全文获取类型
收费全文 | 24474篇 |
免费 | 577篇 |
国内免费 | 141篇 |
专业分类
化学 | 16718篇 |
晶体学 | 123篇 |
力学 | 575篇 |
数学 | 4334篇 |
物理学 | 3442篇 |
出版年
2022年 | 177篇 |
2021年 | 261篇 |
2020年 | 302篇 |
2019年 | 317篇 |
2018年 | 233篇 |
2017年 | 195篇 |
2016年 | 481篇 |
2015年 | 453篇 |
2014年 | 490篇 |
2013年 | 1300篇 |
2012年 | 1137篇 |
2011年 | 1429篇 |
2010年 | 726篇 |
2009年 | 707篇 |
2008年 | 1256篇 |
2007年 | 1279篇 |
2006年 | 1251篇 |
2005年 | 1210篇 |
2004年 | 1053篇 |
2003年 | 912篇 |
2002年 | 818篇 |
2001年 | 347篇 |
2000年 | 278篇 |
1999年 | 248篇 |
1998年 | 276篇 |
1997年 | 307篇 |
1996年 | 358篇 |
1995年 | 239篇 |
1994年 | 279篇 |
1993年 | 262篇 |
1992年 | 237篇 |
1991年 | 240篇 |
1990年 | 191篇 |
1989年 | 231篇 |
1988年 | 239篇 |
1987年 | 202篇 |
1986年 | 195篇 |
1985年 | 291篇 |
1984年 | 338篇 |
1983年 | 218篇 |
1982年 | 366篇 |
1981年 | 336篇 |
1980年 | 316篇 |
1979年 | 319篇 |
1978年 | 324篇 |
1977年 | 292篇 |
1976年 | 280篇 |
1975年 | 250篇 |
1974年 | 248篇 |
1973年 | 234篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Robert Koeber R. Niessner Josep Maria Bayona 《Analytical and bioanalytical chemistry》1997,359(3):267-273
The performance of two liquid chromatography-mass spectrometry (LC/MS) interfacing techniques, thermospray (TSP) and atmospheric
pressure chemical ionization (APCI), for the analysis of benzo[a]pyrene (BaP) metabolites (hydroxy, epoxy and quinone derivatives) was compared. Interface and detection parameters such as
source temperature, eluent composition or flow rate were optimized using negative ion mode. In TSP, the main ions are mostly
[M]-, [M−H2O]- or [M+CH3COO]-, whereas APCI gives mainly the [M]- and [M−H]- ions. Quantification was carried out by flow injection. Calibration graphs were linear in the range of 10 ng to 1000 ng in
TSP and 0.1 ng to 10 ng in APCI. Detection limits were in the range of 1 ng to 20 ng in TSP and 0.002 ng to 0.2 ng in APCI.
The presence of BaP-1,6-dione, BaP-3,6-dione, and BaP-6,12-dione was confirmed in environmental samples of air particulate
matter.
Received: 6 January 1997/Accepted: 18 April 1997 相似文献
972.
Victoria Hunnam David J. Harvey David A. Priestman Robert H. Bateman Robert S. Bordoli Richard Tyldesley 《Journal of the American Society for Mass Spectrometry》2001,12(11):1220-1225
This paper reports the use of a quadrupole time-of-flight (Q-TOF) mass spectrometer fitted with a matrix-assisted laser desorption/ionization (MALDI) ion source for the analysis of neutral and acidic glycosphingolipids. All compounds gave strong [M + Na]+ ions with 2,5-dihydroxybenzoic acid as the matrix, with no loss of sensitivity with increasing mass as was observed from the corresponding ions produced by electrospray. Neutral glycosphingolipids showed negligible in-source fragmentation but sialylated compounds fragmented by loss of sialic acid. However, these losses were not accompanied by unfocused post-source-decay ions as observed with MALDI-reflectron-TOF instruments. The MS/MS spectra were almost identical to those obtained by electrospray. Fragmentation of all compounds was mainly by glycosidic cleavage to give ions, both with and without the ceramide moiety, which defined the carbohydrate chain sequence. Weak ions which defined the sphingosine chain length and abundant ions, produced by loss of the acyl chain, were present when this chain contained a 2-hydroxy group. The technique was applied to the identification of ceramide-trihexosides present in tissues from mice genetically modified to model one of the glycolipid storage diseases (Fabry disease). 相似文献
973.
Synthesis of variously 5-substituted-4-carbaldehyde-1,2,3-triazole derivatives 2 was accomplished by reacting sodium azide with ,β-acetylenic aldehydes 1 in DMSO at room temperature. Therefore, the reaction remains basic avoiding the generation of the hazardous explosive HN3, resulting in a safe process. This mild and general reaction was instantaneous and was essentially quantitative. 相似文献
974.
