全文获取类型
收费全文 | 8022篇 |
免费 | 259篇 |
国内免费 | 62篇 |
专业分类
化学 | 5346篇 |
晶体学 | 41篇 |
力学 | 277篇 |
数学 | 1596篇 |
物理学 | 1083篇 |
出版年
2023年 | 51篇 |
2022年 | 170篇 |
2021年 | 189篇 |
2020年 | 155篇 |
2019年 | 162篇 |
2018年 | 139篇 |
2017年 | 114篇 |
2016年 | 276篇 |
2015年 | 234篇 |
2014年 | 240篇 |
2013年 | 444篇 |
2012年 | 530篇 |
2011年 | 594篇 |
2010年 | 361篇 |
2009年 | 352篇 |
2008年 | 517篇 |
2007年 | 486篇 |
2006年 | 486篇 |
2005年 | 411篇 |
2004年 | 352篇 |
2003年 | 281篇 |
2002年 | 292篇 |
2001年 | 99篇 |
2000年 | 73篇 |
1999年 | 68篇 |
1998年 | 79篇 |
1997年 | 87篇 |
1996年 | 98篇 |
1995年 | 58篇 |
1994年 | 79篇 |
1993年 | 55篇 |
1992年 | 56篇 |
1991年 | 43篇 |
1990年 | 53篇 |
1989年 | 48篇 |
1988年 | 39篇 |
1987年 | 44篇 |
1986年 | 30篇 |
1985年 | 44篇 |
1984年 | 63篇 |
1983年 | 40篇 |
1982年 | 45篇 |
1981年 | 31篇 |
1980年 | 40篇 |
1979年 | 29篇 |
1978年 | 30篇 |
1977年 | 27篇 |
1976年 | 14篇 |
1974年 | 15篇 |
1973年 | 14篇 |
排序方式: 共有8343条查询结果,搜索用时 20 毫秒
851.
Weiss NG Hayes MA Garcia AA Ansari RR 《Langmuir : the ACS journal of surfaces and colloids》2011,27(1):494-498
A novel approach to molecular separations is investigated using a technique termed droplet-based isoelectric focusing. Drops are manipulated discretely on a superhydrophobic surface, subjected to low voltages for isoelectric focusing, and split-resulting in a preparative separation. A universal indicator dye demonstrates the generation of stable, reversible pH gradients (3-10) in ampholyte buffers, and these gradients lead to protein focusing within the drop length. Focusing was visually characterized, spectroscopically verified, and assessed quantitatively by noninvasive light scattering measurements. It was found to correlate with a quantitative model based on 1D steady-state theory. This work illustrates that molecular separations can be deployed within a single open drop, and the differential fractions can be separated into new discrete liquid elements. 相似文献
852.
The interaction of bovine serum albumin (BSA) with the ionic surfactants sodium dodecylsulfate (SDS, anionic), cetyltrimethylammonium chloride (CTAC, cationic) and N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS, zwitterionic) was studied by electron paramagnetic resonance (EPR) spectroscopy of spin label covalently bound to the single free thiol group of the protein. EPR spectra simulation allows to monitor the protein dynamics at the labeling site and to estimate the changes in standard Gibbs free energy, enthalpy and entropy for transferring the nitroxide side chain from the more motionally restricted to the less restricted component. Whereas SDS and CTAC showed similar increases in the dynamics of the protein backbone for all measured concentrations, HPS presented a smaller effect at concentrations above 1.5mM. At 10mM of surfactants and 0.15 mM BSA, the standard Gibbs free energy change was consistent with protein backbone conformations more expanded and exposed to the solvent as compared to the native protein, but with a less pronounced effect for HPS. In the presence of the surfactants, the enthalpy change, related to the energy required to dissociate the nitroxide side chain from the protein, was greater, suggesting a lower water activity. The nitroxide side chain also detected a higher viscosity environment in the vicinity of the paramagnetic probe induced by the addition of the surfactants. The results suggest that the surfactant-BSA interaction, at higher surfactant concentration, is affected by the affinities of the surfactant to its own micelles and micelle-like aggregates. Complementary DLS data suggests that the temperature induced changes monitored by the nitroxide probe reflects local changes in the vicinity of the single thiol group of Cys-34 BSA residue. 相似文献
853.
