全文获取类型
收费全文 | 718篇 |
免费 | 36篇 |
国内免费 | 16篇 |
专业分类
化学 | 570篇 |
晶体学 | 6篇 |
力学 | 7篇 |
数学 | 64篇 |
物理学 | 123篇 |
出版年
2024年 | 1篇 |
2023年 | 5篇 |
2022年 | 20篇 |
2021年 | 22篇 |
2020年 | 23篇 |
2019年 | 41篇 |
2018年 | 46篇 |
2017年 | 33篇 |
2016年 | 49篇 |
2015年 | 39篇 |
2014年 | 41篇 |
2013年 | 94篇 |
2012年 | 45篇 |
2011年 | 57篇 |
2010年 | 28篇 |
2009年 | 34篇 |
2008年 | 27篇 |
2007年 | 21篇 |
2006年 | 11篇 |
2005年 | 16篇 |
2004年 | 21篇 |
2003年 | 7篇 |
2002年 | 14篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 9篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1977年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有770条查询结果,搜索用时 0 毫秒
51.
In this work, the ionic liquid (IL)[C6mim][PF6] was used as IL-based extractant for dispersive liquid–liquid microextraction, followed by back-extraction and HPLC/UV–Vis determination of 3-indole acetic acid (IAA) in pea plant. The effects of some crucial factors such as chemical structure and volume of IL, pH adjustment, dissolution temperature, extraction time, centrifugation time, and ionic strength of aqueous sample were studied. The linear range of the HPLC method for IAA quantification was 17.5 × 10?2–36.8 mg L?1. LOD, LOQ, method recovery, and preconcentration factor values were 0.170 mg L?1, 0.175 mg L?1, 98.3, and 40 %, respectively. The RSD for the suggested method was calculated as 0.93 % at 35.04 mg L?1 of IAA and each IL phase was able to be reused for at least four DLLME/back-extraction cycles. To evaluate the applicability of the suggested method, IAA was determined in pea plant samples. 相似文献
52.
Evolved Gas Analysis‐Mass Spectrometry to Identify the Earliest Organic Binder in Aegean Style Wall Paintings 下载免费PDF全文
Dr. Ravit Linn Dr. Ilaria Bonaduce Dr. Georgia Ntasi Prof. Leila Birolo Prof. Assaf Yasur‐Landau Prof. Eric H. Cline Dr. Austin Nevin Dr. Anna Lluveras‐Tenorio 《Angewandte Chemie (International ed. in English)》2018,57(40):13257-13260
An organic binder was identified in the painted fragments from the Canaanite palace of Tel Kabri, Israel. Recently dated to the late 18th century B.C.E. by 14C, Tel Kabri is the most ancient of the Eastern Mediterranean sites in which Aegean style paintings have been found. The application of pigments was suspected to be using an organic binding medium, particularly for the Egyptian Blue pigment. Samples of blue paint were examined using evolved gas analysis‐mass spectrometry (EGA‐MS) in order to overcome the analytical challenges imposed by highly degraded aged proteinaceous materials. Egg was identified as the binder based on the presence of hexadecanonitrile and octadecanonitrile, confirming the use of a secco painting technique. Lysozyme C from Gallus gallus was detected by proteomics analysis, confirming the presence of egg. To our knowledge, this is the earliest use of egg as a binder in Aegean style wall paintings. 相似文献
53.
