全文获取类型
收费全文 | 25218篇 |
免费 | 4662篇 |
国内免费 | 3984篇 |
专业分类
化学 | 19256篇 |
晶体学 | 547篇 |
力学 | 1314篇 |
综合类 | 360篇 |
数学 | 2767篇 |
物理学 | 9620篇 |
出版年
2024年 | 40篇 |
2023年 | 378篇 |
2022年 | 621篇 |
2021年 | 739篇 |
2020年 | 911篇 |
2019年 | 1057篇 |
2018年 | 802篇 |
2017年 | 830篇 |
2016年 | 1151篇 |
2015年 | 1232篇 |
2014年 | 1404篇 |
2013年 | 1850篇 |
2012年 | 2328篇 |
2011年 | 2311篇 |
2010年 | 1856篇 |
2009年 | 1830篇 |
2008年 | 2051篇 |
2007年 | 1726篇 |
2006年 | 1721篇 |
2005年 | 1557篇 |
2004年 | 1232篇 |
2003年 | 959篇 |
2002年 | 1051篇 |
2001年 | 831篇 |
2000年 | 648篇 |
1999年 | 538篇 |
1998年 | 341篇 |
1997年 | 263篇 |
1996年 | 251篇 |
1995年 | 208篇 |
1994年 | 227篇 |
1993年 | 164篇 |
1992年 | 94篇 |
1991年 | 142篇 |
1990年 | 95篇 |
1989年 | 79篇 |
1988年 | 57篇 |
1987年 | 35篇 |
1986年 | 40篇 |
1985年 | 55篇 |
1984年 | 26篇 |
1983年 | 26篇 |
1982年 | 15篇 |
1981年 | 22篇 |
1980年 | 11篇 |
1979年 | 14篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1957年 | 4篇 |
1936年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
981.
982.
983.
采用亚结构拼接法,将苯氧乙酸类结构引入酰基硫脲中,设计并合成了8个新型的硫脲类化合物——N-(取代苯氧乙酰胺基)-N'-(1-甲基环己基酰基)硫脲(5a~5h);5a~5h在酸性条件下关环形成8个新型的含1,3,4-噻二唑的酰胺类化合物——1-甲基-N-(5-取代基-1,3,4-噻二唑-2-基)环氧甲酰胺(6a~6h),其结构经1H NMR和ESI-MS表征。初步的生物活性测试结果表明,在用药量为500mg·L-1时,N-(2,4-二氯苯氧乙酰胺基)-N'-(1-甲基环己基酰基)硫脲对粘虫的抑制率为80%。 相似文献
984.
Nonlinear Dynamics - To achieve accurate moving target tracking in the presence of multiple disturbances, this paper presents a desired compensation robust integral of the sign of the error... 相似文献
985.
986.
Principal component analysis (PCA) is a canonical tool that reduces data dimensionality by finding linear transformations that project the data into a lower dimensional subspace while preserving the variability of the data. Selecting the number of principal components (PC) is essential but challenging for PCA since it represents an unsupervised learning problem without a clear target label at the sample level. In this article, we propose a new method to determine the optimal number of PCs based on the stability of the space spanned by PCs. A series of analyses with both synthetic data and real data demonstrates the superior performance of the proposed method. 相似文献
987.
Pharmacokinetics and metabolism study of veratramine in mice after oral administration using LC‐MS/MS
下载免费PDF全文
![点击此处可从《Biomedical chromatography : BMC》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Yue Cong Jun‐Li Zhang Sha‐Sha Li Shan Shen Jiang‐Ying Wang Zongwei Cai 《Biomedical chromatography : BMC》2016,30(9):1515-1522
A simple and sensitive high‐performance liquid chromatography coupled with hybrid triple quadrupole–linear ion trap mass spectrometry (Q‐trap‐MS) method was developed and validated for the determination of veratramine, the major bioactive and neurotoxic component in Veratrum nigrum L. Veratramine and the internal standard (IS) were separated with a Waters Symmetry C18 column and eluted with a gradient mobile phase system containing acetonitrile and 0.1% aqueous formic acid. The analysis was performed by using positive electrospray ionization mode with multiple reaction monitoring (MRM). Transition ions of m/z 410.2 → 295.2 for veratramine and m/z 426.1 → 113.8 for the IS were monitored. The method was validated with a good linearity in the range of 1–1000 ng/mL and lower limit of quantification of 1 ng/mL. The precision (CV) of intra‐ and inter‐day ranged from 3.92 to 7.29%, while the accuracy (bias) intra‐ and inter‐day were between ?4.78 and 1.65%. The recovery, stability and matrix effect were within the acceptable ranges. Five metabolites of veratramine, including four hydroxylated and one sulfated metabolites, were tentatively identified using predictive MRM–information dependent acquisition–enhanced product ion mode (predictive MRM‐IDA‐EPI). The developed method was successfully applied to the pharmacokinetic and metabolic study of veratramine in mice after oral administration of veratramine. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
988.
Two new compounds, 19-hydroxy-melodinine K (1) and melodiside (2), and 25 known compounds were isolated from leaves and twigs of Melodinus suaveolens. Their structures were elucidated based on 1- and 2-D NMR, FTIR, UV and MS spectroscopic data. 19-hydroxy-melodinine K showed cytotoxic activity against MDA-MB-231 breast, BCG-823 gastric, SW480 colon and Hela cancer cells. 相似文献
989.
Hui Li Zejun Jiang Xiaolin Cao Hang Su Hua Shao Fen Jin Lufei Zheng A. M. Abd El-Aty Jing Wang 《Chromatographia》2018,81(2):359-364
990.
A simple LC‐MS method for determination of cyasterone in rat plasma: application to a pilot pharmacokinetic study
下载免费PDF全文
![点击此处可从《Biomedical chromatography : BMC》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Fuqiang Li Guangyu Li Jinsong Zhao Jun Xiao Zaoxia Liu Guanfang Su 《Biomedical chromatography : BMC》2016,30(6):867-871
A simple, specific, and sensitive liquid chromatography–mass spectrometry (LC‐MS) method for determination of cyasterone in rat plasma was developed in our laboratory. Cucurbitacin B was used as an internal standard (IS). After protein precipitation with twofold volume of acetonitrile, the analyte and IS were separated on a Luna C18 column (100 × 4.6 mm, i.d., 3.0 µm; Phenomenex) by isocratic elution with acetonitrile–water (80:20, v/v) as the mobile phase at a flow rate of 0.4 mL/min. An electrospray ionization source was applied and operated in the positive ion mode; selected ion monitoring scan mode was used for quantification, and the target ions m/z 543.3 for cyasterone and m/z 581.3 for IS were chosen. Good linearity was observed in the concentration range of 0.40–400 ng/mL for cyasterone in rat plasma. Intra‐day and inter‐day precision were both <7.4%. This method was proved to be suitable for pharmacokinetic studies after oral (5.0 mg/kg) or intravenous (0.5 mg/kg) administration of cyasterone in rats. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献