全文获取类型
收费全文 | 1007篇 |
免费 | 66篇 |
国内免费 | 17篇 |
专业分类
化学 | 572篇 |
晶体学 | 1篇 |
力学 | 27篇 |
综合类 | 1篇 |
数学 | 186篇 |
物理学 | 303篇 |
出版年
2024年 | 1篇 |
2023年 | 14篇 |
2022年 | 29篇 |
2021年 | 25篇 |
2020年 | 16篇 |
2019年 | 25篇 |
2018年 | 20篇 |
2017年 | 18篇 |
2016年 | 45篇 |
2015年 | 40篇 |
2014年 | 56篇 |
2013年 | 58篇 |
2012年 | 45篇 |
2011年 | 59篇 |
2010年 | 41篇 |
2009年 | 66篇 |
2008年 | 66篇 |
2007年 | 84篇 |
2006年 | 61篇 |
2005年 | 29篇 |
2004年 | 43篇 |
2003年 | 36篇 |
2002年 | 26篇 |
2001年 | 22篇 |
2000年 | 31篇 |
1999年 | 37篇 |
1998年 | 21篇 |
1997年 | 29篇 |
1996年 | 15篇 |
1995年 | 6篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1979年 | 2篇 |
排序方式: 共有1090条查询结果,搜索用时 15 毫秒
101.
Lívia Riberti Rodrigues Diogo Noin de Oliveira Mônica Siqueira Ferreira Rodrigo Ramos Catharino 《Analytica chimica acta》2014
The analysis of impurities and degradation products in pharmaceutical preparations are usually performed by chromatographic techniques such as high-performance liquid chromatography (HPLC). This approach demands extensive analysis time, mostly due to extraction and separation phases. These steps must be carried out in samples in order to adapt them to the requirements of the analytical method of choice. In the present contribution, matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) was employed to quantify an important degradation product in atorvastatin calcium 80 mg tablets: the atorvastatin lactone. Through the standard of the impurity, it was possible to perform quantitative analysis directly on the drug tablet, using a quick and novel approach, suitable for quality control processes in the pharmaceutical industry. 相似文献
102.
Guidelines ISO 17025 and ISO 15189 aim to improve the quality-assurance scheme of laboratories. Reliable analytical results are of central importance due to the critical decisions that are taken with them. ISO 17025 and ISO 15189 therefore require that analytical methods be validated and that laboratories can routinely provide the measurement uncertainty of the results of measurements. To evaluate the fitness of purpose of analytical methods, total error is increasingly applied to assess the reliability of results generated by analytical methods. However, the ISO requirement to estimate measurement uncertainty seems opposed to the concept of total error, leading to delays in laboratories implementing ISO 17025 and ISO 15189 and confusion for the analysts. This article therefore aims to clarify the divergences between total error and measurement uncertainty, but also to discuss their main similarities and emphasize their implementation. 相似文献
103.
Sampling and analytical framework for routine environmental monitoring of organic pollutants 总被引:1,自引:0,他引:1
Hong-Gang NiHui Zeng 《Trends in analytical chemistry : TRAC》2011,30(10):1549-1559
Dubious data would lead to incorrect interpretations and consequently faulty conclusions. Environmental monitoring results therefore have to be unambiguous to avoid misunderstanding the problems under investigation. Representative sampling and appropriate laboratory procedures are keys to acquiring quality data in order to draw unbiased conclusions.Although a large number of studies on organic pollutants have been published, few efforts have been directed towards instituting a systematic framework from sampling design to instrumental analysis. Generally, the main components in such a framework should include sampling design, sample preparation, sample extraction, extract purification and fractionation, and quantification (including qualitative and quantitative analyses).This review outlines the sampling and analytical framework appropriate for routine monitoring of organic pollutants, particularly persistent organic pollutants widely occurring in the environment. We emphasize statistically-based sampling schemes and quality-assurance and quality-control measures desirable for environmental monitoring programs.By way of demonstrating their importance, we especially review procedures for collecting unconventional environmental samples (e.g., human blood, breast milk, human hair, fish and bird tissues, and ice and snow) and analytical methods for typical emerging organic chemicals. 相似文献
104.
B. Roig I. DelplaE. Baurès A.V. Jung O. Thomas 《Trends in analytical chemistry : TRAC》2011,30(8):1243-1251
Heavy rainfall events, increasing in frequency and intensity with climate change, impact on the quality of the water resource used for drinking-water production. Small-scale water suppliers are particularly sensitive because of their management and the related difficulties of adapting treatment to variations. Decision-support systems, based on monitoring and analytical tools, need to be developed to improve crisis-management procedures related to such events. After presenting the issues related to heavy rainfall events, the article summarizes the tools currently used for quality control of drinking water within this framework, the need for developments and other requirements. 相似文献
105.
