全文获取类型
收费全文 | 28859篇 |
免费 | 3713篇 |
国内免费 | 3153篇 |
专业分类
化学 | 21074篇 |
晶体学 | 365篇 |
力学 | 1410篇 |
综合类 | 235篇 |
数学 | 3562篇 |
物理学 | 9079篇 |
出版年
2024年 | 53篇 |
2023年 | 444篇 |
2022年 | 783篇 |
2021年 | 802篇 |
2020年 | 910篇 |
2019年 | 933篇 |
2018年 | 792篇 |
2017年 | 755篇 |
2016年 | 1128篇 |
2015年 | 1152篇 |
2014年 | 1358篇 |
2013年 | 1906篇 |
2012年 | 2279篇 |
2011年 | 2527篇 |
2010年 | 1770篇 |
2009年 | 1737篇 |
2008年 | 1942篇 |
2007年 | 1762篇 |
2006年 | 1584篇 |
2005年 | 1393篇 |
2004年 | 1109篇 |
2003年 | 903篇 |
2002年 | 883篇 |
2001年 | 741篇 |
2000年 | 625篇 |
1999年 | 580篇 |
1998年 | 505篇 |
1997年 | 448篇 |
1996年 | 477篇 |
1995年 | 390篇 |
1994年 | 371篇 |
1993年 | 343篇 |
1992年 | 325篇 |
1991年 | 278篇 |
1990年 | 232篇 |
1989年 | 192篇 |
1988年 | 167篇 |
1987年 | 129篇 |
1986年 | 142篇 |
1985年 | 154篇 |
1984年 | 101篇 |
1983年 | 64篇 |
1982年 | 71篇 |
1981年 | 53篇 |
1980年 | 52篇 |
1979年 | 58篇 |
1978年 | 40篇 |
1976年 | 37篇 |
1974年 | 40篇 |
1973年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
Lin LC Tyan YC Shih TS Chang YC Liao PC 《Rapid communications in mass spectrometry : RCM》2004,18(12):1310-1316
An isotope-dilution electrospray ionization tandem mass spectrometry (ESI-MS/MS) method with an on-line sample clean-up device, for the quantitative analysis of human urine for the benzene exposure biomarker S-phenylmercapturic acid (SPMA), was developed and validated. The sample clean-up system was constructed from an autosampler, a reversed-phase C18 trap cartridge, a two-position switching valve, and controlling computer software and hardware. The sample clean-up system was interfaced via 1/20 splitting to the ESI source of a triple-quadrupole mass spectrometer using negative ion mode and multiple reaction monitoring for SPMA and the isotope-labeled internal standard. A strategy was adopted to acquire pooled blank urine matrix and quality control samples spiked with standards. Validated procedures and data on method specificity, detection limits, standard curves, precision and recovery, sample storage stability, and inter-laboratory comparison are presented. The analytical system was fully automated. No tedious manual sample clean-up procedures are required. With the selectivity and the sensitivity provided by ESI-MS/MS detection, the analytical system can be used for high-throughput and accurate determination of SPMA levels in human urine samples, as a biomarker for environmental as well as occupational benzene exposure. 相似文献
952.
Using the ns pulse radolysis, we studied the characteristic absorption spectrum and kinetic decay of cytosine anion radical (Cyt-). Results showed that the characteristic absorption of Cyt- was located at λ=355±5 nm, and decayed following the first order kinetics with τ1/2=265 ns at pH=7.0. The decay became slower and τ1/2 rapidly rised with the increment of pH value, Cyt- protonated at C6 in acidic solution, and the characteristic absorption was located at λ=310±5 nm, and decayed following the second order kinetics: Cyt- protonated at N3 in aqueous solution of pH≥7, and the characteristic absorption was located at λ=295±5 nm, and decayed following the second order kinetics. 相似文献
953.
Hon Man Lee Chi Ying Lu Chih Yuan Chen Wen Ling Chen Hung Ching Lin Pei Ling Chiu Pi Yun Cheng 《Tetrahedron》2004,60(27):5807-5825
A series of new ethylene-bridged bis(imidazolium) halides with various N-substitutions were synthesized. Complexation of these imidazolium halides with Pd(OAc)2 produced new Pd(II) ethylene-bridged bis(carbene) complexes. Crystallographic analyses of some of the new imidazolium salts and Pd(II) complexes were determined. Applications of these seven-member palladacycles in Suzuki and Heck coupling reactions produced comparable catalytic activities to those of six-member analogs. 相似文献
954.
