全文获取类型
收费全文 | 4777篇 |
免费 | 177篇 |
国内免费 | 255篇 |
专业分类
化学 | 4640篇 |
晶体学 | 6篇 |
力学 | 235篇 |
综合类 | 14篇 |
数学 | 46篇 |
物理学 | 268篇 |
出版年
2024年 | 1篇 |
2023年 | 61篇 |
2022年 | 80篇 |
2021年 | 73篇 |
2020年 | 105篇 |
2019年 | 105篇 |
2018年 | 83篇 |
2017年 | 122篇 |
2016年 | 103篇 |
2015年 | 100篇 |
2014年 | 98篇 |
2013年 | 192篇 |
2012年 | 190篇 |
2011年 | 195篇 |
2010年 | 176篇 |
2009年 | 203篇 |
2008年 | 158篇 |
2007年 | 258篇 |
2006年 | 258篇 |
2005年 | 288篇 |
2004年 | 215篇 |
2003年 | 182篇 |
2002年 | 141篇 |
2001年 | 113篇 |
2000年 | 123篇 |
1999年 | 134篇 |
1998年 | 150篇 |
1997年 | 124篇 |
1996年 | 111篇 |
1995年 | 88篇 |
1994年 | 78篇 |
1993年 | 61篇 |
1992年 | 52篇 |
1991年 | 59篇 |
1990年 | 55篇 |
1989年 | 51篇 |
1988年 | 73篇 |
1987年 | 68篇 |
1986年 | 73篇 |
1985年 | 107篇 |
1984年 | 81篇 |
1983年 | 40篇 |
1982年 | 65篇 |
1981年 | 43篇 |
1980年 | 31篇 |
1979年 | 30篇 |
1978年 | 10篇 |
1977年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有5209条查询结果,搜索用时 15 毫秒
91.
As an extension of previous reports, the barium carbonate procedure has been optimized in detail for the preparation of apolar columns. The aim was to produce optimum overall column characteristics, and to maintain them unchanged under the prolonged influence of the highest possible temperature. The main parameters under optimization were glass variety, leaching of glass surface with aqueous HCl, and amount of barium carbonate deposited, while deactivation and coating were kept constant. The basic column characteristics were adsorption properties and thermostability of deactivation, acid/base behaviour and separation efficiency. They were determined by a new, quantitative testing procedure. Intense leaching was able to eliminate almost totally the differences between glass varieties and to create a well-defined glass surface. While untreated glass, leached glass, and barium carbonate treated glass showed specific weak points in the respective column quality, the combination of leaching and barium carbonate treatment yielded the highest and most stable quality. Some technical modifications of the preparation procedure are described, including deactivation in the gas phase, and use of pentane as a solvent for static coating. 相似文献
92.
Summary The feasibility of using columns prepared by mixing together ion exchange and reversed-phase packing materials, for the simultaneous separation of ionised and unionised molecules, has been examined using a range of test solutes. Columns prepared in this way showed chromatographic properties which were intermediate between those of the individual phases. The dual nature of the retention mechanism allowed the retention of ionisable molecules to be adjusted using pH whilst that of uncharged compounds was unaffected. The simultaneous chromatography of model compounds and their glucuronide, sulphate and glycine conjugates, under a variety of conditions, was demonstrated. 相似文献
93.
Summary The capabilities of sulfur hexafluoride as a mobile phase for supercritical-fluid chromatography are investigated. An evaluation
of its overall utility on the basis of separations of standard aromatic hydrocarbon odel mixtures performed on a variety of
bonded-phase, packed columns with UV detection is presented. The dependence of separation performance upon operational parameters
is also examined. A comparative evaluation of the chroamtographic properties of supercitrical sulfur hexafluoride and those
of supercritical carbon dioxide is developed from these separations under corresponding supercritical state conditions. 相似文献
94.
In sample-stacking techniques, the detection limit cannot be improved by simply increasing the length of the sample solution, because the individual electrophoretic parameters must be optimized. In an attempt to increase the amount of sample injected, as well as to focus them onto a small zone, two novel methods are proposed. One of these employs an "ultra-high conductivity zone", which was inserted between the sample zone and background solution to build an unequal conductivity gradient. The other employs a "low temperature bath". A portion of the capillary (near the junction between the sample solution and the background solution) was immersed in a low temperature bath, which served as a "pseudo-high-conductivity zone" due to the fact that conductivity would increases when the temperature is decreased. As a result, a large volume of sample injection can be achieved. Using 3,4-methylenedioxymethamphetamine as a model compound, the detection limit was determined to be 1.6 x 10(-6) M (S/N = 3) by means of normal non-aqueous capillary electrophoresis (NACE). This could be improved to 3.0 x 10(-8) M, 4.8 x 10(-9) M and 5.0 x 10(-9) M, respectively, when the normal stacking, ultra-high conductivity zone NACE-stacking and the low-temperature zone NACE-stacking methods were applied. 相似文献
95.
