全文获取类型
收费全文 | 17041篇 |
免费 | 872篇 |
国内免费 | 716篇 |
专业分类
化学 | 17197篇 |
晶体学 | 9篇 |
力学 | 7篇 |
综合类 | 45篇 |
数学 | 52篇 |
物理学 | 1319篇 |
出版年
2024年 | 32篇 |
2023年 | 228篇 |
2022年 | 441篇 |
2021年 | 400篇 |
2020年 | 540篇 |
2019年 | 540篇 |
2018年 | 487篇 |
2017年 | 670篇 |
2016年 | 815篇 |
2015年 | 777篇 |
2014年 | 719篇 |
2013年 | 1042篇 |
2012年 | 1159篇 |
2011年 | 1053篇 |
2010年 | 865篇 |
2009年 | 1067篇 |
2008年 | 868篇 |
2007年 | 1083篇 |
2006年 | 862篇 |
2005年 | 778篇 |
2004年 | 713篇 |
2003年 | 595篇 |
2002年 | 445篇 |
2001年 | 246篇 |
2000年 | 252篇 |
1999年 | 239篇 |
1998年 | 222篇 |
1997年 | 203篇 |
1996年 | 207篇 |
1995年 | 189篇 |
1994年 | 149篇 |
1993年 | 132篇 |
1992年 | 118篇 |
1991年 | 162篇 |
1990年 | 77篇 |
1989年 | 52篇 |
1988年 | 44篇 |
1987年 | 27篇 |
1986年 | 27篇 |
1985年 | 23篇 |
1984年 | 17篇 |
1983年 | 11篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1973年 | 3篇 |
1967年 | 2篇 |
1959年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
82.
Vora A. Riga A. Dollimore D Alexander K. 《Journal of Thermal Analysis and Calorimetry》2004,75(3):709-717
This study attempts to identify the degradative process which folic acid undergoes in the solid-state under thermal stress.
In order to facilitate the process, the various pieces of the chemical structure, namely, p-amino benzoic acid, pterin and glutamic acid as both its d- and l-isomers were investigated as separate entities. These structured solid-state pieces were then compared to the composite solid
state folic acid degradative curves in order to identify the peaks seen and provide direction for the interpolation of the
degradative mechanism. It was observed that none of the structural pieces could be superimposed as assumed earlier and hence
an attempt was made to identify the decomposition products using various analytical techniques such as infrared spectroscopy,
mass spectroscopy and X-ray diffraction which suggested that the glutamic acid fragment is lost first as evidenced by acid
loss and amide enhancement in the IR spectra. The vitamin was ultimately degrading to carbon fragments and that further identification
was not necessary.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
83.
LC-MS/MS method for the confirmatory determination of aromatic amines and its application in textile analysis 总被引:1,自引:0,他引:1
Sutthivaiyakit P Achatz S Lintelmann J Aungpradit T Chanwirat R Chumanee S Kettrup A 《Analytical and bioanalytical chemistry》2005,381(1):268-276
A confirmation method for the determination of 18 aromatic amines originating from azo dyes after reductive cleavage was developed. The method is based on the use of high-performance liquid chromatography/tandem mass spectrometry with atmospheric-pressure chemical ionization. For the identification of the analytes one precursor ion and two daughter ions (multi-reaction monitoring, MRM) were selected and the LC-MS/MS parameters optimized to obtain high sensitivity and selectivity. The linear ranges varied from 0.1–1 to 30–50 g mL–1 with correlation coefficients of 0.99 or better. The applicability of the method to determine o-tolidine (3,3-dimethylbenzidine) and 3,3-dimethoxybenzidine in textiles following reductive cleavage of acid red 114, trypan blue, and Chicago sky blue 6B was demonstrated. 相似文献
84.
85.
86.
87.
Angelica pubescens and Angelica sinensis belong to the Umbelliferae family and both are used as traditional Chinese medicines. In the present study, headspace solid-phase microextraction (HS-SPME) with gas chromatography-mass spectrometry (GC-MS) was used for the analysis of the volatile constituents present in their roots. Eighty-seven compounds in Angelica pubescens and thirty-six compounds in Angelica sinensis were identified by GC-MS. Their relative contents were calculated by the peak area ratio. HS-SPME was compared to steam distillation (SD) by analyzing the volatile constituents of Angelica sinensis root. A good agreement between results obtained with both techniques was found. As a conclusion, HS-SPME is a powerful tool for determining the volatile constituents present in the TCMs. 相似文献
88.
R. Hoelzl Christian Hoelzl Ludwig Kotz Lazlo Fabry 《Accreditation and quality assurance》1998,3(5):185-188
Inductively coupled plasma mass spectrometry (ICP-MS) and high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS)
are powerful methods of determining metallic impurities in the low- and sub-ppt level in process media such as ultra-pure
water used in semiconductor and wafer manufacturing. By using mass spectrometers for analysis, an isotope dilution analysis
(IDMS) is possible. The reproducibility of an IDMS is unmatched. For concentration levels near the instrument detection limit
a novel method is reported to find the optimal amount of isotopic spike solution. This optimal value can be derived by the
law of propagation of uncertainty combined with the Poisson statistics of the measured number of counts. Generally, an excess
of isotopic spike solution should be used to provide results of lowest possible uncertainty. The results are presented in
a diagram for easy practical use.
Received: 14 October 1997 · Accepted: 13 February 1998 相似文献
89.
Fast atom bombardment mass spectra of a series of arenediazonium salts with various substituents complexed with 18-crown-6 and dibenzo-24-crown-8 ethers were examined. The correlation between quasi-molecular ion abundance of various complexes and the properties, coordination ability of donor-acceptor linkages in the complexes were studied. By using molecular orbital calculation we have successfully demonstrated that the correlation cited is related to the electron cloud densities on the heteroatoms α-N and β-N of arenediazonium salts. 相似文献
90.
A decision scheme for the interpretation of spectra from wavelength dispersive X-ray fluorescence spectrometry is described that encompasses elements from three areas of artificial intelligence: fuzzy logic, rule based expert systems and neural net technology.After transforming the recorded spectra to line spectra by appropriate background correction a reasoning scheme is applied that takes into account not only the observed spectra, but also the recording conditions and prior spectroscopic information regarding the relative emission probabilities and the usefulness of the different lines for the purpose of element identification. The latter is done on the basis of a previously described scheme to compute conditional a posteriori Bayes probabilities for a mean matrix. These different pieces of information are then assembled into a battery of fuzzy rules. The importance of the rules as well as the importance of the X-ray lines is determined in a training process, similar to the one in a feedforward back-propagation network.To further stabilize the results this network is pruned in a second training cycle. This, however, had little effect on the quality of interpretation.The advantages of this approach to the interpretation of X-ray spectra over older ones are numerous: the system adapts itself to better interpret spectra that are of greater importance to a laboratory as these are better represented in the training set; the fuzzy logic is capable of working with incomplete and uncertain knowledge, and the neural network results based on these fuzzy rules is readily interpretable by the X-ray spectroscopist as every rule can be expressed also in natural language as in any classical rule based system.On leave from Silesian University, Katowice, Poland 相似文献