全文获取类型
收费全文 | 1705篇 |
免费 | 121篇 |
国内免费 | 89篇 |
专业分类
化学 | 1418篇 |
力学 | 28篇 |
综合类 | 40篇 |
数学 | 100篇 |
物理学 | 329篇 |
出版年
2024年 | 3篇 |
2023年 | 25篇 |
2022年 | 71篇 |
2021年 | 52篇 |
2020年 | 84篇 |
2019年 | 111篇 |
2018年 | 72篇 |
2017年 | 112篇 |
2016年 | 93篇 |
2015年 | 43篇 |
2014年 | 69篇 |
2013年 | 72篇 |
2012年 | 77篇 |
2011年 | 74篇 |
2010年 | 84篇 |
2009年 | 95篇 |
2008年 | 66篇 |
2007年 | 92篇 |
2006年 | 76篇 |
2005年 | 70篇 |
2004年 | 72篇 |
2003年 | 67篇 |
2002年 | 68篇 |
2001年 | 45篇 |
2000年 | 35篇 |
1999年 | 37篇 |
1998年 | 27篇 |
1997年 | 27篇 |
1996年 | 18篇 |
1995年 | 31篇 |
1994年 | 12篇 |
1993年 | 7篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有1915条查询结果,搜索用时 437 毫秒
131.
132.
金融资产收益分布的混合高斯分析 总被引:3,自引:0,他引:3
本文研究了金融资产收益的混合高斯分布模型 ,给出了混合高斯分布的 Kolmogorov-Smirnov检验的方法 ,分析了金融资产收益的非高斯性及市场价格运动的有效性 .此外 ,用成分数目 K*、拟合误差 DK*n和主成分系数 p*k 描述金融资产收益的性质 ,对外汇银行同业拆借市场和中国股票市场实证分析 相似文献
133.
The distribution of aluminium (Al) species was investigated in the roots of Al-tolerant Chinese cabbage (Brassica rapa L. ssp. pekinensis) by employing fast protein liquid chromatography (FPLC) with inductively coupled plasma optical emission spectrometry (ICP-OES) and electrospray tandem mass spectrometry (ES-MS-MS) detection. The cabbage was exposed to a nutrient solution that contained 10 μg cm−3 of Al3+. The results demonstrated that after 24 h of exposure, Al was quantitatively taken up by the cabbage and was distributed in different parts of the plant. 36 ± 6% of total Al was located in the roots, while the remaining 64 ± 10% was transferred to the leaves. It was found that in the roots Al was partially present in the root sap (15.5%), while the majority (84.5%) was accumulated in its apoplasmic compartments. It was further demonstrated that the proportion of Al that entered the symplasm formed a complex with organic acid. Speciation analysis by FPLC with ICP-OES detection and ES-MS-MS identification of the binding ligand indicated that Al-citrate complex was the prevailing species in the root sap.The results of the present study showed that both immobilization of Al in the apoplasmic compartments of the roots and transformation of Al3+ to Al-citrate are most likely responsible for the tolerance of Chinese cabbage (B. rapa L. ssp. pekinensis) to the toxic effects of Al3+. 相似文献
134.
润滑油的压粘系数和固化压力是弹流理论应用于摩擦学工程设计的关键试验数据。本文报道了20种国产润滑油的压粘系数和其中16种润滑油的最小固化压力的测定结果,并对不同油田的基础润滑油、不同化学结构的合成润滑油和不同用途的工业润滑油的压粘系数和固化压力进行了分析比较。 相似文献
135.
A simple and rapid method was established and validated for the simultaneous quantification of 10 saponins, namely ginsenosides-Rb1, Rb2, Rb3, Rc, Rd, Rg1, Rg2, Re, Rf and Notoginsenside R1, in Chinese Shizhu Panax by ultra performance liquid chromatography coupled with an electrospray mass spectrometry (UPLC-ESI-MS). In addition, the contents of the analytes in different parts of Chinese Shizhu Panax were also analysed. The results showed that the concentration of saponins had a reference to the different parts of Chinese Shizhu Panax. The established method could be used as a new analytical approach for assessment of the quantity of Chinese Shizhu Panax. 相似文献
136.
