全文获取类型
收费全文 | 16218篇 |
免费 | 874篇 |
国内免费 | 1437篇 |
专业分类
化学 | 12804篇 |
晶体学 | 124篇 |
力学 | 325篇 |
综合类 | 10篇 |
数学 | 2210篇 |
物理学 | 3056篇 |
出版年
2023年 | 111篇 |
2022年 | 73篇 |
2021年 | 284篇 |
2020年 | 283篇 |
2019年 | 322篇 |
2018年 | 261篇 |
2017年 | 262篇 |
2016年 | 594篇 |
2015年 | 496篇 |
2014年 | 556篇 |
2013年 | 1231篇 |
2012年 | 1342篇 |
2011年 | 1436篇 |
2010年 | 825篇 |
2009年 | 1049篇 |
2008年 | 1012篇 |
2007年 | 985篇 |
2006年 | 847篇 |
2005年 | 888篇 |
2004年 | 1163篇 |
2003年 | 812篇 |
2002年 | 721篇 |
2001年 | 343篇 |
2000年 | 260篇 |
1999年 | 200篇 |
1998年 | 190篇 |
1997年 | 169篇 |
1996年 | 175篇 |
1995年 | 155篇 |
1994年 | 113篇 |
1993年 | 136篇 |
1992年 | 115篇 |
1991年 | 76篇 |
1990年 | 80篇 |
1989年 | 64篇 |
1988年 | 67篇 |
1987年 | 55篇 |
1986年 | 64篇 |
1985年 | 89篇 |
1984年 | 90篇 |
1983年 | 66篇 |
1982年 | 57篇 |
1981年 | 59篇 |
1980年 | 50篇 |
1979年 | 32篇 |
1978年 | 28篇 |
1977年 | 35篇 |
1976年 | 26篇 |
1975年 | 24篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
381.
Laura Herrero Sagrario CalvarroMario A. Fernández Jesús Eduardo Quintanilla-LópezMaría José González Belén Gómara 《Analytica chimica acta》2015
Phthalates (PAEs) are ubiquitous toxic chemical compounds. During the last few years, some phthalate metabolites (MPAEs) have been proposed as appropriate biomarkers in human urine samples to determine PAE human intake and exposure. So, it is necessary to have fast, easy, robust and validated analytical methods to determine selected MPAEs in urine human samples. Two different instrumental methods based on gas (GC) and ultra-high performance liquid (UHPLC) chromatography coupled to mass spectrometry (MS) have been optimized, characterized and validated for the simultaneous determination of nine primary and secondary phthalate metabolites in urine samples. Both instrumental methods have similar sensitivity (detection limits ranged from 0.03 to 8.89 pg μL−1 and from 0.06 to 0.49 pg μL−1 in GC–MS and UHPLC–MS2, respectively), precision (repeatability, expressed as relative standard deviation, which was lower than 8.4% in both systems, except for 5OH-MEHP in the case of GC–MS) and accuracy. But some advantages of the UHPLC–MS2 method, such as more selectivity and lower time in the chromatographic runs (6.8 min vs. 28.5 min), have caused the UHPLC–MS2 method to be chosen to analyze the twenty one human urine samples from the general Spanish population. Regarding these samples, MEP showed the highest median concentration (68.6 μg L−1), followed by MiBP (23.3 μg L−1), 5cx-MEPP (22.5 μg L−1) and MBP (19.3 μg L−1). MMP (6.99 μg L−1), 5oxo-MEHP (6.15 μg L−1), 5OH-MEHP (5.30 μg L−1) and MEHP (4.40 μg L−1) showed intermediate levels. Finally, the lowest levels were found for MBzP (2.55 μg L−1). These data are within the same order of magnitude as those found in other similar populations. 相似文献
382.
Laura Núñez‐Pons Rosa María Nieto Conxita Avila Carlos Jiménez Jaime Rodríguez 《Journal of mass spectrometry : JMS》2015,50(1):103-111
Taking into account the broad biological activities found in the meridianin indole alkaloids isolated to date, we have re‐examined the organic extracts of an Antarctic collection of the tunicates Aplidium meridianum and A. falklandicum (Chordata: Ascidiacea) by HPLC in conjunction with a high‐resolution mass spectrometer (HPLC‐MS). A new set of analogs of meridianins A–G has been detected, and their structures are proposed on the basis of the molecular formulae identified by LC‐HRMS analysis using a C18 column with a gradient of water/acetonitrile and an LTQ‐FT‐MS Orbitrap detector. Remarkably, dimers derived from meridianin A and from meridianin B or E were also detected. Our findings provide further evidence of the broad variability within the meridianin‐like derivatives of this highly bioactive alkaloid family. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
383.
