全文获取类型
收费全文 | 1034篇 |
免费 | 28篇 |
国内免费 | 100篇 |
专业分类
化学 | 997篇 |
晶体学 | 4篇 |
力学 | 34篇 |
综合类 | 1篇 |
数学 | 46篇 |
物理学 | 80篇 |
出版年
2023年 | 8篇 |
2022年 | 12篇 |
2021年 | 7篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 9篇 |
2017年 | 12篇 |
2016年 | 23篇 |
2015年 | 32篇 |
2014年 | 25篇 |
2013年 | 55篇 |
2012年 | 22篇 |
2011年 | 69篇 |
2010年 | 42篇 |
2009年 | 75篇 |
2008年 | 59篇 |
2007年 | 61篇 |
2006年 | 40篇 |
2005年 | 57篇 |
2004年 | 47篇 |
2003年 | 22篇 |
2002年 | 35篇 |
2001年 | 26篇 |
2000年 | 28篇 |
1999年 | 30篇 |
1998年 | 17篇 |
1997年 | 29篇 |
1996年 | 33篇 |
1995年 | 36篇 |
1994年 | 29篇 |
1993年 | 32篇 |
1992年 | 12篇 |
1991年 | 31篇 |
1990年 | 15篇 |
1989年 | 17篇 |
1988年 | 30篇 |
1987年 | 16篇 |
1986年 | 20篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1980年 | 1篇 |
1978年 | 8篇 |
排序方式: 共有1162条查询结果,搜索用时 31 毫秒
11.
Summary This paper reports the qualitative and quantitative effects of the column pressure drop on the retention of lipid components in a serially coupled capillary column SFC system. The contribution of the pressure drop consists of two components, the density effect and the flow effect. The magnitude of the flow effect,i. e. the change in retention which results from changes in the flow-rate when column pressures are changed, is determined by the difference in single column analyte k values. The effect will be positive compared with the uncorrected retention values when the column with largest k value is closest to the injector. With the columns in reversed order, the effect will be negative. The contribution from the density effect always resulted in larger coupled column k values and was in most instances of more significance than the flow effect component. Values calculated with and without pressure drop correction have been compared and it has been shown that for most of the eighteen model lipid compounds investigated, the deviations from the experimental retention factors were smaller when pressure drop corrections were made. 相似文献
12.
Summary The application of capillary SFC for SIMDIST investigations of paraffins and waxes is reported. Pressure, density and temperature of the mobile phase are optimized to obtain high chromatographic resolution of complex mixtures of industrial products. Isothermal linear pressure programming and asymptotic pressure and density programming are used in the range from 15 to 35 MPa in the isothermic mode above 100°C. SFC chromatograms of natural and synthetic paraffins, microcrystalline wax and candle wax are compared. The retention behaviour of paraffin mixtures and waxes has been investigated on methyl/phenyl and biphenyl capillary columns. The polarity of the stationary phases influences the absolute retention time more than it influences the chromatographic resolution. 相似文献
13.
The performance of single-drop microextraction (SDME), coupled with gas chromatography/mass spectrometry, was assessed for the determination of tributyltin compounds in water and solid samples. Experimental parameters impacting the performance of SDME, such as microextraction solvent and sampling and stirring time, were investigated. Analytical results obtained by SDME were compared with those generated by conventional solid phase microextraction (SPME) and liquid-liquid extraction (LLE) for the determination of TBT in PACS-2 sediment certified reference material (CRM). 相似文献
14.
15.
16.
17.
A method combining the techniques of liquid – solid disk extraction (LSDE) and supercritical fluid elution (SFE) has been developed for the phenols regulated by the Clean Water Act. LSDE uses a disk or membrane made of polytetrafluoroethylene (PTFE) fibrils impregnated with small particles, e.g. styrene divinylbenzene (SDB) resin, to extract phenols from water. After disk extraction the retained analytes are eluted from the disk using SFE. SFE is used as an alternative to liquid solvent elution with an organic solvent. Analytes are separated, identified, and quantified using gas chromatography – ion trap detector mass spectrometry (GC-ITDMS). The method is capable of sub parts per billion detection limits, and precision of 5–28% RSD. Evaluation of various disks or membranes, such as C18-silica disks, SDB disks, and ion exchange membranes, has also been performed for the extraction of phenols from water. The results obtained from the in-situ aqueous acetylation of phenols and extraction of their acetates are quantitative. The utilization of LSDE and SFE techniques has proven to be a more effective approach than liquid – liquid extraction in minimizing air pollution and solvent waste. 相似文献
18.
Supercritical fluid extraction (SFE) of the fluoroquinolones norfloxacin and ofloxacin from chicken breast muscles was examined. A liquid chromatography with fluorescence detection was used for the determination of the fluoroquinolones. Extraction conditions of the SFE were optimized by determining the extraction parameters to achieve a sufficiently high recovery of each fluoroquinolone in fortified-muscle samples. Recovery values for the extraction of the fluoroquinolones using the SFE ranged from 70 to 87%. Chickens were treated orally with each fluoroquinolone and their muscles were extracted at set time intervals for time-course determination of the fluoroquinolones in chickens. The SFE combined with liquid chromatographic analysis showed that the concentrations of the fluoroquinolones decreased gradually with time in the chicken muscles after oral treatment, giving a concentration less than 5 ng/ml in 120 h. No further sample cleanup procedures were required after the SFE. These results suggest that SFE method is an extraction method for the determination of norfloxacin and ofloxacin in chicken muscle. 相似文献
19.
The kinetic analysis of the degradation of polystyrene (PS) in supercritical acetone has been studied using the nonisothermal weight loss technique with heating rates of 3, 5 and 7 °C/min. The weight loss data according to degradation temperature have been analyzed using the integral method based on Arrhenius form to obtain the kinetic parameters such as apparent activation energy and overall reaction order. The kinetic parameters obtained from this work were also compared with those of the thermal degradation of PS in nitrogen atmosphere. From this work, it was found that the activation energies of PS degradation in supercritical acetone were 73.3-200.7 kJ/mol and lower than those of the thermal degradation in nitrogen atmosphere. 相似文献
20.
Density of supercritical CO_2-tetrahydrofuran binary mixture and the partial molar volume of the cosolvent 总被引:1,自引:0,他引:1
The densities of supercritical CO2-tetrahydrofuran (cosolvent) binary mixture weremeasured at temperatures in range of 308.15 to 323.15 K and at pressure up to 16.5 MPa.The concentrations of tetrahydrofuran were from 0 to 0.57 mol/L.The partial molar volume of tetrahydrofuran was calculated based on the relationship between the density of the mixture and the concentration of the cosolvent.It is observed that the partial molar volume of the cosolvent is negative and the absolute value decreases with increasing pressure and the concentration of the cosolvent. 相似文献