全文获取类型
收费全文 | 7584篇 |
免费 | 632篇 |
国内免费 | 774篇 |
专业分类
化学 | 7192篇 |
晶体学 | 30篇 |
力学 | 146篇 |
综合类 | 14篇 |
数学 | 75篇 |
物理学 | 1533篇 |
出版年
2023年 | 87篇 |
2022年 | 120篇 |
2021年 | 128篇 |
2020年 | 147篇 |
2019年 | 136篇 |
2018年 | 103篇 |
2017年 | 154篇 |
2016年 | 221篇 |
2015年 | 308篇 |
2014年 | 240篇 |
2013年 | 361篇 |
2012年 | 402篇 |
2011年 | 503篇 |
2010年 | 452篇 |
2009年 | 504篇 |
2008年 | 403篇 |
2007年 | 491篇 |
2006年 | 357篇 |
2005年 | 342篇 |
2004年 | 315篇 |
2003年 | 277篇 |
2002年 | 180篇 |
2001年 | 138篇 |
2000年 | 164篇 |
1999年 | 126篇 |
1998年 | 131篇 |
1997年 | 125篇 |
1996年 | 154篇 |
1995年 | 118篇 |
1994年 | 119篇 |
1993年 | 116篇 |
1992年 | 81篇 |
1991年 | 86篇 |
1990年 | 68篇 |
1989年 | 78篇 |
1988年 | 112篇 |
1987年 | 97篇 |
1986年 | 100篇 |
1985年 | 127篇 |
1984年 | 116篇 |
1983年 | 74篇 |
1982年 | 116篇 |
1981年 | 116篇 |
1980年 | 108篇 |
1979年 | 119篇 |
1978年 | 73篇 |
1977年 | 16篇 |
1976年 | 28篇 |
1973年 | 19篇 |
1972年 | 16篇 |
排序方式: 共有8990条查询结果,搜索用时 31 毫秒
261.
河豚鱼脂肪酸的气相色谱—质谱分析 总被引:10,自引:0,他引:10
采用GC/MS技术对2种河豚鱼(黄鳍、红鳍)脂肪提取物中的脂肪酸进行定性定量分析,共鉴别出24种脂肪酸。多不饱和脂肪酸尤其是二十二碳六烯酸(DHA)的含量很高,在黄猪中相对含量达32.32%,在红鳍中达24.11%。检出了在其它海产品中较少发现的15-二十四烯酸(神经酸),是人体生长发育不可缺少的一种脂肪酸。对鱼肉脂肪提取方法及不同极性毛细管柱分离情况进行了讨论。 相似文献
262.
C. C. Camarasu 《Chromatographia》2002,56(1):S131-S135
Summary A sensitive headspace SPME method for the extraction of residual solvents from pharmaceutical products has been developed
and optimized. It was found that minimizing sample and headspace volume has a beneficial effect on extraction efficiency.
At the same time the method reproducibility was seriously affected by reducing sample and headspace volume. The added air
volume was not found to have any significant influence on method sensitivity. The method showed reproducibilities of less
than 10% and detection limits as low as 1 ppb for benzene and dichloromethane. The headspace SPME method is around 1000 times
more sensitive than static headspace. The optimized parameters were headspace volume 1.5 mL, sample volume 10 μL, and extraction
time 30 min. The method was successfully applied to the identification of unknown residual solvents in three different proprietary
active drug substances and was successfully applied to the confirmation of the presence of benzene in a proprietary drug substance.
Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001 相似文献
263.
Using geometry optimization and DFT method at the B3LYP/6-31G* level of theory for C30H20, an equilibrium geometry is identified that has the form of polyhedral hydrocarbon with five carbon–carbon single bonds linking a dodecahedrane cage and a pentaprismane cage. Thus, this molecule is a tri-cage molecule with two pentaprismane cages and one dodecahedrane cage. Vibrational frequencies and the infrared spectrum are computed at the same level of theory. The heat of formation for C30H20 has been estimated in this paper. The heat of formation of C30H20 as well as the vibrational analysis indicates that this molecule enjoys sufficient stability to allow for its experimental preparation. 相似文献
264.
