全文获取类型
收费全文 | 2993篇 |
免费 | 13篇 |
国内免费 | 243篇 |
专业分类
化学 | 2775篇 |
晶体学 | 2篇 |
力学 | 8篇 |
数学 | 23篇 |
物理学 | 441篇 |
出版年
2022年 | 2篇 |
2021年 | 1篇 |
2020年 | 1篇 |
2018年 | 2篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2015年 | 2篇 |
2014年 | 7篇 |
2013年 | 1篇 |
2012年 | 262篇 |
2011年 | 326篇 |
2010年 | 43篇 |
2009年 | 129篇 |
2008年 | 434篇 |
2007年 | 305篇 |
2006年 | 351篇 |
2005年 | 353篇 |
2004年 | 248篇 |
2003年 | 188篇 |
2002年 | 145篇 |
2001年 | 90篇 |
2000年 | 112篇 |
1999年 | 38篇 |
1998年 | 14篇 |
1997年 | 8篇 |
1996年 | 37篇 |
1995年 | 20篇 |
1994年 | 23篇 |
1993年 | 47篇 |
1992年 | 24篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有3249条查询结果,搜索用时 15 毫秒
71.
Capillary zone electrophoretic separation and determination of imidazolic antifungal drugs 总被引:1,自引:0,他引:1
Capillary zone electrophoresis (CZE) was adapted to the simultaneous determination of a mixture of three imidazolic antifungal drugs. Separation was achieved by using a fused-silica capillary column with an acetic acid-Tris buffer at pH 5.18 and UV detection at 196 nm. Several electrophoretic parameters were investigated: pH and buffer concentration, applied voltage, temperature and injection conditions. The optimized CZE method was applied to the individual determination of ketoconazole, clotrimazole and econazole in pharmaceutical forms, after a previous single extraction step in methanol, with recoveries of 98.00, 99.96 and 99.58% respectively. The antifungal drugs can be determined at a concentration level lower than 1.0 x 10(-7) M. 相似文献
72.
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
73.
Quezada E Delogu G Viña D Santana L Picciau C Podda G Uriarte E 《Magnetic resonance in chemistry : MRC》2008,46(7):701-705
The synthesis of four pyranocoumarins starting from phloroglucinol and the complete (1)H and (13)C NMR assignment of seven pyranocoumarins has been performed using 1D and 2D NMR techniques including COSY, HMQC and HMBC experiments. 相似文献
74.
75.
Alvarez S Aullón G Fandos R García Fierro JL Ocón P Otero A Rojas S Terreros P 《Dalton transactions (Cambridge, England : 2003)》2005,(5):938-944
The reaction of [Rh(micro-Cl)(COD)]2 with 4,6-dimethyl-pyrimidinethiolate (Me2-pymt) and subsequent substitution of COD by CO yields [Rh(Me2-pymt)(CO)2]. The stacking pattern found in this compound is in contradiction with previously studied comparable square-planar complexes of type d8-[M(chelate)(monodentate)2] in which each ligand has different pi-acidic character. A theoretical study of the intermolecular interactions and conformation of the title compound has been carried out, combining semi-empirical band calculations on the real chains and ab initio(MP2 level) calculations on a model dimer. The combination of electronic and steric effects determines the rotation of the successive monomers and the deviation from linearity of the one-dimensional stacks. Its behaviour in solution is also special, developing a blue colour and forming micelles, when adding water to acetone solutions. 相似文献
76.
Luque Ruiz I García GC Gómez-Nieto MA 《Journal of chemical information and computer sciences》2004,44(6):1903-1911
The increase in the size and complexity of chemical databases necessitates the proposal and development of efficient methods of classification and recovery of information, which supposes proposal of a model of classification of database records and the use of a compatible model of screening for inspection of clusters and recovery of the molecules that satisfy the search criterion. The cycle graphs model based on consideration of all the cycles and chains (and equivalent cycles and chains) present in the molecular structure has been proven appropriate for classification of chemical databases, giving rise to a generation of different classification levels depending on the structural elements (cycles and chains) that are considered. In this paper we propose a screening model, compatible with the cycle graphs model, based on a hierarchy of levels of abstraction. The set of molecules that satisfies a screening model (or selection criterion) diminishes as we advance in the hierarchy of levels of the model, which allows filtering of records and, therefore, an increase in the efficiency of the screening process. In the following work of this series we describe and validate the screening tool developed. 相似文献
77.
Pérez-Ruiz T Martínez-Lozano C Martín J García MD 《Analytical and bioanalytical chemistry》2006,384(1):280-285
A very simple, rapid and highly sensitive flow injection fluorimetric method was developed for the determination of phylloquinone.
The assay was based on the on-line reduction of phylloquinone in dodecylsulfate micelles after irradiation with UV light.
The micellar medium enhanced the fluorescence and stability of the reduced phylloquinone. Under optimum experimental conditions,
the range of application of the technique was between 0.09 and 45.0 μg mL−1 and the detection limit was 0.05 μg mL−1. The sample throughput was 90 injections per hour. The reliability of the method for the routine analysis of phylloquinone
in vegetables and fruits is demonstrated. Extractions were made with hexane, and an automated solid phase extraction system
was used to purify the sample extracts prior to injection into the flow injection manifold. 相似文献
78.
Costa M Areias F Castro M Brea J Loza MI Proença F 《Organic & biomolecular chemistry》2011,9(11):4242-4249
A one-pot procedure was developed for the synthesis of novel 3-[amino(methoxy)methylene]-2-oxo-3,4-dihydro-2H-chromen-4-yl)-3-cyanoacetamides and chromeno[3,4-c]pyridine-1-carbonitriles from the reaction of 2-oxo-2H-chromene-3-carbonitriles and cyanoacetamides. These chromene derivatives were identified as new scaffolds for adenosine receptors and the hits 3a, 3c, 5a, and 5b were found. 相似文献
79.
García F García JM García-Acosta B Martínez-Máñez R Sancenón F Soto J 《Chemical communications (Cambridge, England)》2005,(22):2790-2792
A sensory polymeric material for the colorimetric sensing of cyanide in water has been developed based on the reactivity of this anion with the pyrylium cation. 相似文献
80.
The development of analytical methods for routine simultaneous identification and quantification of carboxylic fatty acids (CFAs) are required in different fields, such as, pharmaceutical cosmetics, food products and formulations of water–microemulsion–oil systems. Determination of CFAs has been developed mainly by gas chromatography (GC). As an alternative to GC, liquid chromatography (LC) has better sensitivity and selectivity. However, most CFAs show no useful absorption in ultraviolet–violet (UV–Vis) region, one of the more used detection technique in high-performance liquid chromatography (HPLC). In order to allow the use of UV–Vis detection, the use of pre-column derivatization has been reported to increase sensitivity and selectivity. Therefore, establishment of a simpler and faster on-line method with complete separation is needed for the screening of large numbers of samples. 2,4-Dinitrophenylhydrazine (2,4-DNPH.), benzoil chloride (BC), and phenylhydrazine (PH) were used for derivatization of different FAs by microwaves radiation (MW). After the on-line derivatization, products were separated and quantified by HPLC. Reactor coil was placed inside of microwaves oven at 450 W. Parameters as flow, amount of reagents, irradiation time, and chromatographic conditions were optimized. The continuous analysis using the MW–HPLC–UV system provided high sensitivity and reduced both the amount of reagent used and the analysis times. This proposed method can be used for the routine analysis of FAs contained in water–microemulsion–oil systems, to quantify the total acid fraction in each phase. 相似文献