Simultaneous chemical fingerprint and quantitative analysis of Rhizoma Smilacis Glabrae by accelerated solvent extraction and high‐performance liquid chromatography with tandem mass spectrometry 下载免费PDF全文
Weiquan Dai Weiquan Zhao Fangyuan Gao Jingjing Shen Diya Lv Yunpeng Qi Guorong Fan 《Journal of separation science》2015,38(9):1466-1475
Rhizoma Smilacis Glabrae (RSG) is a well‐known herbal medicine with the homology of medicine and food. In this study, simultaneous chemical fingerprint and quantitative analysis of the bioactive flavonoid components of RSG were developed using accelerated solvent extraction and high‐performance liquid chromatography coupled with ion trap tandem mass spectrometry. The operational parameters of accelerated solvent extraction including extraction solvent, extraction temperature, static extraction time, solid‐to‐liquid ratio, and extraction cycles were optimized. Hierarchical cluster analysis, similarity analysis, and principal component analysis were performed to evaluate the similarity and variation of the samples collected from several provinces in China. Subsequently, high‐performance liquid chromatography fingerprints were established for the discrimination of 16 batches of RSG samples, and the major six flavonoids, namely, toxifolin, neoastilbin, astilbin, neoisoastilbin, isoastilbin, and engeletin were then quantitatively determined. The calibration curves for all the six analytes showed good linearity (r2 > 0.999), and the limits of detection and quantification were less than 0.10 and 0.27 μg·mL?1, respectively. Therefore, the proposed extraction and determination methods were proved to be robust and reliable for the quality control of RSG. 相似文献
137.
Systematic chemical profiling of a multicomponent Chinese herbal formula Huo Luo Xiao Ling Dan by ultra high performance liquid chromatography coupled with electrospray ionization quadrupoletime‐of‐flight mass spectrometry 下载免费PDF全文
Fenrong Wang Yu Ai Yun Wu Wen Ma Qiaoxia Bian David Y.‐W. Lee Ronghua Dai 《Journal of separation science》2015,38(6):917-924
Huo Luo Xiao Ling Dan, a Chinese herbal formula consisting of 11 different herbs, has been used in folk medicine for the treatment of arthritis and other chronic inflammatory diseases. However, the chemical compositions of Huo Luo Xiao Ling Dan are not completely characterized. In the present study, an ultra high performance liquid chromatography coupled with electrospray ionization quadrupole time‐of‐flight mass spectrometry method in positive and negative ion modes was employed to identify biochemical constitutes in Huo Luo Xiao Ling Dan. As a result, a total of 76 compounds including alkaloids, monoterpene glycosides, iridoids, phenolic acids, and tanshinones, coumarins, lactones, flavones, and their glycosides, triterpenes, and triterpene saponins were characterized by comparing the retention time and mass spectrometry data with reference standards within 5 ppm error or by reference to the reference literature. These results would provide the basis for a further in vivo study of Huo Luo Xiao Ling Dan and information for potential new drug candidates for treating arthritis and other chronic inflammatory diseases. 相似文献
138.
Determination of hexavalent chromium in traditional Chinese medicines by high‐performance liquid chromatography with inductively coupled plasma mass spectrometry 下载免费PDF全文
Peng Li Li‐min Li Jing Xia Shuai Cao Xin Hu Hong‐Zhen Lian Shen Ji 《Journal of separation science》2015,38(23):4043-4047
An analytical method that combined high‐performance liquid chromatography with inductively coupled plasma mass spectrometry has been developed for the determination of hexavalent chromium in traditional Chinese medicines. Hexavalent chromium was extracted using the alkaline solution. The parameters such as the concentration of alkaline and the extraction temperature have been optimized to minimize the interconversion between trivalent chromium and hexavalent chromium. The extracted hexavalent chromium was separated on a weak anion exchange column in isocratic mode, followed by inductively coupled plasma mass spectrometry determination. To obtain a better chromatographic resolution and sensitivity, 75 mM NH4NO3 at pH 7 was selected as the mobile phase. The linearity of the proposed method was investigated in the range of 0.2–5.0 μg L?1 (r2 = 0.9999) for hexavalent chromium. The limits of detection and quantitation are 0.1 and 0.3 μg L?1, respectively. The developed method was successfully applied to the determination of hexavalent chromium in Chloriti lapis and Lumbricus with satisfactory recoveries of 95.8–112.8%. 相似文献
139.
