全文获取类型
收费全文 | 584篇 |
免费 | 14篇 |
国内免费 | 4篇 |
专业分类
化学 | 353篇 |
晶体学 | 1篇 |
力学 | 8篇 |
数学 | 69篇 |
物理学 | 171篇 |
出版年
2023年 | 4篇 |
2020年 | 3篇 |
2019年 | 10篇 |
2018年 | 5篇 |
2017年 | 7篇 |
2016年 | 11篇 |
2015年 | 8篇 |
2014年 | 10篇 |
2013年 | 23篇 |
2012年 | 22篇 |
2011年 | 39篇 |
2010年 | 13篇 |
2009年 | 13篇 |
2008年 | 26篇 |
2007年 | 32篇 |
2006年 | 31篇 |
2005年 | 25篇 |
2004年 | 26篇 |
2003年 | 23篇 |
2002年 | 34篇 |
2001年 | 35篇 |
2000年 | 22篇 |
1999年 | 9篇 |
1998年 | 10篇 |
1997年 | 6篇 |
1996年 | 9篇 |
1995年 | 13篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 12篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1980年 | 12篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1975年 | 3篇 |
1973年 | 6篇 |
1937年 | 2篇 |
1932年 | 2篇 |
1917年 | 2篇 |
1908年 | 2篇 |
排序方式: 共有602条查询结果,搜索用时 593 毫秒
11.
Sanders L Halder M Xiao TL Ding J Armstrong DW Petrich JW 《Photochemistry and photobiology》2005,81(1):183-186
We report the first separation of the enantiomers of hypericin. Their steady-state optical spectra and ultrafast primary photoprocesses are investigated in chiral environments. Within experimental error, there is no difference between the two enantiomers in any of the systems considered. This is consistent with the emerging picture that the rich and extended absorption spectrum of hypericin is not a result of ground-state heterogeneity. It is also consistent with the observation that the spectra and photophysics of hypericin are generally insensitive to environments in which it does not aggregate. 相似文献
12.
Dyer JA Trivedi P Sanders SJ Scrivner NC Sparks DL 《Journal of colloid and interface science》2004,270(1):66-76
Previous studies demonstrated the environmental and economic benefits of treating lead(II)-contaminated water streams with ferrihydrite in multiple equilibrium sorption stages. In this work, multistage ferrihydrite sorption systems were evaluated for their effectiveness in reducing single-solute zinc(II) (Zn(II)) concentrations in contaminated water streams to very low levels. As for lead(II) (Pb(II)), experimental data and modeling results indicate that a multistage sorption system can significantly reduce Zn(II) effluent concentrations for the same total amount of sorbent or, alternatively, dramatically lower total sorbent consumption for the same effluent Zn(II) concentration. Compared to Pb(II), however, Zn(II) removal requires on the order of 10 times more sorbent to achieve the same target effluent concentration for the same pH and number of stages. Model predictions were made using a steady-state, multistage, equilibrium adsorber model that was previously developed for and integrated into OLI Systems' Environmental Simulation Program (ESP). The modified triple-layer model was used to simulate Zn(II) surface-liquid equilibria within the adsorber model. Engineering screening evaluations again indicate that a 2- to 3-stage sorption process can provide significant economic savings when compared to a 1-stage process operating with the same target effluent Zn(II) concentration. Additional equilibrium stages beyond 2 or 3 provide diminishing economic returns. The major economic driver for multiple contacting stages is reduced capital investment and operating costs for sludge handling, dewatering, and disposal. 相似文献
13.
Determination of tamoxifen and metabolites in mouse fetal tissue using nonaqueous capillary electrophoresis 总被引:1,自引:0,他引:1
Tamoxifen (TAM), an antiestrogen, has been approved for use by women at risk for developing hormone-dependent breast cancer. Administration of TAM to pregnant CD-1 mice apparently results in reproductive tract toxicity in female offspring. However, there is little or no data describing potential TAM-induced fetal toxicity to women who may become pregnant while receiving prophylactic TAM treatment. In support of the National Toxicology Program's characterization of reproductive and developmental effects of TAM, the present work describes a capillary electrophoresis (CE)-based analytical technique used for detection of TAM and two major metabolites, N-desmethyltamoxifen (DMT), and 4-hydroxytamoxifen (4-HT) in CD-1 mouse fetal tissue. TAM-derived material was extracted from CD-1 mouse fetuses 2-12 h following TAM administration (100 mg/kg) to dams on gestation day 16. The presence of TAM, DMT, and 4-HT was confirmed in the solvent extracts by nonaqueous CE. The limit of detection of TAM by UV absorption was approximately 675 amol at a signal-to-noise ratio of 2:1. This work demonstrates both transplacental transport of TAM in CD-1 mice and a sensitive analytical technique for detecting low concentrations of TAM and similar compounds in biological tissues. 相似文献
14.
Gillespie KM Sanders CJ O'Shaughnessy P Westmoreland I Thickitt CP Scott P 《The Journal of organic chemistry》2002,67(10):3450-3458
Racemic 2,2'-diamino-6,6'-dimethylbiphenyl is resolved using simulated moving bed chromatography, and the absolute configuration of the enantiomers is confirmed via the X-ray crystal structure of a derivative. The diamine is condensed with a range of aldehydes to give bidentate aldimine proligands L. Molecular structures of the complexes formed between L and Cu(I) fall into two classes; bimetallic double helices ([Cu(2)L(2)](2+)) and monometallic ([CuL](+)). The latter are strikingly more efficient in the aziridination of alkenes than are the former in terms of rate, turnover, and enantioselection. In particular, the imine ligand formed from the diamine and 2,6-dichlorobenzaldehyde gives, in combination with Cu(I) or Cu(II), up to 99% ee in the aziridination of 6-acyl-2,2-dimethylchromene and 88-98% ee for a range of cinnamate esters. Styrenic and other alkenes are converted with lower selectivities (5-54%). The catalytic system shows a linear response in product ee to catalyst ee, and the product ee does not vary significantly during the reaction. UV spectrophotometric investigations indicate that conversion of Cu(I) to Cu(II) is not essential for catalysis but that Cu(II) is probably also a competent system. 相似文献
15.
