全文获取类型
收费全文 | 2752篇 |
免费 | 108篇 |
国内免费 | 9篇 |
专业分类
化学 | 2485篇 |
晶体学 | 7篇 |
力学 | 17篇 |
数学 | 212篇 |
物理学 | 148篇 |
出版年
2023年 | 14篇 |
2022年 | 58篇 |
2021年 | 80篇 |
2020年 | 37篇 |
2019年 | 48篇 |
2018年 | 29篇 |
2017年 | 41篇 |
2016年 | 68篇 |
2015年 | 93篇 |
2014年 | 83篇 |
2013年 | 166篇 |
2012年 | 202篇 |
2011年 | 212篇 |
2010年 | 106篇 |
2009年 | 131篇 |
2008年 | 169篇 |
2007年 | 180篇 |
2006年 | 206篇 |
2005年 | 205篇 |
2004年 | 149篇 |
2003年 | 132篇 |
2002年 | 121篇 |
2001年 | 26篇 |
2000年 | 25篇 |
1999年 | 18篇 |
1998年 | 20篇 |
1997年 | 29篇 |
1996年 | 22篇 |
1995年 | 20篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 16篇 |
1991年 | 15篇 |
1990年 | 14篇 |
1989年 | 10篇 |
1988年 | 3篇 |
1987年 | 12篇 |
1986年 | 12篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1970年 | 2篇 |
1927年 | 2篇 |
排序方式: 共有2869条查询结果,搜索用时 15 毫秒
161.
Liquid chromatography with ultraviolet absorbance detection for the analysis of tetracycline residues in honey 总被引:5,自引:0,他引:5
Viñas P Balsalobre N López-Erroz C Hernández-Córdoba M 《Journal of chromatography. A》2004,1022(1-2):125-129
The separation of tetracyclines (TCs) using reversed-phase liquid chromatography (LC) is proposed. The use of an amide-based stationary phase prevents the interaction of tetracyclines with the residual silanol groups and thus avoids the appearance of tailed peaks. Detection was based on using an UV spectrophotometer and gradient elution with acetonitrile-oxalic acid as mobile phase permitted good separation of all the peaks. Specificity was demonstrated by the retention characteristics, UV spectra and peak purity index. Linearity, precision, recovery and sensitivity were satisfactory. The procedure was applied to the analysis of tetracycline residues (tetracycline, oxytetracycline (OTC), chlortetracycline (CTC), doxycycline (DC), minocycline (MINO) and methacycline (MTC)) in honey of different types. Extraction involved using a mild acidic solvent containing EDTA to release protein-bound or sugar-bound tetracyclines. For the clean-up step, solid phase extraction using phenyl cartridges was applied. Detection limits in the honey using the proposed procedure are between 15 and 30 ng g(-1), depending on the tetracycline. 相似文献
162.
Pérez-Sirvent C Martínez-Sánchez MJ García-Lorenzo M López-García I Hernández-Córdoba M 《Analytical and bioanalytical chemistry》2007,388(2):495-498
Use of small membrane pumps, instead of peristaltic pumps, to introduce sample and reagent solutions into the spectrometer
has several advantages in atomic fluorescence spectrometric determination of mercury. This simple modification results in
a substantial saving in the time required for the measurements and so 90% of reagent solution volumes and 95% of sample solution
volumes are saved, with a consequent decrease in the volume of waste generated. The sampling frequency is almost tripled,
with no deterioration in sensitivity, which is similar to that obtained by use of peristaltic pumps. The relative standard
deviation for ten consecutive measurements of a 1 μg L−1 mercury solution was approximately 2%.
Figure Small membrane pumps for the atomic fluorescene spectro metric determination of mercury 相似文献
163.
Molecularly imprinted capillary electrochromatography for selective determination of thiabendazole in citrus samples 总被引:3,自引:0,他引:3
In this work, the suitability of the combination of molecular imprinting and capillary electrochromatography (MIP-CEC) to be used as powerful tool in environmental or food analysis has been for the first time studied and successfully demonstrated. A molecularly imprinted monolith (MIM) has been synthesised and evaluated as stationary phase for the selective determination of the fungicide thiabendazole (TBZ) in citrus samples by non-aqueous capillary electrochromatography. The influence of the mobile phase composition, the voltage of the power supply and the separation temperature on the recognition of TBZ by the imprinted polymer has been evaluated, and the imprint effect in the MIM was clearly demonstrated. Once optimum recognition conditions were established, other variables affecting mechanical properties and chromatographic performance of MIM were adjusted using computational approach. The high selectivity achieved by the MIP-CEC developed procedure allowed unambiguous detection and quantification of TBZ in citrus samples by direct injection of the crude sample extracts, without any previous clean-up, in less than 6 min. The developed method was properly validated and the calculated detection limits were below the established maximum residue limits (MRLs), clearly demonstrating the suitability of the method to be used for the control of the selected fungicide. 相似文献
164.