Determination of neuroprotective oxysterols in Calculus bovis,human gallstones,and traditional Chinese medicine preparations by liquid chromatography with mass spectrometry 下载免费PDF全文
Yalong Wang Han Jiang Huizhi Huang Yanqi Xie Yunshi Zhao Xiuhua You Lipeng Tang Youqiong Wang Wei Yin Pengxin Qiu Guangmei Yan Haiyan Hu 《Journal of separation science》2015,38(5):796-803
So far, the components responsible for the neuroprotective effects of Calculus bovis are unclear. Cholesterol, one of the major components in Calculus bovis, is easily oxidized into oxysterols, which possess direct or indirect neuroprotective effects proved by our and others’ previous studies. Therefore, a liquid chromatography with mass spectrometry method coupled with ultrasonic extraction and solid‐phase extraction was developed for the determination of neuroprotective oxysterols in Calculus bovis, human gallstones, and traditional Chinese medicine preparations. Chromatographic separation was achieved on a C18 column with isocratic elution at a flow rate of 1 mL/min. The established method showed good linearity (R2 > 0.998), sensitivity with low limits of detection (0.06–0.39 μg/g), acceptable precisions (relative standard deviations ≤ 7.4%), stability (relative standard deviations ≤ 5.9%), and satisfactory accuracy (92.4–102.9%) for all analytes identified by different retention times, which could be applied for the determination of oxysterols. Five kinds of oxysterols proved to function as neuroprotectants were detected at different concentrations. Among them, 7β‐hydroxycholesterol and cholestane‐3β,5α,6β‐triol were rather abundant in the samples. It could be concluded that the potential neuroprotective components in Calculus bovis may be these oxysterols. 相似文献
140.
Single nucleotide polymorphisms of mitochondrial DNA HVS‐I and HVS‐II in Chinese Bai ethnic group 下载免费PDF全文
Feng Chen Cai‐Yong Yin Xiao‐Qin Qian Han‐Ting Fan Ya‐Jun Deng Yu‐Dang Zhang Hao‐Tian Meng Chun‐Mei Shen Chun‐Hua Yang Rui Jin Bo‐Feng Zhu Peng Xu 《Electrophoresis》2015,36(6):930-936
For forensic and population genetic purposes, a total of 125 unrelated volunteers’ blood samples were collected from Chinese Bai ethnic minority group to analyze sequence variation of two hypervariable segments (HVS‐I and HVS‐II) in the mitochondrial DNA control region. Comparing the HVS‐I and HVS‐II sequences of the 125 Chinese Bais to the Anderson reference sequence, we found 86 polymorphic loci in HVS‐I and 40 in HVS‐II in mitochondrial DNA sequences of the Chinese Bai ethnic minority group, which defined 93 and 53 different haplotypes, respectively. Haplotype diversity and the mean pairwise differences were 0.992 ± 0.003 and 6.553 in HVS‐I, and 0.877 ± 0.027 and 2.407 in HVS‐II, respectively. We defined four macrohaplogroups R, M, N and D with the proportions ranging from 9.6% to 40.0%. With the analysis of the hypervariable domain from nucleotide 16 180–16 193 in HVS‐I, our study revealed new haplotypes of sequence variations. In addition, the Fst metric, phylogenetic tree, and principal component analysis demonstrated a close genetic relationship between the Bai group and Chinese Han populations from South China, Changsha, and Guangdong. The results support that the Bai group is a multiorigin ethnic minority that has merged with the Chinese Han population. 相似文献