全文获取类型
收费全文 | 711篇 |
免费 | 22篇 |
国内免费 | 6篇 |
专业分类
化学 | 645篇 |
晶体学 | 1篇 |
力学 | 17篇 |
数学 | 25篇 |
物理学 | 51篇 |
出版年
2024年 | 1篇 |
2023年 | 12篇 |
2022年 | 14篇 |
2021年 | 13篇 |
2020年 | 12篇 |
2019年 | 17篇 |
2018年 | 14篇 |
2017年 | 14篇 |
2016年 | 18篇 |
2015年 | 13篇 |
2014年 | 21篇 |
2013年 | 29篇 |
2012年 | 52篇 |
2011年 | 45篇 |
2010年 | 44篇 |
2009年 | 43篇 |
2008年 | 31篇 |
2007年 | 49篇 |
2006年 | 45篇 |
2005年 | 39篇 |
2004年 | 65篇 |
2003年 | 17篇 |
2002年 | 24篇 |
2001年 | 17篇 |
2000年 | 14篇 |
1999年 | 5篇 |
1998年 | 2篇 |
1997年 | 10篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 7篇 |
1993年 | 4篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有739条查询结果,搜索用时 15 毫秒
41.
42.
Thierry Dana-Picard Aaron Naiman 《International Journal of Mathematical Education in Science & Technology》2013,44(6):865-877
Geometric constructions have previously been shown that can be interpreted as rays of light trapped either in polygons or in conics, by successive reflections. The same question, trapping light in closed Fermat curves, is addressed here. Numerical methods are used to study the behaviour of the reflection points of a triangle when the degree of the curve varies, including a generalization to non integer powers. 相似文献
43.
《Analytical letters》2012,45(3):455-463
ABSTRACT A spectrophotometric method for determination of silica in herbs and herbal pharmaceutical products has been developed and validated. The method showed excellent accuracy and precision with S.D. 1.67 and RSD 1.72%, 2.75%, respectively. The established linearity range was 0.05–0.5 μg/mL(r2 > 0.99974). The recovery of silica from spiked placebo was > 95% over the linear range. The method has been successfully used in the analysis of silica in raw material and herbal pharmaceutical formulations. This validated method proved to be rapid and less costly. 相似文献
44.
AHMED Khan MUHAMMAD Asghar MOHAMMED Yaqoob MASOOD Ahmed Siddiqui SAMAR Ali 《高等学校化学研究》2021,37(3):629-638
Asensitive and selective method employing chemiluminescence(CL) coupled with flow injection(FI) is reported for nalbuphine hydrochloride(NAL) assay in pharmaceutical formulations. The enhancement effect of NAL on the CL reaction between tris(2,2'-bipyridyl)ruthenium(II) chloride-diperiodatocuprate(III) {Ru[(bpy)3]2+-Cu(III) complex} in acidic medium is used as analytical measurement. The optimal conditions of the CL reaction were sulfuric acid 1.0×10-3 mol/L, Ru[(bpy)3]2+ 7.5×10-5 mol/L, Cu(III)/Ag(III) complexes 4.0×10-4/5.0×10-4 mol/L, sample loop volume of 120 μL and flow rate of 2.5 mL/min. The sensitivities of the method in terms of detection(S/N=3) and quantification(S/N=10) limits are 5×10-4 and 0.001 ppm(1 ppm=1 mg/L), respectively. The linear response of the instrument in the form of CL intensity with respect to NAL concentration is over the range 0.001-15.0 ppm(R2=0.9999) with relative standard deviation from 0.8% to 3.2% and injection throughput of 120 injection/h. The applications of the method include the quantitative analysis of NAL in pharmaceutical injection samples. Variations and the average results of the proposed method are not signi-ficantly different from the results of a reported method by applying F- and paired student t-test. The most likely CL reaction mechanism is written in accordance with spectrophotometric and CL studies. 相似文献
45.
《Analytical letters》2012,45(9):1683-1698
For the first time, a simple differential pulse voltammetry methodology for direct determination of benserazide in presence of levodopa in tablets was developed without any redox mediator, modified electrodes, or the aplication of mathematic deconvolution of signals. Benserazide was studied by differential pulse voltammetry using glassy carbon electrode in aqueous media. The drug exhibited a main well-defined oxidation signal in a broad pH range (2–10), and two poorly resolved signals at higher potentials. We have found that levodopa does not interfere on the electrochemical response of benserazide at pH 6.0. Thus, at this pH value, the developed analytical method exhibited adequate repeatability and reproducibility (RSD < 2%), recoveries >98.5%, which permitted its successful application to both the assay and the uniformity content of benserazide. Also, hydrolytic degradation studies of benserazide were carried out by differential pulse voltammetry. 相似文献
46.