Manfred Bortz Robert Bau Jörg J. Schneider Sax A. Mason 《Journal of Cluster Science》2001,12(1):285-291
The structure of H3Co2[C5H2(t-Bu)3]2 has been analyzed by low-temperature single-crystal neutron diffraction techniques, and shown to consist of two CoCp moieties with three hydride ligands bridging the central Co–Co bond. Despite a fairly extensive twinning problem, the structure could be solved and successfully refined to a final R factor of 9.2% for 2024 reflections. Average molecular parameters in the H3Co2 core of the molecule are as follows: Co–Co=2.275(21) Å, Co–H=1.637(16) Å, HH=2.050(20) Å, Co–H–Co=88.0(9)°, H–Co–H=77.0(7)°. Also included in this paper is a discussion on the molecular dimensions of symmetric hydride-bridged dinuclear systems (M(-H)nM, n=1, 2, 3, 4) that have been studied to date by neutron diffraction. 相似文献
975.
Dr. Govind V. Kaigala Robert D. Lovchik Dr. Emmanuel Delamarche 《Angewandte Chemie (International ed. in English)》2012,51(45):11224-11240
Local interactions between (bio)chemicals and biological interfaces play an important role in fields ranging from surface patterning to cell toxicology. These interactions can be studied using microfluidic systems that operate in the “open space”, that is, without the need for the sealed channels and chambers commonly used in microfluidics. This emerging class of techniques localizes chemical reactions on biological interfaces or specimens without imposing significant “constraints” on samples, such as encapsulation, pre‐processing steps, or the need for scaffolds. They therefore provide new opportunities for handling, analyzing, and interacting with biological samples. The motivation for performing localized chemistry is discussed, as are the requirements imposed on localization techniques. Three classes of microfluidic systems operating in the open space, based on microelectrochemistry, multiphase transport, and hydrodynamic flow confinement of liquids are presented. 相似文献
976.
Vivek B. Shah Gregory S. Orf Sean Reisch Lucas B. Harrington Mindy Prado Robert E. Blankenship Pratim Biswas 《Analytical and bioanalytical chemistry》2012,404(8):2329-2338
Photosynthetic organisms have light-harvesting complexes that absorb and transfer energy efficiently to reaction centers. Light-harvesting complexes (LHCs) have received increased attention in order to understand the natural photosynthetic process and also to utilize their unique properties in fabricating efficient artificial and bio-hybrid devices to capture solar energy. In this work, LHCs with different architectures, sizes, and absorption spectra, such as chlorosomes, Fenna–Matthews–Olson (FMO) protein, LH2 complex, and phycobilisome have been characterized by an electrospray-scanning mobility particle-sizer system (ES-SMPS). The size measured by ES-SMPS for FMO, chlorosomes, LH2, and phycobilisome were 6.4, 23.3, 9.5, and 33.4?nm, respectively. These size measurements were compared with values measured by dynamic light scattering and those reported in the literature. These complexes were deposited onto a transparent substrate by electrospray deposition. Absorption and fluorescence spectra of the deposited LHCs were measured. It was observed that the LHCs have light absorption and fluorescence spectra similar to that in solution, demonstrating the viability of the process. 相似文献
977.
978.
Katharine G. Lunny Yanice Benitez Yishai Albeck Prof. Daniel Strasser Prof. John F. Stanton Prof. Robert E. Continetti 《Angewandte Chemie (International ed. in English)》2018,57(19):5394-5397
In an effort to characterize the electronic states of ethylenedione, OCCO, photoelectron‐photofragment coincidence (PPC) spectroscopy was applied to measure anions at m/z 56 and 57 using a pulsed discharge of glyoxal vapor and N2O. PPC measurements at a photon energy of 3.20 eV yield photoelectron spectra in coincidence with either neutral photofragments or stable neutral products. The measurements showed that primarily stable neutral products were formed, with photoelectron spectra consistent with the oxyallyl diradical, C3H4O, and acetone enolate radical, C3H5O. The spectra were also found to have features nearly identical to those reported for OCCO and HOCCO by Sanov and co‐workers. The stability of the neutral products, as well as an examination of spectra reported for the oxyallyl anion and acetone enolate show that the previous assignments of OCCO and HOCCO are in error, and are instead attributed here to the oxyallyl diradical, C3H4O, and the acetone enolate radical, C3H5O. 相似文献
979.
Jamie Hunter Scott Rice Robert Lowe Christopher M. Pask Stuart Warriner Visuvanathar Sridharan 《Tetrahedron letters》2017,58(46):4400-4402
2′-Hydroxyacetophenone was alkylated with a range of substituted benzyl and heteroaryl alcohols to afford the corresponding C-alkylated products in good yields under microwave irradiation. The C-alkylated products were reacted with bromoacetonitrile to afford 2-amino-3-benzyl 1,4-naphthoquinone derivatives in moderate yields. 相似文献
980.
Animesh Roy Maximilian A. Silvestri Robert A. Hall Salvatore D. Lepore 《Tetrahedron letters》2017,58(1):106-108
In the attempt to synthesize substituted allenyl esters through a metathesis coupling of unsubstituted allenyl esters and alkenes using a variety of ruthenium catalysts, it was discovered that allenyl esters themselves cleanly arrested the activity of the catalysts. Further studies suggests possible utility of allene esters as general quenching agents for metathesis reactions. To explore this idea, several representative olefin metathesis reactions, including ring closing, were successfully terminated by the addition of simple allenyl esters for more convenient purification. 相似文献