全文获取类型
收费全文 | 1660篇 |
免费 | 122篇 |
国内免费 | 147篇 |
专业分类
化学 | 1430篇 |
力学 | 35篇 |
综合类 | 39篇 |
数学 | 99篇 |
物理学 | 326篇 |
出版年
2024年 | 3篇 |
2023年 | 28篇 |
2022年 | 103篇 |
2021年 | 61篇 |
2020年 | 99篇 |
2019年 | 131篇 |
2018年 | 77篇 |
2017年 | 108篇 |
2016年 | 91篇 |
2015年 | 43篇 |
2014年 | 66篇 |
2013年 | 70篇 |
2012年 | 69篇 |
2011年 | 69篇 |
2010年 | 80篇 |
2009年 | 94篇 |
2008年 | 64篇 |
2007年 | 88篇 |
2006年 | 74篇 |
2005年 | 67篇 |
2004年 | 65篇 |
2003年 | 66篇 |
2002年 | 60篇 |
2001年 | 44篇 |
2000年 | 33篇 |
1999年 | 34篇 |
1998年 | 26篇 |
1997年 | 24篇 |
1996年 | 17篇 |
1995年 | 29篇 |
1994年 | 11篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1990年 | 2篇 |
1989年 | 2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1929条查询结果,搜索用时 36 毫秒
131.
研究矿物中药钟乳石X射线衍射Fourier指纹图谱.对10批主产地的钟乳石药材X射线衍射Fourier图谱进行分析,建立钟乳石X射线衍射Fourier指纹图谱,并通过相似度计算,区分钟乳石、其他含钙矿物中药、同种化学试剂CaCO3.主成分相同的矿物中药利用X射线衍射Fourier指纹图谱无法完全区分,但钟乳石与同种化学试剂CaCO3的X射线衍射Fourier图谱区别显著.利用X射线衍射Fourier指纹图谱可以很好的区分含有不同化学成分的矿物药,而对于主成分相同的矿物中药,就不能简单地利用它们的X射线衍射Fourier指纹图谱加以区分,即X射线衍射Fourier指纹图谱仅具有相对的专属性. 相似文献
132.
Differentiating parts of Cinnamomum cassia using LC‐qTOF‐MS in conjunction with principal component analysis 下载免费PDF全文
Pei‐Yi Chen Jhe‐Wei Yu Fen‐Ling Lu Mei‐Chih Lin Hwei‐Fang Cheng 《Biomedical chromatography : BMC》2016,30(9):1449-1457
Cinnamon bark (Rou Gui in Chinese), cinnamon twig (Gui Zhi) and shaved cinnamon bark (Gui Sin) have been widely used as spices and in traditional Chinese medicine since ancient times. On‐going issues related to quality and authenticity necessitate the development of analytical methods capable of providing an objective evaluation of samples. In this study, chemical fingerprints of cinnamon bark, cinnamon twigs and shaved cinnamon bark were established using liquid chromatography quadruple time‐of‐flight mass spectrometry in conjunction with principal component analysis (PCA). From 125 samples of cinnamon, we identified the following eight compounds and their the detection ratios: coumarin, cinnamaldehyde, cinnamyl alcohol, cinnamic acid, 2‐hydroxycinnamaldehyde, 2‐hydroxycinnamic acid, 2‐methoxycinnamaldehyde and 4‐methoxycinnamaldehyde. Of these, 4‐methoxycinnamaldehyde presented the largest variations in detection ratio, making up 64.0, 97.4 and 50.0% in cinnamon bark, cinnamon twig, and shaved cinnamon bark, respectively. The quantities of cinnamyl alcohol, coumarin and cinnamaldehyde also varied between the three parts of the plant. Chemical fingerprints of the three cinnamon samples were established using principal component analysis, the results of which indicate that cinnamon bark and shaved cinnamon bark could be easily differentiated, despite a marked similarity in outward appearance. Cinnamon twig was also shown to depart from the other clusters. The proposed method provides a fast and efficient means of identifying cinnamon herbs for quality control purposes. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
133.
134.
A new proof of the characterization of the Chinese postman polyhedra is given. In developing this proof, a theorem of Gomory about homomorphic lifting of facets for group polyhedra is generalized to subproblems. Some results for the Chinese postman problem are generalized to binary group problems. In addition, a connection is made between Fulkerson's blocking polyhedra and a blocking pair of binary group problems. A connection is also developed between minors and lifting of facets for group problems. 相似文献
135.
Determination of rutin and forsythin in fruit ofForsythia suspensa(Thunb.) Vahl by capillary electrophoresis-electrochemical detection 总被引:1,自引:0,他引:1
Summary Capillary electrophoresis-amperometric detection is evaluated for simultaneous determination of rutin and forsythin. The cyclic
voltammogram, hydrodynamic voltammogram, effect of pH, buffer concentration and SDS, and percent organic modifier on separation
and detection were studied. Conditions were optimized as follows: 1.2 V detection potential; separation at 12 kV; 5 s at 15
kV for sample injection time and sample injection voltage; mobile phase 20 mM boric acid buffer; pH 8.4, containing 40 mM
SDS and 10% (v/v) acetontrile. The method gave low detection limit as 0.001 mg mL−1 and 0.0005 mg mL−1 (S/N=3), wide linear range 0.005–0.5 mg mL−1 for rutin and forsythin, respectively. The relative standard deviations of peak current and migration time for 8 consecutive
injections of the standard solution containing 0.1 mg mL−1 each compound were 4.78%, 3.63% and 6.40%, 2.95% for rutin and forsythin, respectively. In addition, levels of the two compounds
in traditional Chinese herbal drugs were easily determined. 相似文献
136.
