全文获取类型
收费全文 | 35884篇 |
免费 | 1675篇 |
国内免费 | 1214篇 |
专业分类
化学 | 15497篇 |
晶体学 | 375篇 |
力学 | 1812篇 |
综合类 | 102篇 |
数学 | 9749篇 |
物理学 | 11238篇 |
出版年
2024年 | 32篇 |
2023年 | 240篇 |
2022年 | 390篇 |
2021年 | 378篇 |
2020年 | 461篇 |
2019年 | 427篇 |
2018年 | 1524篇 |
2017年 | 1758篇 |
2016年 | 1123篇 |
2015年 | 979篇 |
2014年 | 928篇 |
2013年 | 1263篇 |
2012年 | 3796篇 |
2011年 | 2959篇 |
2010年 | 2277篇 |
2009年 | 1980篇 |
2008年 | 957篇 |
2007年 | 962篇 |
2006年 | 892篇 |
2005年 | 4699篇 |
2004年 | 4126篇 |
2003年 | 2479篇 |
2002年 | 643篇 |
2001年 | 432篇 |
2000年 | 196篇 |
1999年 | 305篇 |
1998年 | 211篇 |
1997年 | 156篇 |
1996年 | 132篇 |
1995年 | 115篇 |
1994年 | 105篇 |
1993年 | 64篇 |
1992年 | 203篇 |
1991年 | 198篇 |
1990年 | 175篇 |
1989年 | 132篇 |
1988年 | 120篇 |
1987年 | 74篇 |
1986年 | 55篇 |
1985年 | 49篇 |
1976年 | 79篇 |
1975年 | 37篇 |
1974年 | 40篇 |
1973年 | 49篇 |
1972年 | 39篇 |
1970年 | 34篇 |
1969年 | 38篇 |
1968年 | 35篇 |
1967年 | 31篇 |
1966年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
This work describes the analytical procedures for atomic absorption and inductively coupled plasma (ICP) techniques that have to be used in order to obtain a license to sell drugs in the USA. The qualification of atomic absorption spectrometers and ICP instruments is described. The method validation characteristics, e.g., accuracy, precision, linearity, range, detection limits, and quantification are discussed. The time involved and the quality of documentation are pointed out. The consequences for laboratory personnel and operating costs are also discussed. 相似文献
212.
Amidines can be prepared on a solid support by reducing polymer-bound amidoximes with SnCl2·2H2O. The method has proved to be straightforward and highly efficient. Amidoximes attached to the solid support are readily available by treating resin-bound nitriles with hydroxylamine. 相似文献
213.
Both single-molecule detection (SMD) methods and miniaturization technologies have developed very rapidly over the last ten years. By merging these two techniques, it may be possible to achieve the optimal requirements for the analysis and manipulation of samples on a single molecule scale. While miniaturized structures and channels provide the interface required to handle small particles and molecules, SMD permits the discovery, localization, counting and identification of compounds. Widespread applications, across various bioscience/analytical science fields, such as DNA-analysis, cytometry and drug screening, are envisaged. In this review, the unique benefits of single fluorescent molecule detection in microfluidic channels are presented. Recent and possible future applications are discussed.Dedicated to the memory of Wilhelm Fresenius 相似文献
214.
Gytis-Kazimieras Kupetis Gintautas ?aduikis Ona Nivinskien? Olegas Eicher-Lorka 《Monatshefte für Chemie / Chemical Monthly》2002,71(11):313-321
A series of 1-alkyl-4-dialkylaminopyridinium halides derived from 4-dimethylamino- and 4-morpholinopyridines were synthesized and tested as phase-transfer catalysts in three typical reactions of dichlorocarbene: dehydration of benzamide, N-formylation of diphenylamine, and dichlorocyclopropanation of styrene. The catalytic performance of the above compounds was found comparable or higher than that of conventional quaternary ammonium catalysts. The influence of catalyst structure on the reactivity was evaluated. 相似文献
215.
