全文获取类型
收费全文 | 51354篇 |
免费 | 4955篇 |
国内免费 | 2516篇 |
专业分类
化学 | 35162篇 |
晶体学 | 560篇 |
力学 | 2426篇 |
综合类 | 110篇 |
数学 | 5257篇 |
物理学 | 15310篇 |
出版年
2023年 | 623篇 |
2022年 | 901篇 |
2021年 | 1294篇 |
2020年 | 1423篇 |
2019年 | 1414篇 |
2018年 | 1129篇 |
2017年 | 1016篇 |
2016年 | 1864篇 |
2015年 | 1725篇 |
2014年 | 2075篇 |
2013年 | 3232篇 |
2012年 | 4243篇 |
2011年 | 4488篇 |
2010年 | 2852篇 |
2009年 | 2606篇 |
2008年 | 3325篇 |
2007年 | 3119篇 |
2006年 | 2868篇 |
2005年 | 2564篇 |
2004年 | 2081篇 |
2003年 | 1601篇 |
2002年 | 1564篇 |
2001年 | 1107篇 |
2000年 | 1008篇 |
1999年 | 847篇 |
1998年 | 676篇 |
1997年 | 727篇 |
1996年 | 777篇 |
1995年 | 608篇 |
1994年 | 576篇 |
1993年 | 504篇 |
1992年 | 497篇 |
1991年 | 402篇 |
1990年 | 322篇 |
1989年 | 282篇 |
1988年 | 237篇 |
1987年 | 227篇 |
1986年 | 182篇 |
1985年 | 250篇 |
1984年 | 159篇 |
1983年 | 140篇 |
1982年 | 157篇 |
1981年 | 106篇 |
1980年 | 90篇 |
1978年 | 86篇 |
1977年 | 88篇 |
1976年 | 103篇 |
1975年 | 112篇 |
1974年 | 82篇 |
1973年 | 105篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Summary A thin layer chromatographic method for a qualitative screening-test and a quantitative analysis of 2,4,6-TNT, biodegradation products, octogen and hexogen in ammunition wastes was developed using both polar and non-polar modified sorbents. For enrichment a solidphase extraction on LiChrolut® EN followed by removal with methanol/acetonitrile (11 v/v) was chosen. To imitate real samples, spiked tap water samples of known composition were used. 相似文献
982.
Summary Polyethylene oxide (PEO)-based polymers with hydroxy, methoxy, and aminopropoxy terminal groups were coated on diol functionalized and hexamethyldisilazane end-capped silica particles. Proton-donor and proton-acceptor test solutes, including carboxylic acids, hydroxy-containing compounds, arylamines, and alkylamines were used to evaluate the chromatographic performances of these polymer coated particles under SFC conditions with neat CO2 as mobile phase. It was found that the particles coated with hydroxy-terminated PEO were suitable for the separation of proton-donor compounds such as hydroxy-containing compounds and carboxylic acids, and the particles coated with aminopropoxy-terminated PEO could be used for the separation of amines. That is, the proton-accepting stationary phase is suitable for the separation of proton accepting solutes, including strong basic alkylamines (pKb4), using neat CO2 as mobile phase, while the protondonating stationary phase is suitable for the separation of proton-donating compounds such as carboxylic acids (pKa4). Hydrogen bond basicity was found to be a critical factor for the chromatography of basic amines. Low volatility acidic and basic drugs were chromatographed using the new stationary phases. The stability of the PEO coated particles was determined by measuring the loss of organic carbon under SFC conditions. It was found that approximately 18 % of the coating (average molecular weight of 15,000) was washed out of the particles by supercritical CO2 after 7 h at 350 atm and 50°C 相似文献
983.
984.
The formation and stereoselective quenching of 1-mannopyranosyl radicals by a tributyltin hydride-mediated intramolecular 1,5-hydrogen abstraction sequence is described. A competing process is 1,4-hydrogen atom abstraction leading principally to glucopyran-2-ulosides. Fragmentation of the anomeric radical resulting in the formation of ring opened products is a problem in certain series. The chemistry is dictated to a considerable extent by the nature of the protecting groups employed with the 4,6-benzylidene series and, for rhamnose, the Ley 3,4-dispiroketal, being particularly susceptible to the 1,4-hydrogen atom abstraction but less to the fragmentation. Photochemical conditions are described, in which these side reactions are practically eliminated, and applied to the inversion of an alpha- to a beta-mannoside in a disaccharide. 相似文献
985.
986.
987.
A highly sensitive detection system using a semiconductor laser (SCL) and a thin long flow-through cell (TLFCT) to match flow-based analysis, such as flow injection analysis (FIA), has been developed. The SCL was a GaAlAs semiconductor laser emitting at 780 nm (5 mW), and the TLFTC, which has a 100 mm thin long light path, was made of a poly(tetrafluoroethylene) (PTFE) rod. The sensitivity was essentially improved about 10-fold in comparison to usual FIA-spectrophotometry using a 10 mm conventional flow cell. When this detection system was applied to the determination of total phosphorus (P) in water by molybdenum-blue spectrophotometry, the relative standard deviation, detection limit and linear range were 1.0% (n = 10 at 20 mug P 1(-1)), 0.6 (3 sigma) and 1.0-50 mug P 1(-1), respectively, and the results for the samples were fairly consistent with those by the official method. It may also be possible to apply the present detection system to other flow-based analytical methods such as the liquid chromatography family. 相似文献
988.
989.
990.