The linearity of the relationship between σT,t2 and (1 + k)2 is studied with the view of using it for the evaluation of GC equipment. Based on appropriate analytical expressions it is concluded that in general this relationship is not linear and that practical procedures for column evaluation should be developed that take this non-linearity into account. 相似文献
265.
Summary A comparison of the official EPA method 515.1 for determination of chlorinated acidic pesticides and a modification of it is illustrated. Extraction of the analytes from water and their determination and quantitation is by gas chromatography-electron capture detection (GC-ECD), liquid chromatography-UV detection and liquid chromatography-particle beam mass spectrometry. Although HPLC-PBMS was found to be less sensitive than the GC-ECD method, it was, nevertheless, more sensitive than HPLC-UV. The modified method is simpler, quicker and allows more accurate determination of pesticides in aqueous samples.Deceased July 31, 1996 相似文献
266.
Summary The true meanings of the so-called corrected retention volume and the specific retention volume are explained in the light of the recent paper by Davankov. 相似文献
267.
A method for the determination of trace Cr(III) in aqueous solution by solid-phase microextraction (SPME) coupled with gas chromatography (GC)-flame photometric detection (FPD) was developed. Aqueous Cr(III) was first converted to the volatile chromium trifluoroacetylacetonate (Cr(tfa)3) by derivatization with 1,1,1-trifluoroacetylacetone (Htfa), followed by SPME extraction using a polyimide-coated silica fiber. The distribution constants (K) of derivatized cis- and trans-Cr(tfa)3 between the polyimide phase and aqueous phase were 2012 and 2214, respectively. The two Cr(tfa)3 isomers extracted can be efficiently separated by a DB-210 GC column within 9 min. Selective detection of Cr was performed by a FPD equipped with a 385-nm long-pass filter. Linearity (r> 0.99) over the concentration range 5-300 ng ml(-1) Cr was obtained and the limit of detection was 2 ng ml(-1) Cr. The relative standard deviation was 7% at 10 ng ml(-1) Cr (n = 5). Applicability of this method to water analysis was tested by analyzing the chromium content in a reference standard water sample and an industrial effluent. 相似文献
268.
269.
R. V. Golovnya 《Chromatographia》1979,12(8):533-538
Summary Equations modifying Rohrschneider's relationship between I and thermodynamical characteristics values are derived. Polarity in chromatography is discussed as a possibility of stationary phases or absorbents to enter into intermolecular interactions and is determined on the basis of the partial molar free energies of solution (G) of six test substances. A simple method for the calculation of stationary phase polarity in terms of G from retention indices and McReynolds constants is given. Equations are presented for the calculation of stationary phase selectivity utilizing G data tabulated for selected test substances. The thermodynamical basis of selecting a reference phase for a unified system expressing the selectivity of stationary phases which can also be applied for adsorbents is suggested. The energy equivalent to an index unit, Gi.u., is discussed. 相似文献
270.
K. E. Markides B. J. Tarbet C. M. Schregenberger J. S. Bradshaw M. L. Lee K. D. Bartle 《Journal of separation science》1985,8(11):741-747
A method is described for surface deactivation and modification of fused silica capillary columns with a cyanopropyl-containing reagent. The deactivation procedure involved a dehydrocondensation reaction between a bis(cyanopropyl)methylhydropolysiloxane reagent and surface silanol groups at an optimum temperature of only 250°C. Actual critical surface tension measurements were made using the capillary rise method. Excellent deactivation for acidic and basic compounds at the low ng level, and wettability for nonpolar and polar polysiloxane stationary phases were obtained. A procedure was developed to remove acidic impurities that are present in polar stationary phases. 相似文献