Mirela-Fernanda Zaltariov Angelica VladMaria Cazacu Sergiu ShovaMihaela Balan Carmen Racles 《Tetrahedron》2014
The new dicarboxylic acid, 1,3-bis(p-carboxyphenylene-ester-methylene)tetramethyldisiloxane (H2L, 1) was obtained by treating 1,3-bis(chloromethyl)-1,1,3,3-tetramethyldisiloxane with a mixture of terephthalic acid and terephthalic acid sodium salt in a 1:1 ratio. In this approach, besides the desired compound 1 (33 wt % yield), the condensation cyclic dimer 2 (7 wt % yield) and an oligomer 3 (10 wt % yield) resulted. The reaction between dicarboxylic acid H2L, where L is the carboxylate ligand, along with imidazole as co-ligand, and copper hydroxide resulted in the formation of a coordination compound [Cu(HIm)4(H2O)2]L·4.5H2O (4). Single-crystal X-ray crystallography has revealed that the crystal structure of 4 is a self-assembled H-bonded three-dimensional supramolecular structure. FTIR and NMR spectral techniques were also used to characterize the formed structures. Optical and thermal properties of all compounds were studied. The stability of the supramolecular structure in solution (methanol) and with temperature was studied using ATR-FTIR. The ability of the macrocycle 2 to bind potassium cations in solution was investigated by UV–vis spectrophotometry. 相似文献
165.
An Efficient RuII–RhIII–RuII Polypyridyl Photocatalyst for Visible‐Light‐Driven Hydrogen Production in Aqueous Solution 下载免费PDF全文
Dr. Thibaut Stoll Dr. Marcello Gennari Dr. Jérôme Fortage Dr. Carmen E. Castillo Dr. Mateusz Rebarz Dr. Michel Sliwa Dr. Olivier Poizat Dr. Fabrice Odobel Dr. Alain Deronzier Dr. Marie‐Noëlle Collomb 《Angewandte Chemie (International ed. in English)》2014,53(6):1654-1658
The development of multicomponent molecular systems for the photocatalytic reduction of water to hydrogen has experienced considerable growth since the end of the 1970s. Recently, with the aim of improving the efficiency of the catalysis, single‐component photocatalysts have been developed in which the photosensitizer is chemically coupled to the hydrogen‐evolving catalyst in the same molecule through a bridging ligand. Until now, none of these photocatalysts has operated efficiently in pure aqueous solution: a highly desirable medium for energy‐conversion applications. Herein, we introduce a new ruthenium–rhodium polypyridyl complex as the first efficient homogeneous photocatalyst for H2 production in water with turnover numbers of several hundred. This study also demonstrates unambiguously that the catalytic performance of such systems linked through a nonconjugated bridge is significantly improved as compared to that of a mixture of the separate components. 相似文献
166.
167.
168.
Carmen FestaSimona De Marino Valentina SepeMaria Valeria D’Auria Giuseppe BifulcoRosa Andrés Maria Carmen TerencioMiguel Payá Cécile DebitusAngela Zampella 《Tetrahedron》2011,67(40):7780-7786
Two new cyclopeptides, perthamides E and F were isolated from the polar extracts of the sponge Theonella swinhoei. The new structures, featuring an unprecedented β-amino acid unit (AHMOA), were determined by interpretation of NMR and MS data. The absolute configuration of the AHMOA residue was proposed on the basis of quantum chemical calculation of NMR chemical shifts. Perthamides were proved to inhibit TNF-α and IL-8 release in primary human keratinocytes cells and therefore could represent potentially leads for the treatment of psoriasis. 相似文献
169.
Carmen L. Huffman Melissa Lee Williams D. Michelle Benoist Richard E. Overstreet Emily E. Jellen‐McCullough 《Rapid communications in mass spectrometry : RCM》2011,25(16):2299-2306
In collision‐induced dissociation mass spectrometry experiments, the collision energy required for dissociation linearly depends on the degrees of freedom in the precursor ion. The magnitude of the slope of this relationship previously has been shown to qualitatively correlate to the relative binding strength of a noncovalently bound, monovalent complex. The goal of the work presented here is to determine if a similar methodology can be applied for assessing relative binding strengths in multivalent species. We have tested the method on complexes formed from 18‐crown‐6 and a variety of protonated, primary alkylamines, [CnH2n+1NH3]+ (n = 9, 12, 14, 16 and 18) and alkyldiamines, [H3NCnH2nNH3]2+ (n = 3, 5, 6, 9 and 12), and compared our results with dissociation energies calculated using density functional theory at the B3LYP/6‐31G* level. We found that the method correctly assessed the stronger crown ether/headgroup interaction in the two divalent species (1:1 and 2:1 complexes formed from the diaminoalkanes) compared with the weaker interaction in the monovalent species (1:1 complexes formed from mono‐aminoalkanes). However, the experimental method could not distinguish between the binding strengths of the two divalent complexes, perhaps because their calculated dissociation energies were quite similar. Our preliminary results suggest that this method could potentially be used for a quick and simple analysis of binding strengths in multivalent species if the binding strengths of the species are significantly different from one another. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
170.
Noe Oña Antonio Romero Carmen Assiego Claudia Bello Pierre Vogel M. Soledad Pino-González 《Tetrahedron: Asymmetry》2010,21(17):2092-2099
An approach to the synthesis of polyhydroxyazepane derivatives from sugar-based epoxyamides or epoxyalcohols, in which the total regioselective epoxide opening by nitrogen nucleophiles is the key step, is described. Thus, novel polyhydroxyazepane carboxamides and aminomethyl polyhydroxyazepanes, with potential pharmacological interest, are synthesized from diacetone d-mannose. Configurational assignments of the obtained products were determined. 相似文献