全文获取类型
收费全文 | 1932篇 |
免费 | 92篇 |
国内免费 | 2篇 |
专业分类
化学 | 1323篇 |
晶体学 | 13篇 |
力学 | 33篇 |
数学 | 324篇 |
物理学 | 333篇 |
出版年
2023年 | 12篇 |
2022年 | 9篇 |
2021年 | 26篇 |
2020年 | 35篇 |
2019年 | 30篇 |
2018年 | 53篇 |
2017年 | 45篇 |
2016年 | 59篇 |
2015年 | 56篇 |
2014年 | 82篇 |
2013年 | 115篇 |
2012年 | 133篇 |
2011年 | 144篇 |
2010年 | 84篇 |
2009年 | 68篇 |
2008年 | 109篇 |
2007年 | 123篇 |
2006年 | 84篇 |
2005年 | 105篇 |
2004年 | 80篇 |
2003年 | 76篇 |
2002年 | 65篇 |
2001年 | 56篇 |
2000年 | 55篇 |
1999年 | 37篇 |
1998年 | 21篇 |
1997年 | 13篇 |
1996年 | 23篇 |
1995年 | 19篇 |
1994年 | 17篇 |
1993年 | 26篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 16篇 |
1989年 | 10篇 |
1988年 | 17篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 20篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1935年 | 2篇 |
排序方式: 共有2026条查询结果,搜索用时 31 毫秒
31.
Determination of pesticides in waters by capillary gas chromatography with atomic emission detection 总被引:5,自引:0,他引:5
Viñas P Campillo N López-García I Aguinaga N Hernández-Córdoba M 《Journal of chromatography. A》2002,978(1-2):249-256
Capillary gas chromatography with atomic emission detection (GC-AED) was successfully used to separate and quantify 14 pesticides (organochlorines, organophosphorus compounds and pyrethrins) in water samples after liquid-liquid extraction with ethyl acetate. Monitoring the emission lines for elements such as chlorine, bromine and sulfur ensures nearly specific chromatograms for these elements, and markedly increases selectivity. Calibration curves were obtained by plotting peak area versus concentration and the correlation coefficients relating to linearity were at least 0.999. Each chromatographic separation takes 21 min and, since two injections are necessary, it requires approximately 1 h to analyze one sample. The method shows a precision of 4.3-8.2% (RSD), depending on the compound. The application of liquid-liquid extraction with ethyl acetate led to recoveries from spiked samples ranging from 76 to 113%. The sensitivity and linearity for the elements chlorine, sulfur, nitrogen and phosphorus were checked under the optimized conditions at their customary emission wavelength. The sensitivity and linearity for these compounds decreased in the order (atom and emission wavelength) Cl (479 nm)>S (181 nm)>P (178 nm)>N (174 nm). A study of the GC-AED system's response to chlorine concentration in eight pesticide molecules was performed and a linear relationship was found with a correlation coefficient of 0.987. 相似文献
32.
Patrizia Calaminici Victor D. Domínguez-Soria Gerald Geudtner Elizabeth Hernández-Marín Andreas M. Köster 《Theoretical chemistry accounts》2006,115(4):221-226
The parallelization of the three-center electron repulsion integrals arising from the variational fitting of the Coulomb potential
is presented. A scheme for dynamical load balancing of the corresponding loop structure is discussed. The implementation in
the density functional theory program deMon using the message passing interface is described. The efficiency of the parallelization
is analyzed by selected benchmark calculations 相似文献
33.
M. J. Medina Hernández S. Sagrado Vives M. C. García Alvarez-Coque 《Mikrochimica acta》1992,108(3-6):293-298
Automation by flow injection analysis with Spectrophotometric detection of the determination of total amino acids and proteins witho-phthalaldehyde is not straightforward. The use of spectrophotometry, instead of spectrofluorimetry, and of N-acetyl-L-cysteine, instead of the conventional mercaptoethanol is advantageous because of the lower variability of absorptivities with respect to fluorescence yields, and the larger stability of the derivatives. Under adequate working conditions and with leucine as reference, the procedure can be used for the evaluation of total amino acids. A similar procedure is proposed for the analysis of proteins in a sample. Limits of detection are 1 × 10–5
M for amino acids, and 1 × 10–6
M for proteins, respectively. 相似文献
34.
When excesses of ammonium thiocyanate and tetrabutylammonium chloride are added to a cobalt(II) solution, a water-insoluble ion-pair is formed; this compound is soluble in chloroform, isoamyl alcohol, and other solvents. The variables which affect the extractions by these two solvents are studied in order to obtain the optimal conditions and two alternative extraction procedures are proposed. Thus, the separation of cobalt as a previous step for its spectrophotometric determination is studied. 相似文献
35.
José I. Concepción Cosme G. Francisco Rosendo Hernández José A. Salazar Ernesto Suárez 《Tetrahedron letters》1984,25(18):1953-1956
Photolysis of several hydroxy compounds in presence of iodosobenzene diacetate and iodine leads to alkoxy radical derivatives which undergo intramolecular hydrogen abstraction to produce cyclic ethers in good yield. 相似文献
36.
Hernández de la Peña L Gulam Razul MS Kusalik PG 《The Journal of chemical physics》2005,123(14):144506
Quantum and classical simulations are carried out on ice Ih over a range of temperatures utilizing the TIP4P water model. The rigid-body centroid molecular dynamics method employed allows for the investigation of equilibrium and dynamical properties of the quantum system. The impact of quantization on the local structure, as measured by the radial and spatial distribution functions, as well as the energy is presented. The effects of quantization on the lattice vibrations, associated with the molecular translations and librations, are also reported. Comparison of quantum and classical simulation results indicates that shifts in the average potential energy are equivalent to rising the temperature about 80 K and are therefore non-negligible. The energy shifts due to quantization and the quantum mechanical uncertainties observed in ice are smaller than the values previously reported for liquid water. Additionally, we carry out a comparative study of melting in our classical and quantum simulations and show that there are significant differences between classical and quantum ice. 相似文献
37.
38.
39.
40.
Infant formula and powdered milk samples were suspended in a medium containing 20% ethanol, 0.65% nitric acid, 6% hydrogen peroxide and 0.1% ammonium dihydrogenphosphate and directly introduced in the electrothermal atomizer. Liquid milk samples were diluted with this suspension solution. The build-up of carbonaceous residues inside the atomizer was avoided by using a combination of hydrogen peroxide and nitric acid. Fast programs were used to simplify the heating cycles. Calibration was carried out using aqueous standards and the detection limits for copper and zinc were found to be 70 and 15 pg, respectively. The metal contents of several milk samples obtained by direct introduction of the samples agree with those obtained by means of a conventional procedure based on the total dissolution of the samples. The reliability of the methods was also confirmed by analyzing three certified reference materials. The free and protein binding fractions were measured by ultrafiltration membrane permeability of the milk samples. 相似文献