Summary Flexibility, strength and adsorption inertness of fused-silica capillaries permits their extensive application for the preparation
of micro-packed columns in gas chromatography. Decreasing the column diameter (from 0.5 to 0.15 mm) and the diameter of the
sorbent particles (from 100 to 5 μm) results in a marked reduction of the height equivalent to a theoretical plate (HETP),
as well as in diminishing the dependence of the HETP on the carrier gas flow rate.
The chromatographic characteristics of fused-silica capillary micro-packed columns and open-tubular columns are compared.
The fused-silica capillary micro-packed column can be used to advantage for performing rapid and trace analyses and have been
shown to be adapted for application in gas-solid chromatography. Separation of organic and inorganic compounds on fused-silica
capillary micro-packed columns is illustrated by practical examples.
Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984 相似文献
96.
Summary The retention of 22 ring-substituted phenol derivatives on porous graphitized carbon (PGC) (eluents: acetonitrile — water and methanol — water mixtures) and on octadecylsilica (ODS) (eluents: methanol — 0.025 M KH2PO4 mixtures) was determined, and the relationship between retention and physicochemical parameters were evaluated by principal component analysis followed by two-dimensional nonlinear mapping and by cluster analysis as well as by canonical correlation analysis. Calculations proved that marked differences can be detected between the retention characteristics of PGC and ODS columns, and the electronic parameters of phenol derivatives have the highest impact on their retention. The comparison of various multivariate mathematical-statistical methods indicated that principal component analysis followed by two dimensional non-linear mapping is the most appropriate method for the evaluation of large data matrices in RP-HPLC. 相似文献
97.
František Kvasnička Jaroslav Dobiáš Kamila Klaudisová-Chudáčková 《Central European Journal of Chemistry》2003,1(1):91-97
A simple, rapid and reproducible capillary isotachophoretic on-line coupled with capillary zone electrophoresis (CITP-CZE)
method for the determination of IMz in food packaging extracts and its residues in apples is described. A good separation
of the IMZ from other sample constituents was achieved within 15 minutes without any sample clean up. Method characteristics
(linearity, accuracy, intra-assay and detection limit) were determined. Less amount of time involved, sufficient sensitivity
and low running cost are the important attributes of CITP-CZE method. 相似文献
98.
De-Qi Yuan Yuewei Zhao Makoto Fukudome Kahee Fujita 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):125-129
One or two coumarin units were incorporated to the primary face of γ-cyclodextrin (γ-CD), and the resultant coumarin derivatives
were employed to harvest the chemical energy generated in the reaction of bis(trichlorophenyl)oxalate with hydrogen peroxide.
In comparison with the coumarin without CD cavity for molecular recognition, the coumarin–CD conjugates demonstrated much
higher chemiluminescence inetensity, indicating that the CD moiety remarkably improves the chemical energy transfer. 相似文献
99.
Surface modification of soft-glass capillaries for gas chromatography by treatment with water vapour
“Leaching” or “etching” by strong mineral acids seems to be a necessary pretreatment step for the most commonly used deactivation procedures of glass capillaries by reaction with either polyethylene glycol or silylation reagents. The acidic sites which are formed on the surface during this acid treatment cannot be completely removed by the subsequent deactivation process. This drawback can be overcome by performing the leaching with water vapour, resulting in an accumulation of cations at the surface and a decrease in the number of silanol groups. Capillaries of this type show excellent properties for the chromatography of strongly basic compounds. After the wash-out of the alkaline surface layer, the acidity of the support is suited for the chromatography of strongly basic as well as strongly acidic compounds. Due to a lack of reactive acidic sites, special deactivation procedures have to be applied to capillaries produced in this way. 相似文献
100.
Summary Diaromatics are geochemically significant constituents of crude oils. Their determination is usually achieved by elaborate prefractionation methods, such as medium pressure liquid chromatography and HPLC, prior to capillary gas chromatography. The present contribution describes the quantitative analysis of methylnaphthalenes, ethylnaphthalenes, and dimethylnaphthalenes in selected crude oils by two-dimensional capillary GC. Since the method does not comprise any work-up procedure the determination of geochemical parameters (alkylnaphthalene concentration ratios) is performed on the original, untreated crude oil samples. Accordingly, the analytical results reflect the original composition. The influence by evaporational losses in the laboratory is minimized. 相似文献