The reaction of the rifle cyclic complex (1) with sodium amalgam in THF resulted in the expected cleavage of the Fe-Fe bond to afford his-sodium salt ( Me2SiSiMe2 ) [η^5-C5H4Fe(CO)2]2 (4). The latter was not isolated and was used directly to react with MeI, PhCH2Cl, CH3C(O)Cl, PhC(O)Cl,Cy3SnCl (Cy= cyclohexyl) or Ph3SnCl to afford corresponding ring-opened derivatives (Me2SiSiMe2) [η^5-C5H4Fe(CO)2]2 [5, R=Me; 6, R=PhCH2; 7, R=CH3C(O); 8, R=PhC(O); 9, R = Cy3Sn or 10, R = Ph3Sn ]. The crystal and molecular structures of 10 were determined by X-ray diffraction analysis. The molecule took the desired ant/ conformation around the Si-Si bond. The length of the Si--Si bond is 0.2343(3)nm, which is essentially identical to that in the cyclic structure of 1[0.2346(4) tun]. This result unambiguously demonstrates that the Si--Si bond in the cyclic structure of 1 is not subject to obvious strain. 相似文献
955.
We present a new microfluidic device utilizing multiwavelength detection for high-throughput capillary electrophoresis (CE). In general, different fluorescent dyes are only excited by light sources with appropriate wavelengths. When excited by an appropriate light source, a fluorescent dye emits specific fluorescence signals of a longer wavelength. This study designs and fabricates plastic micro-CE chips capable of performing multiple-wavelength fluorescence detection by means of multimode optic fiber pairs embedded downstream of the separation channel. For detection purposes, the fluorescence signals are enhanced by positioning microfocusing lens structures at the outlets of the excitation fibers and the inlets of the detection fibers, respectively. The proposed device is capable of detecting multiple samples labeled with different kinds of fluorescent dyes in the same channel in a single run. The experimental results demonstrate that various proteins, including bovine serum albumin and beta-casein, can be successfully injected and detected by coupling two light sources of different wavelengths to the two excitation optic fibers. Furthermore, the proposed device also provides the ability to measure the speed of the samples traveling in the microchannel. The developed multiwavelength micro-CE chip could have significant potential for the analysis of DNA and protein samples. 相似文献
956.
O. P. Charkin N. M. Klimenko T. P. Nguyen D. O. Charkin Y. -S. Wang H. -C. Chang S. H. Lin 《Russian Journal of Inorganic Chemistry》2006,51(10):1613-1622
The electronic and geometric structures, energy stability, normal mode frequencies, and spin density distribution have been calculated by the density functional theory B3LYP method with the Gen = 6-31+G*(Fe) + 6-31G(C,H,N,O), 6-31G*, and 6-311++G** basis sets for the deep fragmentation products of the free hemin ion with successive removal of methyl and vinyl groups in the electronic states with different multiplicities. The computation results are compared with the available experimental data and previous computation results for the fragmentation products of the isolated heme molecule and hemin ion with removal of carboxymethyl groups. The trends in the behavior of these properties are analyzed as a function of multiplicity, external charge, and the number of peripheral substituents at the porphyrin core. 相似文献
957.
Adrenaline was found to inhibit strongly the electrochemiluminescence (ECL) from the Ru(bpy)32+/tripropylamine system when a working Pt electrode was maintained at 1.05 V (versus Ag/AgCl) in pH 8.0 phosphate buffer. On this basis, a flow injection (FI) procedure with inhibited electrochemiluminescence detection has been developed for determination of adrenaline. The method exhibited a good reproducibility, sensitivity, and stability with a detection limit (signal-to-noise ratio = 3) of 7.0×10−9 mol l−1 and dynamic concentration range of 2×10−8 to 1×10−4 mol l−1. The relative standard deviation was 2.2% for 1.0×10−6 mol l−1 adrenaline (n=11). The method was successfully applied to the determination of adrenaline in pharmaceutical samples. Moreover, ECL emission spectra, UV-Vis absorption spectra and cyclic voltammograms of Ru(bpy)32+/tripropylamine/adrenaline were studied. The inhibition mechanism has been proposed as the interaction of electrogenerated Ru(bpy)32+* and the o-benzoquinone derivatives, adrenochrome and adrenalinequinone, at the electrode surface. 相似文献
958.
Tong Lin Michael R. Asam Gary L. Glish 《Journal of the American Society for Mass Spectrometry》1996,7(9):930-937
The major dissociation pathways of the [M-H]+ (loss of NH3 or CH4) and the [M+H]+ (loss of NH3 or CH3) ions from dimethylpyrroles have been determined to occur from isomeric parent ions. For the [M-H]+ ion (formed by loss of a methyl hydrogen), loss of NH3 leads to the formation of the phenylium ion and is preceded by consecutive carbon ring expansions followed by a ring contraction to form protonated aniline. Loss of CH4 occurs after the first carbon ring expansion, which forms protonated picoline. The relative partitioning between the two dissociation paths depends upon the internal energy content of the parent ion; the highest point on the potential energy surface is the second ring expansion step. The [M+H]+ ion reacts through a similar pathway via dihydro analogs of picoline and aniline. The proposed reaction pathways are supported by results of semiempirical molecular orbital calculations. 相似文献
959.
960.