Vitaliy Chagovets Zhihao Wang Alexey Kononikhin Natalia Starodubtseva Anna Borisova Dinara Salimova Igor Popov Andrey Kozachenko Konstantin Chingin Huanwen Chen Vladimir Frankevich Leila Adamyan Gennady Sukhikh 《Journal of the American Society for Mass Spectrometry》2018,29(2):323-330
Recent research revealed that tissue spray mass spectrometry enables rapid molecular profiling of biological tissues, which is of great importance for the search of disease biomarkers as well as for online surgery control. However, the payback for the high speed of analysis in tissue spray analysis is the generally lower chemical sensitivity compared with the traditional approach based on the offline chemical extraction and electrospray ionization mass spectrometry detection. In this study, high resolution mass spectrometry analysis of endometrium tissues of different localizations obtained using direct tissue spray mass spectrometry in positive ion mode is compared with the results of electrospray ionization analysis of lipid extracts. Identified features in both cases belong to three lipid classes: phosphatidylcholines, phosphoethanolamines, and sphingomyelins. Lipids coverage is validated by hydrophilic interaction liquid chromatography with mass spectrometry of lipid extracts. Multivariate analysis of data from both methods reveals satisfactory differentiation of eutopic and ectopic endometrium tissues. Overall, our results indicate that the chemical information provided by tissue spray ionization is sufficient to allow differentiation of endometrial tissues by localization with similar reliability but higher speed than in the traditional approach relying on offline extraction. 相似文献
54.
Gas chromatographic determination of some phenolic compounds in fuels and engine oil after simultaneous derivatization and microextraction 下载免费PDF全文
Mir Ali Farajzadeh Adeleh Yadeghari Leila Khoshmaram Houshang Ghorbanpour 《Journal of separation science》2014,37(20):2966-2973
In this study, a simultaneous derivatization/air‐assisted liquid–liquid microextraction method has been developed for sample preparation of some phenolic compounds in fuels and engine oil. Analytes are transferred by back liquid–liquid extraction into NaOH solution and then are derivatized with butyl chloroformate and extracted simultaneously into carbon tetrachloride. The extracted derivatized analytes are analyzed using gas chromatography with flame ionization detection. The effect of extracting solvent type, derivatization agent and extraction solvent volumes, ionic strength of the aqueous solution, number of extraction cycles, etc., on the extraction efficiency is investigated. The calibration graphs are linear in the range of 3–10 000 μg/L. Enhancement factors, enrichment factors, and extraction recoveries are in the ranges of 497 to 1471, 571 to 991, and 60 to 109%, respectively. Detection limits are obtained in the range of 0.8 to 2.0 μg/L. Relative standard deviations for the extraction of each selected phenols are in the ranges of 2–4% for intraday (n = 6) and 3–6% (n = 5) for interday precisions for 200 μg/L. This technique is successfully applied for the extraction, preconcentration, and determination of the selected phenols in gasoline, kerosene, gas oil, and engine oil. 相似文献
55.
A new diamine monomer containing fluorene unit, 3,5‐diamino‐N‐(9H‐fluoren‐2‐yl)benzamide was successfully synthesized via the condensation of 2‐aminofluorene and 3,5‐dinitrobenzoyl chloride and subsequent reduction of the dinitro compound. A series of novel aromatic polyimides having pendent fluorenamide moieties were prepared from the reaction of the diamine monomer and various tetracarboxylic dianhydrides by a conventional two‐step polymerization process. The polyimides were obtained in quantitative yields with inherent viscosities of 0.33–0.44 dl/g. The resulting polymers dissolved in N‐methyl‐2‐pyrrolidinone, N,N‐dimethylacetamide, N,N‐dimethylformamide, and dimethyl sulfoxide. The glass transition temperature of these polymers was in the range of 261–289°C. They were fairly stable up to a temperature around 450°C and lost 10% weight in the range of 498–556°C in nitrogen. The UV–vis absorption spectra showed that all of the polymers had absorption maxima around 320 nm. Cyclic voltammograms of the polyimides revealed an oxidation wave with a peak around 1.3 V. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
56.
Maha Chaieb Héla Habli Leila Mejrissi Brahim Oujia Florent Xavier Gadéa 《International journal of quantum chemistry》2014,114(11):731-747
A wide adiabatic study is performed for NaRb molecule, involving 151Σ+ electronic states including the ionic state Na?Rb+, as well as 143Σ+, 1–91,3Π, and 1–51,3Δ states. This investigation is performed using an ab initio approach which involves the effective core potential, the core polarization potential with l‐dependent cut‐off functions. The NaRb system has been treated as a two‐electron system and the full valence configuration interaction is easily achieved. The spectroscopic constants Re, De, Te, ωe, ωexe, Be, and D0 for all these states are derived. We have also computed the vibrational levels as well their spacing for different values of J. In addition, permanent and transition dipole moments are determined and analyzed. The Dunham coefficients have been used to perform experimental spacing to compare directly with our results. The present calculations on NaRb extend previous theoretical works to numerous electronic excited states in the various symmetries. © 2014 Wiley Periodicals, Inc. 相似文献
57.