《Analytical letters》2012,45(9):1905-1917
Abstract The determination of rifampin in the presence of its main degradation products, 3-formyl rifampin and rifampin quinone using two spectrophotometric methods is described. Both Glenn's method and first derivative spectrophotometry were successfully adopted. No preliminary separation steps were required in either cases. Both methods gave accurate and reproducible results for the determination of the drug in dosage forms. The percentage recoveries ranged from 99.33% ±0.63 to 100.2% ± 0.44. The proposed methods are more simple, rapid than other existing methods and can be readily adopted in control laboratory. 相似文献
47.
The characteristics,performance and application of membrane sensors based on ion-pair Brilliant Green mefenamate are described.The sensor’s response to the mefenamate ion has the sensitivity of (86.0±2.0) mV/pC over the range of 9×10-5-1×10-2moI/Land the detection limit of 4.5×10-5mol/L at pH 8.5-12.The sensor is easily assembled at a relatively low cost and has fast response time(5-10 s).The proposed sensor displayed good selectivity for mefenamate ion in the presence of different substances.It was used to determine mefenamic acid in pharmaceuticals. 相似文献
48.
A simple stopped-flow injection system with spectrophotometric detection was proposed for the determination of nicotinamide (NAM) in pharmaceutical formulations. In this system cyanogen chloride formed from the combination of an acidic KSCN with the NaClO streams reacts with injected NAM to form glutaconic aldehyde. Then the product of these three components was coupled with another buffered (pH 3.5) stream of barbituric acid and directed towards the detector. A 45 s after sample injection the pump was stopped for 130 s. During this time the reactants in the flow cell were provided with the required temperature (40 °C) by placing the cell in a home made cell jacket to increase the yield of the polymethine dye product. Eventually, the absorbance of the formed pink color dye was monitored spectrophotometrically at 560 nm and NAM in the concentration range of 1.0–25.0 μg/mL (R = 0.9974 and D.L = 0.5 μg/mL) was determined. The results obtained by this method were compared statistically and agree with those obtained by the method described in the British Pharmacopoeia. 相似文献
49.
Jiangman Liu 《Analytical letters》2013,46(11):1804-1815
A sensitive method for the determination of total chromium in real samples by flow injection–chemiluminescence (FI–CL) analysis was proposed. It was found that the CL intensity from luminol–lysozyme reaction could be markedly quenched, and the decrease of CL intensity was linear with the logarithm of Cr(III) concentrations over the range of 5.0 to 4000 pg mL?1 with a detection limit of 2.0 pg mL?1 (3σ) and relative standard deviations (RSDs) of 3.0, 2.6, and 2.0% for 10, 100, and 1000 pg mL?1 Cr(III) (n = 7), respectively. At a flow rate of 2.0 mL min?1, the whole process including sampling and washing could be accomplished within 36 s. The proposed CL method was successfully applied to the determination of total chromium in pharmaceutical capsules, a dietary supplement, and spiked human serum samples, with recoveries from 92.2 to 108.4% and RSDs of less than 4.0%. Using the homemade FI–CL model, the binding constant (K = 4.38 × 106 L mol?1) and the binding sites (n ≈ 1) of Cr(III) to lysozyme were given. 相似文献
50.
Armand Gellis Charline Kieffer Nicolas Primas Gilles Lanzada Michel Giorgi Pierre Verhaeghe Patrice Vanelle 《Tetrahedron》2014
We report herein a new methodology for synthesizing quinazoline derivatives bearing a heteroarylamino moiety at position-4 of the quinazoline ring. As an alternative to the Buchwald–Hartwig cross-coupling reaction, which appears, until now, as the only efficient way to react 4-chloroquinazolines with numerous amino nitrogen-containing heterocycles displaying poor nucleophilicity, we developed a DMAP-catalyzed reaction involving microwave irradiation. Optimization of the reaction conditions led to the use of 30 mol % of DMAP in toluene, using a monomode microwave reactor and sealed vials. Moreover, the SNAr reaction intermediate salt was isolated and fully characterized. Finally, the procedure was extended to two different 2-substituted-quinazoline series and also to various anilines, demonstrating that this approach was a general efficient way to access to such 4-substituted quinazoline scaffolds of high pharmaceutical interest. 相似文献