Three ion-pair complexes, [RbzPy](+)[Ni(mnt)(2)](-) (mnt(2)(-) = maleonitriledithiolate; [RbzPy](+) = 4-R-benzylpyridinium; R = Br (1), Cl (2), and NO(2) (3)), with unusual magnetic properties have been synthesized and characterized. The crystal structures of 1 and 2 have been solved. The two complexes belong to the P2(1)/c space group with Z = 4 and C(20)H(11)BrN(5)NiS(4), a = 12.0744(17) A, b = 26.369(4) A, c = 7.440(3) A, and beta = 102.63(3) degrees for 1 and C(20)H(11)ClN(5)NiS(4), a = 12.105(2) A, b = 26.218(4) A, c = 7.374(2) A, and beta = 102.55(2) degrees for 2, respectively. The [Ni(mnt)(2)](-) anions in 1-3 form uniformly spaced one-dimensional (1-D) magnetic chains of s = 1/2 at room temperature. The temperature dependences of the susceptibility for 1-3 show that they undergo phase transitions. All three complexes are paramagnetic in their high-temperature (abbreviation HT) phase and diamagnetic in the low-temperature (abbreviation LT) phase because of strong dimerization along the stacking direction. The results of thermal analysis (DSC) further confirm that the phase transition for 1 and 2 is first-order but maybe second-order for 3. The phenomena observed in this study are similar to those of the 1-D radical systems. 相似文献
216.
A fluorimetric assay for cortisol 总被引:2,自引:0,他引:2
Appel D Schmid RD Dragan CA Bureik M Urlacher VB 《Analytical and bioanalytical chemistry》2005,383(2):182-186
A simple, rapid and sensitive fluorimetric assay for the quantitative determination of cortisol is reported. The assay is
based on the formation of a fluorescent dye when cortisol is incubated with a mixture of sulfuric acid and acetic acid. The
fluorescence spectrum recorded for the resulting dye shows a maximum extinction at 475 nm and a maximum emission at 525 nm.
The solvent 2-methyl-4-pentanone was used for extraction and was found to act as a fluorescence amplifier. A limit of detection
of 2.7 μM was achieved, making it possible to forego solvent evaporation. The assay suffers minor interference from 11-deoxycortisol
which exhibits low fluorescence at λ
ex: 460 nm; λ
em: 505 nm. Typical standard deviations were below 4%. We validated the assay using a biotransformation with recombinant Schizosaccharomyces pombe which regioselectively hydroxylates 11-deoxycortisol to cortisol. The method described herein is suitable for preliminary
screening of microorganisms capable of steroid hydroxylation. 相似文献
217.
Davor Margeti? 《Tetrahedron》2007,63(20):4338-4346
Four new substituted methano-bridged or heteroatom-bridged norbornadienomaleic anhydrides have been prepared and converted to sesquinorbornadiene anhydrides by reaction with cyclic 1,3-dienes. The versatility of parity reversal, in conjunction with N-substituent steric effects, has been used to produce all three possible stereoisomers of the N,O-sesquinorbornadiene anhydrides in separate, stereoselective cycloadditions. The anhydrides have been synthesized by cyclization of their diacids (in situ production) or by flash vacuum pyrolysis of their furan adducts (yielding crystalline products); further fragmentation occurs at these or higher temperatures to produce five-membered carbocyclic or heterocyclic anhydrides. Activation energies have been evaluated for the fragmentation and cycloaddition processes using DFT calculations (B3LYP/6-31G∗) and these calculations correctly predict, which reaction can be intercepted at the norbornadienomaleic anhydride stage and preferred stereochemistry of cycloadducts. 相似文献
218.
This report describes a new type of intra-intermolecular criss-cross cycloadditions. Thermal reactions of unsymmetrical allenylazines in the presence of alkynes led to three fused five-membered heterocycles in some cases. In the case of unsymmetrical substituted alkynes, a regioselectivity was observed. The molecular structures of all products are discussed. One X-ray crystal structure is also reported. 相似文献
219.
A new highly sensitive and selective chromogenic reagent dibromo-p-methyl-carboxysulfonazo (DBMCSA) was synthesized and studied for the spectrophotometric determination of lead in detail. In 0.25 M phosphoric acid medium, which greatly increases the selectivity, lead reacts with DBMCSA to form a 1:2 blue complex, having a sensitive absorption peak at 648 nm. Under the optimal conditions, Beer's law is obeyed over the range from 0 to 0.8 μg ml−1 Pb(II) and the apparent molar absorptivity is 1.04×105 l mol−1 cm−1. The detection limit and the variation coefficient were found to be 2.14 ng ml−1 and 1.0%, respectively. It is found that, except for Ca(II) and Ba(II), all foreign ions studied do not interfere with determination. The interference caused by Ca(II) and Ba(II) can be easily eliminated by prior extraction with potassium iodide-methylisobutylketone. The method has been applied to the determination of lead in vegetables with satisfactory results. 相似文献
220.