A novel method combining high performance liquid chromatography (HPLC) fingerprint and simultanous quantitative analysis of multiple acitve components was developed and validated for quality evaluation of one type of traditional Chinese medicine preparations: Shuang-huang-lian (SHL) oral liquid formulation. For fingerprint analysis, 45 peaks were selected as the common peaks to evaluate the similarities among several different SHL oral liquid preparations collected from manufacturers. Additionally, simultanous quantification of eleven markers, including chlorogenic acid, caffeic acid, rutin, forsythiaside, scutellarin, baicalin, forsythin, luteoloside, apigenin, baicalein and wogonin, was performed. Statistical analysis of the obtained data demonstrated that our method has achieved desired linearity, precision and accuracy. Finally, concentrations of these eleven markers in SHL oral liquid prepared by different manufacturers in China were determined. These results demonstrated that the combination of HPLC chromatographic fingerprint and simultaneous quantification of multi-ingredients offers an efficient and reliable approach for quality evaluation of SHL oral liquid preparations. 相似文献
106.
A new HPLC-DAD-ESI-MS(n) method was developed for rapid separation, characterization and quantitation of flavonoids in Epimedium wushanense, a popular Chinese herbal medicine. For qualitative identification, a total of 37 compounds were characterized from the underground and aerial parts of E. wushanense. Among them, 28 compounds were prenylated flavonoids, and 23 were confirmed by comparing with reference standards. For quantitative analysis, 12 major flavonoids including kaempferol glycosides, desmethylicaritin glycosides, and icaritin glycosides were simultaneously determined by HPLC/UV. Samples were separated on a Waters Symmetry C(18) column at 35 °C eluted with a gradient three-component mobile phase of acetonitrile, methanol, and water containing 0.03% v/v formic acid. All the flavonoids showed good linearity (r(2) ≥0.9997). The recoveries varied from 92.6 to 106.1% at three concentration levels. This method was applied to the determination of 20 samples of different geographical sources, harvesting time, and plant parts. Contents of the predominant flavonoid, epimedin C, ranged from 1.4 to 5.1% in aerial parts and 1.0 to 2.8% in underground parts. The methods established in this paper were simple and reliable and could be used for the quality control of E. wushanense. 相似文献
107.
Mollerup JM 《Journal of chromatography. A》2011,1218(49):8869-8873
Oligomer forms of proteins are formed by self and cross association or complex formation with ligands. Modelling studies using an ion-exchange adsorbent have demonstrated that the formation of an oligomer form of a target protein can improve a chromatographic separation because the oligomer form displaces the impurities. The results of the investigation show that the process is very robust, that the purity, the yield, and the productivity increase with increasing load and increasing salt concentration in the eluant. An impurity level less than 1 ppm is easy to achieve having a yield of 98%. A universal model for the adsorption equilibria of mono and oligomer forms of proteins on ion-exchange, hydrophobic, and bimodal adsorbents has been developed. 相似文献
108.
109.
Protection and restoration are critical network design issues for optical networks since even a single failure for a short duration may result in huge data loss due to the large capacity of optical fibers. However, few studies have been done on these issues for optical burst switching (OBS) networks. Protection and restoration are essential mechanisms for guaranteeing more reliable traffic delivery services. But it is not easy to apply existing mechanisms to optical burst switching (OBS) networks due to its one-way reservation signaling and the statistical burst multiplexing. Thus, to achieve the high transmission performance and reliability simultaneously, unique properties of OBS must be considered in the design of protection scheme. In this paper, an optimal protection and restoration scheme (OPARS) has been introduced that not only optimizes the number of provisioned protection wavelengths adaptively based on the traffic load as well as the quality of service (QoS) requirements of bursts in high speed networks but also minimizes burst loss rates. In addition, the proposed scheme has been used as an efficient contention resolution technique. The simulation results verify that the proposed scheme improve the network resource and channel utilization while guaranteeing the targeted protection reliability and QoS requirements of bursts. 相似文献
110.
In this paper we study the variability reduction gain that results from the use of multiweighing proceedings in packing processes
with the objective of sales at constant nominal weights. According to this, we have proposed a packing algorithm that includes
the formation of units with variable number of pieces and we have obtained the value of the percentage variability reduction
index.
Collaborator in the Catalan Institute of Tecnology (ICT). 相似文献