Esposito Serena Setaro Antonio Maddalena Pasqualino Aronne Antonio Pernice Pasquale Laracca Marco 《Journal of Sol-Gel Science and Technology》2011,60(3):388-394
A modified sol–gel method was used to prepare cobalt doped silica thin film with a cobalt content of 10, 20 and 30 mol% (10Co, 20Co and 30Co). The prepared films were annealed at different temperatures in the range 400–1,000 °C, and their structural evolution examined.
The mixed valence cobalt oxide, Co3O4, crystallizes only in the sample with the higher cobalt content, while cobalt silicate is the only crystalline phase detected
in the sample 10Co and 20Co. Both the cobalt content and the temperature of heat treatment resulted to affect the nature of cobalt species dispersed
in the silica matrix. The 30Co was selected for further investigations by FTIR spectroscopy to follow the structural evolution of 30Co film as function of the temperature and UV–Vis to get information on the cobalt valence state. The optical gas-sensing properties
of 30Co films, containing Co3O4 as the major cobalt phase, were studied through the measuring of the film transmittance in dry air and in presence of dry
air containing variable concentrations of polluting gases, CO and NO2. The 30Co samples resulted to be highly sensitive to CO at room temperature. An explanation for the CO sensing characteristics, at
low temperature, was proposed by referring to the physisorption-related mechanics of CO. 相似文献
854.
855.
Vega ED Lomsadze K Chankvetadze L Salgado A Scriba GK Calvo E López JA Crego AL Marina ML Chankvetadze B 《Electrophoresis》2011,32(19):2640-2647
The enantiomer migration order (EMO) of ephedrine was investigated in the presence of various CDs in CE. The molecular mechanisms of chiral recognition were followed for the ephedrine complexes with native α- and β-CD and heptakis(2,3-di-O-acetyl-6-O-sulfo)-β-CD (HDAS-β-CD) by CE, NMR spectroscopy and high-resolution MS. Minor structural differences were observed between the complexes of ephedrine with α- and β-CD although the migration order of enantiomers was opposite when these two CDs were applied as chiral selectors in CE. The EMO was also opposite between β-CD and HDAS-β-CD. Significant structural differences were observed between ephedrine complexes with the native CDs and HDAS-β-CD. The latter CD was advantageous as chiral CE selector not only due to its opposite electrophoretic mobility compared with that of the cationic chiral analyte, but also primarily due to its enhanced chiral recognition ability towards the enantiomers of ephedrine. 相似文献
856.
Domínguez-Vega E Crego AL Lomsadze K Chankvetadze B Marina ML 《Electrophoresis》2011,32(19):2700-2707
A novel polysaccharide-based chiral stationary phase (CSP), cellulose tris(3-chloro-4-methylphenylcarbamate), also known as Sepapak-2 or Lux Cellulose-2, has been evaluated for the enantiomeric separation of FMOC derivatives of amino acids. After mobile-phase optimization in nano liquid chromatography (nano-LC) the column enabled the enantiomeric separation of 19 out of 23 amino acids tested, indicating the high chiral recognition power of this new CSP. Subsequently, a comparison of the driving force employed (pressure or voltage) was carried out comparing nano-LC and CEC under the same conditions. Better peak efficiencies and resolution were observed by using CEC experiments, which enabled the chiral discrimination of 20 out of 23 amino acids tested. Finally, in order to show the potential of this new CSP, the determination of the content and the enantiomeric purity of the non-protein amino acid citrulline in food supplements was performed. For that purpose, the method was optimized, evaluated and applied to different commercial samples. 相似文献
857.
