全文获取类型
收费全文 | 250篇 |
免费 | 13篇 |
国内免费 | 10篇 |
专业分类
化学 | 240篇 |
晶体学 | 4篇 |
力学 | 1篇 |
数学 | 24篇 |
物理学 | 4篇 |
出版年
2023年 | 1篇 |
2022年 | 8篇 |
2021年 | 2篇 |
2020年 | 3篇 |
2019年 | 5篇 |
2017年 | 1篇 |
2016年 | 6篇 |
2015年 | 9篇 |
2014年 | 13篇 |
2013年 | 8篇 |
2012年 | 8篇 |
2011年 | 14篇 |
2010年 | 7篇 |
2009年 | 10篇 |
2008年 | 19篇 |
2007年 | 10篇 |
2006年 | 16篇 |
2005年 | 18篇 |
2004年 | 14篇 |
2003年 | 8篇 |
2002年 | 3篇 |
2001年 | 3篇 |
2000年 | 5篇 |
1999年 | 16篇 |
1998年 | 3篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1976年 | 1篇 |
1973年 | 1篇 |
1969年 | 2篇 |
排序方式: 共有273条查询结果,搜索用时 15 毫秒
31.
Li Q Zhao C Li Y Chang Y Wu Z Pang T Lu X Wu Y Xu G 《Journal of separation science》2011,34(2):119-126
An approach was developed for extracting and analyzing the chemical components of tobacco leaves based on solvent extraction and rapid & resolution liquid chromatography/quadrupole time-of-flight mass spectrometry analysis. Two solvents with different polarities were used to extract hydrophilic components and hydrophobic components, respectively, the combined analytical data can provide a "global" view of metabolites. Based on the evaluation of parallel samples, it was found that this approach provided good repeatability, accurate and reliable profiling data, and is suitable for the metabolomics study of tobacco leaves. In order to find the chemical component differences of tobacco leaves, 56 samples from Zimbabwe and China were analyzed using the developed method. The metabolite data were processed by multivariate statistic technique; an obvious group classification between Zimbabwe and China was observed, 14 significantly changed compounds were found, and 9 of them were identified. 相似文献
32.
Gas chromatography-mass spectrometric method for metabolic profiling of tobacco leaves 总被引:1,自引:0,他引:1
A gas chromatography-mass spectrometric method was developed for profiling of tobacco leaves. The differentiation among tobacco leaves planted in two different regions was investigated. Prior to analysis, the extraction solvent formulation was optimized and a combination of water, methanol and acetonitrile with a volume ratio of 3:1:1 was found to be optimal. The reproducibility of the method was satisfactory. Kendall tau-b rank correlation coefficients were equal to 1 (p<0.05) for 82% of the resolved peaks (up to 95% of the overall peak areas), indicating the good response correlation. Forty-four compounds including 9 saccharides, 9 alcohols, 9 amino acids, 16 organic acids and phosphoric acid were identified based on standard compounds. The method was successfully applied for profiling of tobacco leaves from Zimbabwe and Yunnan of China. Our result revealed that levels of saccharides and their derivatives including xylose, ribose, fructose, glucose, turanose, xylitol and glyceric acid were more abundant while sucrose, glucitol and D-gluconic acid were less abundant in tobacco leaves from Yunnan as compared to those from Zimbabwe. Amino acids such as L-alanine, L-tyrosine and L-threonine were found to be richer in Zimbabwe tobacco than in Yunnan tobacco. 相似文献
33.
Zhao C Wu Z Xue G Wang J Zhao Y Xu Z Lin D Herbert G Chang Y Cai K Xu G 《Journal of chromatography. A》2011,1218(23):3669-3674
Nanoflow liquid chromatography/mass spectrometry (nano-LC/MS) has attracted increasing interest in virtue of high sensitivity, low sample consumption, and minimal matrix effect. In this work a HPLC-Chip/quadrupole time-of-flight (Q-TOF) MS device with a new ultra-high capacity small molecule chip (UHC-Chip) which features a 500 nL enrichment column and a 150 mm × 75 μm analytical column, was evaluated with a drug mixture covering a wide range of polarities. Excellent chromatographic precision with 0.1-0.5% RSD for retention time and 1.7-9.0% RSD for peak area, low limit of detection, good chip-to-chip reproducibility and linearity were obtained by using this UHC-Chip. Compared with the standard HPLC-Chip with 40 nL trapping column, the UHC-Chip showed higher enrichment capability and hence gave a higher response in signal detection. Additionally, 4-30 times increase in sensitivity was obtained compared with conventional LC/MS, which indicated that UHC-Chip/MS was a valuable tool for the quantitative analysis of low level impurities and degradation products in pharmaceuticals. Moreover, satisfactory results obtained from trace drug analysis of serum samples further proved its practicality and potential for use in drug testing and development. 相似文献
34.
Summary The information of distribution analyses considerably exceeds that of integral analyses because of the spatial resolution. Distribution-analytical procedures are characterized by the ideal information amount which is determined by the ideal spatial resolving power A* that is theoretically attainable by actual scanning or mapping. Compared with it, the real information amount of distribution-analytical results are determined by a practical spatial resolving power and therefore, among others, by the number of measuring points. In dependence on the stochastical properties of multi-phase materials an optimum area can be specified for distribution-analytical investigations with a maximum information content. Methods of testing homogeneity can be characterized by means of the information theory in dependence on the arrangement of measuring points, on the evaluation of concentration or measuring values, and on their univariate, multiple or multivariate character.Part II: see [5] 相似文献
35.
36.
37.
该文应用Galerkin方法证明人口问题中一广义 Ginzburg-Landau模型方程的时间周期问题广义时间周期解与古典时间周期解的存在性与唯一性. 相似文献
38.
39.
40.
Comprehensive 2-D GC (GC x GC) coupled with TOF MS or flame ionization detector (FID) was employed to characterize and quantify the chemical composition of volatile oil in the radixes of Panax ginseng C. A. Mey. (ginseng) at different ages. Thirty-six terpenoids were tentatively identified based on the MS library search and retention index in a ginseng sample at the age of 3 years. An obvious group-type separation was obtained in the GC x GC-TOF MS chromatogram. The data collected by GC x GC-FID were processed using a principal component analysis (PCA) method to classify the samples at different ages. The compounds responsible for the significant differentiation among samples were defined. It was found that the relative abundances of alpha-cadinol, alpha-bisabolol, thujopsene, and n-hexadecanoic acid significantly rise with the increase in age. 相似文献