The need for validated analytical methods and reference materials to identify and measure constituents in dietary supplements is essential. Such methods allow for the documentation that products meet manufacturer's specifications and contain what their labels declare. In March 2003, the U.S. Food and Drug Administration issued a proposed rule that would establish specific current good manufacturing practices for dietary supplements. This proposed rule requires that companies create a quality control unit to set specifications and ensure compliance with these specifications using scientifically valid procedures. This report provides insights and lessons learned from 3 meetings the Office of Dietary Supplements (ODS) at the National Institutes of Health helped organize. These meetings were structured to (1) educate individuals about the importance and need for validated analytical methods and reference materials to identify and quantify constituents of dietary supplements; (2) identify resources required to fulfill this need; and (3) serve as a platform to obtain input from interested parties to help frame the research agenda for the Dietary Supplements Methods and Reference Materials Program within ODS. Stakeholder's opinions and views expressed at these 3 meetings are outlined in this report. 相似文献
58.
A Calculation of the Culler-shalen Seminorms Associated to Small Seifert Dehn Fillings 总被引:1,自引:0,他引:1
We determine the relationship between the multiplicities ofthe zeros of certain rational functions defined on the SL2(C)and PSL2C character varieties of knot exteriors. This leadsto a formula for the CullerShalen seminorms associatedto small Seifert Dehn fillings of such manifolds. 2000 MathematicsSubject Classification: 57M05, 57M27, 57R65. 相似文献
59.
Mahboob Nemati Mir Ali Farajzadeh Ali Mohebbi Fariba Khodadadeian Mohammad Reza Afshar Mogaddam 《Journal of separation science》2020,43(6):1119-1127
A stir bar sorptive extraction method coupled with deep eutectic solvent based solidification of floating organic droplets–dispersive liquid–liquid microextraction has been used for the simultaneous derivatization and extraction of some acidic pesticides in tomato samples. In this method, initially the analytes are adsorbed on a coated stir bar from tomato juice filled in a narrow tube. After extraction, the stir bar is removed and a water–miscible deep eutectic solvent is used to elute the analytes. Afterward, a derivatization agent and a water–immiscible deep eutectic solvent (as an extraction solvent) with melting point near to room temperature are added to the obtained eluant at µL–levels and the obtained mixture is rapidly injected into deionized water. Under the optimum conditions, the introduced method indicated high enhancement (1543–3353) and enrichment (2530–2999) factors, low limits of detection (7–14 ng/L) and quantification (23–47 ng/L), good linearity (r2 ≥ 0.9982), and satisfactory repeatabilities (relative standard deviation ≤12% for intra– and inter–day precisions at a concentration of 100 ng/L of each analyte). Finally, the proposed method was applied in analysis of the analytes in tomato samples. 相似文献
60.
Cinabro D Henderson S Liu T Saulnier M Wilson R Yamamoto H Bergfeld T Eisenstein BI Gollin G Ong B Palmer M Selen M Thaler JJ Sadoff AJ Ammar R Ball S Baringer P Bean A Besson D Coppage D Copty N Davis R Hancock N Kelly M Kwak N Lam H Kubota Y Lattery M Nelson JK Patton S Perticone D Poling R Savinov V Schrenk S Wang R Alam MS Kim IJ Nemati B O'Neill JJ Severini H Sun CR Zoeller MM Crawford G Daubenmier CM Fulton R Fujino D Gan KK Honscheid K Kagan H Kass R Lee J Malchow R Morrow F Skovpen Y 《Physical review letters》1994,72(10):1406-1410