Villar-Pulido M Gilbert-López B García-Reyes JF Martos NR Molina-Díaz A 《Talanta》2011,85(3):1419-1427
A fast liquid chromatography time-of-flight mass spectrometry (LC-TOFMS) method has been developed for simultaneous quantitative multiclass determination of residues of selected antibiotics and other veterinary drugs (benzalkonium chloride, ethoxyquin, leucomalachite green (LMG), malachite green (MG), mebendazole, sulfadiazine, sulfadimethoxine, sulfamethazine, sulfamethizole, sulfanilamide, sulfapyridine, sulfathiazole and trimethoprim) in shrimps. Different sample treatment methodologies were tested for the extraction of the targeted species based on either liquid partitioning with different solvents, solid-phase extraction or and matrix solid-phase dispersion. The final selected extraction method consisted of solid-liquid extraction protocol using acetonitrile as solvent followed by a clean-up step with primary secondary amine (QuEChERS). Recovery rates for the extraction of the selected multiclass chemicals were in the range 58-133%. Subsequent identification, confirmation and quantitation were carried out by LC-TOFMS analysis using a reverse-phase C18 column with 1.8 μm particle size. The confirmation of the target species was based on accurate mass measurements of the protonated molecules ([M+H]+) and their fragment ions, obtaining routine accuracy errors lower than 2 ppm in most cases. The optimized LC-TOFMS method displayed excellent sensitivity for the studied analytes, with limits of detection (LODs) in the range 0.06-7 μg kg−1. Finally, the proposed method was successfully applied to the analysis of 12 shrimp samples collected from different supermarkets, showing the potential applicability of the method for ultratrace detection of these chemicals in such complex matrix. 相似文献
858.
A new approach for target quantitative analysis for comprehensive two-dimensional gas chromatography (GC × GC), interval Multi-way Partial Least Square (iNPLS) is presented and evaluated in this paper. In iNPLS, the two-dimensional chromatogram is split in small sections; each of these pieces is treated as an independent new chromatogram. Separated conventional NPLS calibration models for the concentration of the target analyte are built for each of the pieces of the whole chromatogram, and the best model is selected for quantitative analysis. An algorithm for iNPLS running on MatLab platform was written, preliminarily evaluated with using solutions of model compounds with different chemical properties and subsequently applied to quantify some allergens in perfume samples. The results were found to be adequate, and good precision and accuracy was obtained even for poorly resolved peaks. 相似文献
859.
Resveratrol (3,5,4′-trihydroxystilbene) is a phytoalexin that plays a central role in the human diet because of its antioxidant, anticarcinogenic and antimutagenic properties. This paper shows the development of a multicommuted optosensing device for the determination of resveratrol in beer. The method is based on the measurement of the fluorescence (277/382 nm, λex/λem) of the photoproduct on-line generated by UV-irradiation of resveratrol. The fluorescent photoproduct is monitored once it is retained on a solid support (Sephadex QAE A-25) in the detection area, which improves both sensitivity and selectivity. The sample was delipidated with toluene and cyclohexane and resveratrol was extracted by solid-phase extraction (SPE) on C18 cartridges, using methanol as eluent. This pre-treatment allowed recovering about an 82% resveratrol and removing its 3-O-β-d-glycoside (piceid) and other interfering substances present in beer. The method provides a detection limit (DL) of 1.0 ng mL−1 and a linear dynamic range (LDR) of 3.3-100 ng mL−1. It was satisfactorily applied to the determination of resveratrol in top- and bottom-fermented beers by standard addition calibration. Resveratrol concentrations in the analysed samples varied from 4.1 to 14.1 ng mL−1. This is the first proposed spectroscopic method for determination of resveratrol in beer. 相似文献
860.
Alberto Fraile M. Rosario MartínJosé Luis García Ruano Juan Antonio DíazEsther Arranz 《Tetrahedron》2011,67(1):100-105
The efficient synthesis of novel 3-heteroaryl N-1-functionalized indazoles, via palladium cross-coupling reactions of ethyl (3-iodo-1H-indazol-1-yl)acetate with 2- and 3-pyrrolylboronic acids, 2-, 4- and 5-thiazolylstannanes, and other heteroarylmetallated derivatives are reported. 相似文献