全文获取类型
收费全文 | 82978篇 |
免费 | 430篇 |
国内免费 | 388篇 |
专业分类
化学 | 27463篇 |
晶体学 | 833篇 |
力学 | 6786篇 |
数学 | 32795篇 |
物理学 | 15919篇 |
出版年
2018年 | 10448篇 |
2017年 | 10275篇 |
2016年 | 6125篇 |
2015年 | 893篇 |
2014年 | 361篇 |
2013年 | 456篇 |
2012年 | 3909篇 |
2011年 | 10646篇 |
2010年 | 5718篇 |
2009年 | 6114篇 |
2008年 | 6726篇 |
2007年 | 8883篇 |
2006年 | 374篇 |
2005年 | 1445篇 |
2004年 | 1679篇 |
2003年 | 2101篇 |
2002年 | 1137篇 |
2001年 | 335篇 |
2000年 | 406篇 |
1999年 | 232篇 |
1998年 | 272篇 |
1997年 | 216篇 |
1996年 | 290篇 |
1995年 | 237篇 |
1994年 | 179篇 |
1993年 | 168篇 |
1992年 | 140篇 |
1991年 | 146篇 |
1990年 | 125篇 |
1989年 | 148篇 |
1988年 | 151篇 |
1987年 | 132篇 |
1986年 | 133篇 |
1985年 | 156篇 |
1984年 | 124篇 |
1983年 | 134篇 |
1982年 | 152篇 |
1981年 | 136篇 |
1980年 | 134篇 |
1979年 | 143篇 |
1978年 | 110篇 |
1977年 | 95篇 |
1976年 | 83篇 |
1975年 | 77篇 |
1974年 | 83篇 |
1973年 | 97篇 |
1972年 | 61篇 |
1971年 | 64篇 |
1968年 | 50篇 |
1966年 | 55篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
In the course of investigations on equestrian supplemental products for the presence of doping substances, two products were found to contain forbidden substances. As reported earlier a plant extract (Mexican cactus extract) named “Energy 5” contained the anabolic androgenic steroids (AAS) stanozolol, 17β-hydroxy-17α-methyl-5α-androstane-3β-ol (3β,5α-THMT) as well as mestanolone not declared on the label. In the present study, a product called “Super Kalm Paste” was tested. Analysis by gas chromatography - mass spectrometry (GC-MS) revealed that the preparation contained the class I anti-arrhythmics quinine (trade names KinidinTM, Durules) and cinchonine. The samples were prepared according to a sample preparation procedure established for anabolic steroids in nutritional supplements for humans. The sample treatment comprised the extraction and purification of the analytes as well as the chemical conversion with N-methyl-N-trimethylsilyl-trifluoracetamide (MSTFA) to yield the trimethylsilyl (TMS)-derivatives. To verify whether the administration of such products could lead to positive doping tests, a pilot excretion study on “Energy 5” was conducted with two geldings, and urine samples were collected. Gas chromatography - high resolution mass spectrometry (GC-HRMS) after solid phase extraction and mixed derivatisation has demonstrated the presence of the stanozolol metabolite 16β-hydroxy-stanozolol in urine samples after “Energy 5” application. 相似文献
992.
Robert?WeisEmail author Armin?Presser Werner?Seebacher 《Monatshefte für Chemie / Chemical Monthly》2003,134(8):1129-1136
Summary. 3-Carboxamides and 3-carboxanilides of 6-alkyl and 6-aryldihydropyridin-2(1H)-ones have been prepared via different reaction pathways. All synthesized amides show hydrogen bonds in their NMR spectra. The 4-hydroxy compounds were obtained as a mixture of tautomers. Their configurations were elucidated by NMR experiments.Received December 16, 2002; accepted December 20, 2002
Published online June 2, 2003 相似文献
993.
Dusanka Ž. Obadović Anikó Vajda Maya Garić A. Bubnov Vera Hamplová M. Kašpar Katalin Fodor-Csorba 《Journal of Thermal Analysis and Calorimetry》2005,80(2):519-523
Summary Thermal properties of a homologous series of ferroelectric liquid crystals S-(-)-[4-(2-n-alkoxy-propionyloxy)]biphenyl-4'-[n-alkoxy-(3,5-dimethyl)]benzoate have been investigated by polarizing optical microscopy and differential scanning calorimetry.
The mesophases were identified and confirmed by X-ray too. Three binary mixtures were prepared from the individual homologues.
In one of the mixtures (Mix1), the ferroelectric SmC* phase has broadened and became enantiotropic. This mesophase remained
monotropic in the other two mixtures (Mix2, Mix3). The chiral nematic N* phase did not appear in Mix1, but remained monotropic
for the other two mixtures. Two molecular parameters, the layer spacing and the average intermolecular distance have been
calculated from the X-ray results for the homologues and their mixtures. An intercalated tail-to-tail packing of molecules
was found both in the single compounds and their mixtures resulting in the layer spacing about half of the molecular length
of the single compounds. 相似文献
994.
A practicable and selective liquid chromatography-mass spectrometry assay for the determination of two cyclooxygenase-2 inhibitors, etoricoxib and valdecoxib, in human plasma is presented. The analytical technique is based on reversed-phase high-performance liquid chromatography (HPLC) coupled to atmospheric pressure chemical ionisation (APCI) mass spectrometry (Finnigan Mat LCQ ion trap). Mass analysis was performed in the positive ion mode. The ion trap was operated in the tandem MS mode (MS2) and the transitions of etoricoxib (m/z 359.2 --> 280.3) and valdecoxib (m/z 315.1 --> 235.1) were followed by selected reaction monitoring. Retention times of etoricoxib and valdecoxib were 1.05 and 1.08 min, respectively. The method was validated over a linear range 10-2500 and 5-1000 microg/L using the other substrate as internal standard. After validation, the method was used to study the pharmacokinetic pro fi le of etoricoxib or valdecoxib in a healthy volunteer after administration of a single oral dose (valdecoxib, 20 mg; etoricoxib, 90 mg). The presented method was suf fi cient to cover more than 90% of the area under the plasma concentration time curve. 相似文献
995.
Miniaturizing all dimensions of apparatus, such as electronics and computers, is the current trend followed by scientists
in various fields. The idea of Lab-on-a-Chip has significantly expanded and found its broad applications in analytical chemistry.
Microplasmas can act as a sample excitation source and are the miniaturized versions of full-sized plasmas. These can be created
in various forms, such as direct current, microwave induced, capacitively coupled and inductively coupled plasmas. Scaling
down the size would reduce the amount of gases, liquids and consumables required, as well as the sample analysis time, which
in turn would decrease the operating costs. Therefore, several research groups are involved in the development of microplasmas
for utilisation in analytical instruments. 相似文献
996.
997.
998.
999.
1000.