use of a Pd-oxazoline catalyst as a mediator of a regio-selective (Heck) C-C bond formation reaction. A simple isolation step is then used to obtain the title material. The compound has been further characterised in the solid-state by X-ray diffraction methods. 相似文献
67.
A method was developed for determination of total dietary fiber (TDF) in foods containing resistant maltodextrin (RMD) which includes nondigestible carbohydrates that are not fully recovered as dietary fiber by conventional TDF methods such as AOAC 985.29 or 991.43. Because the average molecular weight (MW) of RMD is 2000 daltons, lower MW soluble dietary fiber components do not precipitate in 78% ethanol; therefore, RMD is not completely quantitated as dietary fiber by current AOAC methods. The accuracy and precision of the method was evaluated through an AOAC collaborative study. Ten laboratories participated and assayed 12 test portions (6 blind duplicates) containing RMD. The 6 test pairs ranged from 1.5 to 100% RMD. The method consisted of the following steps: (1) The insoluble dietary fiber (IDF) and high MW soluble dietary fiber (HMWSDF) were determined by AOAC 985.29. (2) Ion exchange resins were used to remove salts and proteins contained in the AOAC 985.29 filtrates (including ethanol and acetone). (3) The amount of low MWRMD (LMWRMD) in the filtrates were determined by liquid chromatography. (4) The TDF was calculated by summation of the IDF, HMWSDF, and LMWRMD fractions having nondigestible carbohydrates with a degree of polymerization of 3 and higher. Repeatability standard deviations (RSDr) were 1.33-7.46%, calculated by including outliers, and 1.33-6.10%, calculated by not including outliers. Reproducibility standard deviations (RSDR) were 2.48-9.39%, calculated by including outliers, and 1.79-9.39%, calculated by not including outliers. This method is recommended for adoption as Official First Action. 相似文献
68.
Edie M. Rasmussen Peter Willett Terence Wilson Gordon A. Manson George A. Wilson 《Analytica chimica acta》1990
The use of two types of parallel computer hardware for increasing the efficiency of processing in chemical structure data bases is discussed. The distributed array processor can be used for the clustering of 2-D chemical structure data bases by using the Jarvis—Patrick clustering method and for the ranking of output in an experimental system for substructure searching in the 3-D macromolecules in the Protein Data Bank. The Inmos transputer can be used in the construction of PC-based systems for 2-D substructure searching and in the identification of the maximal substructures common to pairs of 3-D molecules. 相似文献
69.
Due to the widespread uses of Ni and Ni alloys, patients undergoing medical treatments can experience inadvertent exposure to the metal, present as a contaminant in fluids for intravenous administration or released from surgical implants and other medical devices. Because of the risk of acute allergic reactions in Ni-sensitive subjects, sources of metal exposure within medical care and its concentrations in biological fluids of potentially exposed patients should be periodically monitored, using reliable analytical procedures, which include strict measures of contamination control. The results of a recent survey on the concentrations of Ni and other potentially toxic metals in human albumin solutions are reported. 相似文献
70.
Bulloch EM Jones MA Parker EJ Osborne AP Stephens E Davies GM Coggins JR Abell C 《Journal of the American Chemical Society》2004,126(32):9912-9913
(6S)-6-Fluoroshikimate has antimicrobial activity. The molecular basis of this effect had not been identified, but there was speculation that (6S)-6-fluoroshikimate is first converted in vivo into 2-fluorochorismate, which then could inhibit 4-amino-4-deoxychorismate synthase (ADCS). 2-Fluorochorismate was prepared from E-fluorophosphoenolpyruvate and erythose-4-phosphate by the sequential reactions of DAHP synthase, dehydroquinate synthase, dehydroquinase, shikimate dehydrogenase, EPSP synthase, and chorismate synthase. Inhibition studies on ADCS showed that it was inhibited rapidly and irreversibly by 2-fluorochorismate. Electrospray mass spectrometry of the inactivated enzyme showed an additional mass of 198 +/- 10 Da. A novel peptide of 1087.6 Da was identified in the HPLC trace for the tryptic digest of 2-fluorochorismate-inactivated ADCS. Sequencing of this peptide by MS/MS showed that the peptide corresponded to residues 272-279 with a modification of 206.1 Da on Lys-274. This observation is particularly exciting in the context of a recent proposal for the catalytic mechanism of ADCS. 相似文献