16.
Arisaka K Auerbach LB Axelrod S Belz J Biery KA Buchholz P Chapman MD Cousins RD Diwan MV Eckhause M Ginkel JF Guss C Hancock AD Heinson AP Highland VL Hoffmann GW Horvath J Irwin GM Joyce D Kaarsberg T Kane JR Kenney CJ Kettell SH Kinnison WW Knibbe P Konigsberg J Kuang Y Lang K Lee DM Margulies J Mathiazhagan C McFarlane WK McKee RJ Melese P Milner EC Molzon WR Ouimette DA Riley PJ Ritchie JL Rubin P Sanders GH Schwartz AJ Sivertz M Slater WE Urheim J Vulcan WF Wagner DL Welsh RE Whyley RJ 《Physical review letters》1993,71(24):3910-3913
17.
Traceability of the measurement of analytical parameters capable of evaluating the performance of methods is an important
concept for the assessment of quality for the routine control, especially for residue monitoring of non-authorized medicinal
substances in food from animal origin. The European Decision no. 657/2002/EC recommends to calculate two statistical limits,
CCα and CCβ, which allow to evaluate the critical concentrations above which the method reliably distinguish and quantify
a substance taking into account the variability of the method and the statistical risk to take a wrong decision. The calculation,
which can be derived from the ISO standard no. 11843 is applied on a routine basis. An example displays a very simple way
for evaluating the performance of an LC-MSMS method which has been validated a few years ago and is qualified onto a Micromass
Quattro LCZ tandem mass spectrometer to monitor and confirm the nitrofuran metabolite residues in food from animal origin.
Community Reference Laboratory for Antimicrobial Veterinary Drug Residue Control in Food from Animal Origin 相似文献
18.
Tiggelaar RM Berenschot JW de Boer JH Sanders RG Gardeniers JG Oosterbroek RE van den Berg A Elwenspoek MC 《Lab on a chip》2005,5(3):326-336
In this paper the fabrication and electrical characterization of a silicon microreactor for high-temperature catalytic gas phase reactions, like Rh-catalyzed catalytic partial oxidation of methane into synthesis gas, is presented. The microreactor, realized with micromachining technologies, contains silicon nitride tubes that are suspended in a flow channel. These tubes contain metal thin films that heat the gas mixture in the channel and sense its temperature. The metal patterns are defined by using the channel geometry as a shadow mask. Furthermore, a new method to obtain Pt thin films with good adhesive properties, also at elevated temperatures, without adhesion metal is implemented in the fabrication process. Based on different experiments, it is concluded that the electrical behaviour at high temperatures of Pt thin films without adhesion layer is better than that of Pt/Ta films. Furthermore, it is found that the temperature coefficient of resistance (TCR) and the resistivity of the thin films are stable for up to tens of hours when the temperature-range during operation of the microreactor is below the so-called "burn-in" temperature. Experiments showed that the presented suspended-tube microreactors with heaters and temperature sensors of Pt thin films can be operated safely and in a stable way at temperatures up to 700 degrees C for over 20 h. This type of microreactor solves the electrical breakdown problem that was previously reported by us in flat-membrane microreactors that were operated at temperatures above 600 degrees C. 相似文献
19.
An interlaboratory study was organised in 1996 in order to determine laboratory testing performance for measuring oxytetracycline (OTC) and 4-epioxytetracycline (4-epiOTC) residues in pig muscle. The organisation was designed according to the 'International Harmonised Protocol for Proficiency Testing of (Chemical) Analytical Laboratories' produced by the Association of Official Analytical Chemists (AOAC International) and the International Union of Pure and Applied Chemistry (IUPAC). Fourteen National Reference Laboratories (NRLs) out of the 15 asked to participate agreed to analyse the samples. These laboratories were from 13 different European Union (EU) Member States and each participant was allowed to use the extraction method of their own choice but had to use liquid chromatography as the analytical technique. Most of the methods used were based on UV detection (simple wavelength or diode-array detection) but some involved fluorimetric detection. The production of incurred samples was prepared on-site. The OTC and 4-epiOTC concentrations present in the samples were determined by taking the mean of the results (excluding outliers) and the deviation of each result from the assigned value was measured. The performance of the laboratories was evaluated by calculating the 'z-scores'. The results were globally satisfactory and showed that all 14 laboratories were capable of determining OTC and 4-epiOTC in pig muscle with satisfactory accuracy. Only two laboratories obtained a questionable result in terms of repeatability. 相似文献
20.
The Curtin-Hammett (C-H) principle and the Winstein-Holness (W-H) equation approximate the product ratio and overall rate constant of reaction for systems involving a starting material which exists in two forms, each of which reacts via first-order kinetics to give a different product. The C-H/W-H approximations are valid when the rates of isomer interconversion are significantly faster than the rates of product formation. The present treatment encompasses non-first-order reactions to product. A numerical predictor-corrector technique is used to show (1) that relative reagent concentration can affect both the product ratio and the observed rates of product formation; (2) that the absolute concentration of reagent and substrate can affect the kinetics; and (3) that factors (1) and (2) above can affect the validity of the C-H/W-H approximations for non-first-order C-H/W-H schemes. 相似文献