384.
A reversed‐phase compatible thin‐layer chromatography autography for the detection of acetylcholinesterase inhibitors 下载免费PDF全文
I. Ayelen Ramallo Paula García Ricardo L. E. Furlan 《Journal of separation science》2015,38(21):3788-3794
A dual readout autographic assay to detect acetylcholinesterase inhibitors present in complex matrices adsorbed on reversed‐phase or normal‐phase thin‐layer chromatography plates is described. Enzyme gel entrapment with an amphiphilic copolymer was used for assay development. The effects of substrate and enzyme concentrations, pH, incubation time, and incubation temperature on the sensitivity and the detection limit of the assay were evaluated. Experimental design and response surface methodology were used to optimize conditions with a minimum number of experiments. The assay allowed the detection of 0.01% w/w of physostigmine in both a spiked Sonchus oleraceus L. extract chromatographed on normal phase and a spiked Pimenta racemosa (Mill.) J.W. Moore leaf essential oil chromatographed on reversed phase. Finally, the reversed‐phase thin‐layer chromatography assay was applied to reveal the presence of an inhibitor in the Cymbopogon citratus (DC.) Stapf essential oil. The developed assay is able to detect acetylcholinesterase inhibitors present in complex matrixes that were chromatographed in normal phase or reversed‐phase thin‐layer chromatography. The detection limit for physostigmine on both normal and reversed phase was of 1×10?4 μg. The results can be read by a change in color and/or a change in fluorescence. 相似文献
385.
Dehydrogenative TEMPO‐Mediated Formation of Unstable Nitrones: Easy Access to N‐Carbamoyl Isoxazolines 下载免费PDF全文
Andrea Gini Marwin Segler Dominik Kellner Prof. Dr. Olga García Mancheño 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(34):12053-12060
N‐carbamoyl nitrones represent an important class of reagents for the synthesis of a variety of natural and biologically active compounds. These compounds are generally converted into valuable 4‐isoxazolines upon cyclization reaction with dipolarophiles. However, these types of N‐protected nitrones are highly unstable, which limits their synthesis, storage and practical use, enforcing alternative lengthy or elaborated synthetic routes. In this work, a 2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO)‐mediated formal “dehydrogenation” of N‐protected benzyl‐, allyl‐ and alkyl‐substituted hydroxylamines followed by in situ trapping of the generated unstable nitrones into N‐carbamoyl 4‐isoxazolines is presented. A plausible mechanism is also proposed, in which the dipolarophile shows an important assistant role in the generation of the active nitrone intermediate. This simple protocol avoids the problematic isolation of N‐carbamoyl protected nitrones, providing new synthetic possibilities in isoxazoline chemistry. 相似文献
386.
Tibor Rozsa Mihaela Duma Laurian Vlase Ioana Ionuţ Adrian Pîrnău Brînduşa Tiperciuc Ovidiu Oniga 《Journal of heterocyclic chemistry》2015,52(4):999-1006
Thiazole and bisthiazole derivatives represent a prevalent scaffold in the antimicrobial drug discovery. Therefore, we have decided to synthesize some new series of 4,5′‐bisthiazoles. A total of 17 compounds were synthesized, their structural elucidation being based on elemental analysis (C,H,N,S) and spectroscopic data (MS and 1H NMR). Their in vitro antimicrobial activities were assessed against several Gram‐positive and Gram‐negative bacteria strains and also against one fungal strain (Candida albicans) using the difusimetric method. Some of the compounds showed modest to good antibacterial activity against Gram‐negative Escherichia coli and Salmonella typhimurium and Gram‐positive Staphylococcus aureus and Bacillus cereus bacterial strains. All of the synthesized compounds showed moderate to very good antifungal activity against C. albicans. 相似文献
387.
388.
389.
390.
Leandro F. Gil‐Silva Raquel Santamaría‐Fernández Prof. Dr. Armando Navarro‐Vázquez Prof. Roberto R. Gil 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(2):472-476
A new DMSO‐compatible aligning gel based on cross‐linked poly(2‐hydroxylethyl methacrylate) (poly‐HEMA) has been developed. Due to a significant difference in bulk magnetic susceptibility between the DMSO inside and outside the gel, it is possible to simultaneously collect isotropic and anisotropic NMR data, such as residual dipolar couplings (RDC), in the same NMR tube. RDC‐assisted structural analysis of menthol and the alkaloid retrorsine is reported as proof of concept. 相似文献