Suspension culture is an essential large-scale cell culture technique for biopharmaceutical development and regenerative medicine. To transition from monolayer culture on the culture surface of a flask to suspension culture in a bioreactor, a pre-specified cell number must first be reached. During this period of preparation for suspension culture, static suspension culture in a flask is generally performed because the medium volume is not large enough to use a paddle to circulate the medium. However, drawbacks to this static method include cell sedimentation, leading to high cell density near the bottom and resulting in oxygen and nutrient deficiencies. Here, we propose a suspension culture method with acoustic streaming induced by ultrasonic waves in a T-flask to create a more homogeneous distribution of oxygen, nutrients, and waste products during the preparation period preceding large-scale suspension culture in a bioreactor. To demonstrate the performance of the ultrasonic method, Chinese hamster ovary cells were cultured for 72 h. Results showed that, on average, the cell proliferation was improved by 40% compared with the static method. Thus, the culture time required to achieve a 1000-fold increase could be reduced by 32 h (a 14% reduction) compared with the static method. Furthermore, the ultrasonic irradiation did not compromise the metabolic activity of the cells cultured using the ultrasonic method. These results demonstrate the effectiveness of the ultrasonic method for accelerating the transition to large-scale suspension culture. 相似文献
137.
More than 60% commonly used pharmaceutical active ingredients are chiral compounds. Developing more effective and safe chiral compounds has become a focus in the pharmaceutical industry. Chiral compounds widely exist in traditional Chinese medicine and include alkaloids, flavonoids, volatile oils, and amino acids. The characterization of chiral compounds used in traditional Chinese medicine remains limited. Here, the characterization of chiral compounds commonly used in traditional Chinese medicine is reviewed focusing upon their separation and pharmacology. 相似文献
138.
五种中药中微量元素及重金属元素的含量分析 总被引:3,自引:0,他引:3
用微波消解-原子吸收分光光度法对巴戟天、大黄、陈皮、板蓝根、茯苓等5种中药材中的7种金属微量元素进行了含量分析。结果表明,该法的线性良好,重现性在3.3%~11%之间,精密度在2.6%以内,回收率满足定量分析要求;5种药材中铅均未超标,茯苓中镉超出国家标准;5种药材里的铜含量均有超标,其中,巴戟天超标最严重;铁在5种药材中的含量最高,其中以巴戟天含量最为丰富,达到了746.750 mg/kg,其次是锌和锰,铬的含量最少。结论:对5种药材中铜均出现超标,虽然在边缘位置,但应该予以重视;巴戟天中丰富的铁元素,可能在防止癌变发生和发展中起到一定功效。 相似文献
139.
陈德荣 《广东微量元素科学》2010,17(1):57-66
通过对各种化学元素、化合物在人体中的作用与影响作出分析与总结,从现代科学的角度直观地解释了中医学中如阴虚、阳虚、正、邪、湿、热等的多个抽象概念和理论。为研究抽象而深奥难懂的中医学概念和理论提供了一个新的思路,使之转化为具体而直观的符合现代科学概念的理论。 相似文献
140.
Ionic liquid-based microwave-assisted extraction of rutin from Chinese medicinal plants 总被引:1,自引:0,他引:1
An ionic liquid-based microwave-assisted extraction (ILMAE) method has been developed for the effective extraction of rutin from Chinese medicinal plants including Saururus chinensis (Lour.) Bail. (S. chinensis) and Flos Sophorae. A series of 1-butyl-3-methylimidazolium ionic liquids with different anions were investigated. The results indicated that the characteristics of anions have remarkable effects on the extraction efficiency of rutin and among the investigated ionic liquids, 1-butyl-3-methylimidazolium bromide ([bmim]Br) aqueous solution was the best. In addition, the ILMAE procedures for the two kinds of medicinal herbs were also optimized by means of a series of single factor experiments and an L9 (34) orthogonal design. Compared with the optimal ionic liquid-based heating extraction (ILHE), marinated extraction (ILME), ultrasonic-assisted extraction (ILUAE), the optimized approach of ILMAE gained higher extraction efficiency which is 4.879 mg/g in S. chinensis with RSD 1.33% and 171.82 mg/g in Flos Sophorae with RSD 1.47% within the shortest extraction time. Reversed phase high performance liquid chromatography (RP-HPLC) with ultraviolet detection was employed for the analysis of rutin in Chinese medicinal plants. Under the optimum conditions, the average recoveries of rutin from S. chinensis and Flos Sophorae were 101.23% and 99.62% with RSD lower than 3%, respectively. The developed approach is linear at concentrations from 42 to 252 mg L−1 of rutin solution, with the regression coefficient (r) at 0.99917. Moreover, the extraction mechanism of ILMAE and the microstructures and chemical structures of the two researched samples before and after extraction were also investigated. With the help of LC-MS, it was future demonstrated that the two researched herbs do contain active ingredient of rutin and ionic liquids would not influence the structure of rutin. 相似文献