全文获取类型
收费全文 | 1234篇 |
免费 | 42篇 |
国内免费 | 6篇 |
专业分类
化学 | 727篇 |
晶体学 | 14篇 |
力学 | 44篇 |
数学 | 248篇 |
物理学 | 249篇 |
出版年
2021年 | 13篇 |
2020年 | 13篇 |
2019年 | 13篇 |
2018年 | 16篇 |
2017年 | 15篇 |
2016年 | 24篇 |
2015年 | 32篇 |
2014年 | 42篇 |
2013年 | 65篇 |
2012年 | 62篇 |
2011年 | 74篇 |
2010年 | 44篇 |
2009年 | 51篇 |
2008年 | 47篇 |
2007年 | 70篇 |
2006年 | 68篇 |
2005年 | 52篇 |
2004年 | 62篇 |
2003年 | 48篇 |
2002年 | 52篇 |
2001年 | 32篇 |
2000年 | 23篇 |
1999年 | 17篇 |
1998年 | 18篇 |
1997年 | 13篇 |
1996年 | 23篇 |
1995年 | 18篇 |
1994年 | 19篇 |
1993年 | 16篇 |
1992年 | 20篇 |
1991年 | 14篇 |
1990年 | 9篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 13篇 |
1985年 | 10篇 |
1984年 | 30篇 |
1983年 | 7篇 |
1982年 | 13篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 6篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1973年 | 3篇 |
1970年 | 3篇 |
排序方式: 共有1282条查询结果,搜索用时 130 毫秒
51.
Torres-Lapasió JR García-Alvarez-Coque MC Bosch E Rosés M 《Journal of chromatography. A》2005,1089(1-2):170-186
The problems associated to the modelling and optimisation of the chromatographic resolution of mixtures involving ionisable solutes at varying pH and acetonitrile content are discussed. Several retention models that separate the contributions of solute, column and stationary phase, were used. The retention was predicted with low errors in large pH domains (2-12), which was an essential requirement to face the optimisation of resolution. The selected mixture was particularly problematic under the viewpoint of resolution, owing to the excessively diverse acid-base behaviour of solutes. This variety led to sudden drops in retention at different pH for each solute, yielding numerous peak crossing, which made finding shared regions of high resolution especially difficult. Conventional resolution diagrams for these situations are scarcely informative, since both the overall and the worst elementary resolutions drop to zero if at least two compounds remain overlapped, even when all the others are baseline resolved. A new chromatographic objective function is proposed to address this drawback. This function, called "limiting peak count", is based on the limiting peak purity concept, and measures the success in the resolution focusing on the resolved solutes, in contrast to conventional resolution assessments that attend mainly to the least resolved solutes. Limiting peak count yields the same result as conventional assessments when full resolution is possible, but it is also able to discriminate the maximal resolving power in low-resolution situations. It offers a different perspective to that given by the complementary mobile phases approach, and the computation is far simpler. 相似文献
52.
Summary A global LSER model that relates HPLC retention to mobile phase composition and pH is tested for a varied group of solutes,
both neutral and ionizable, in a polymeric column and methanol-water mobile phases. It is compared to the local LSER model
developed only for a given mobile phase, i.e., a fixed organic modifier content, and to the global LSER model set only for
neutral solutes. The global LSER model for neutral and ionizable solutes requires a few supplementary parameters over the
other models tested, but it accounts for retention under any experimental conditions for a given column and methanol-water
mobile phases, describing properly the interactions established in the HPLC system (hydrophobicity, hydrogen-bond acidity
and basicity, dipolarity/polarizability…).
This paper is number 13 of a series with the same general title: “Retention of Ionizable Compounds on HPLC” published in various
journals. 相似文献
53.
F. Bosch Reig V. Peris Martinez F. Bosch Mossi J. V. Gimeno Adelantado 《Fresenius' Journal of Analytical Chemistry》1992,342(4-5):295-298
Summary An algorithm for quantifying interelemental effects in X-ray fluorescence techniques is developed. By applying an addition process, the ratio between the mass absorption coefficients of the analyte and the unknown sample (
i
*
/
s
*
) is calculated to correct the fluorescence intensity of the element to be determined and linearize the I-c calibration plot. This coefficient can be calculated graphically and numerically. The method is applied to the determination of tin in lead alloys with good results over wide concentration ranges.Notation used Q
Constant of proportionality in Eq. (4)
X-ray fluorescence intensity of the I
i
o
standard
- I
i
s
unknown sample
- I
i
ns
dilute unknown sample
- I
i
ms
unknown sample after addition of i analyte
Corrected fluorescence intensity of the I
i
cs
unknown sample
- I
i
msc
unknown sample after addition of i analyte
Relationship of fluorescence intensity between Ri
sample and standard
- R
i
dilute sample and standard
Factor of fm
addition
- fx
addition equivalent to the mass fraction of the i analyte in the unknown sample
Mass absorption coefficient of
i
*
analyte
-
s
*
unknown sample
-
ms
*
unknown sample after addition
Mass fraction of c
i
s
analyte
- c
i
ms
unknown sample after addition 相似文献
54.
A method for the simultaneous spectrophotometric determination of the divalent ions of iron, cobalt, nickel and copper based on the formation of their complexes with 1,5-bis(di-2-pyridylmethylene) thiocarbonohydrazide (DPTH) is proposed. The resolution of quaternary mixtures of these metallic ions was accomplished by several chemometric approaches. A comparative study of the results obtained for simultaneous determinations in mixture by using principal component regression (PCR) and partial least-squares regression (PLS-1 and PLS-2) for absorbance, first-derivative and second-derivative data is presented. In general, the best recovery values are obtained by the PLS-2 method for absorbance data. This procedure allows the simultaneous determination of the cited ions in alloys and biological materials Good reliability of the determination was proved. 相似文献
55.
T. López P. Bosch R. Gómez E. Basaldella A. Kikot E. Pereyra 《Reaction Kinetics and Catalysis Letters》1991,43(2):461-466
The impregnation preparation of H-zeolite over amorphous aluminosilicate solids (cracking catalysts) is compared with new synthesis procedures based on sol-gel process. The characterization results show that the H-zeolite reacts with the intermediates of the amorphous aluminosilicate.
H- ( ) , -. , H- .相似文献
56.
The potassium hydroxide-induced (Stevens) rearrangement of 1,3,4-trimethyl-1-(3,4,5-trimethoxybenzyl)-1,2,5,6-tetrahydropyridinium chloride (I) gives the desired 1,3,4-trimethyl-2-(3,4,5-trimethoxybenzyl)-1,2,5,6-tetrahydropyridine (III) and the Hofmann elimination product, N-methyl-N-(3,4,5-trimethoxybenzyl)-2,3-dimethyl-2,4-pentadienamine (II). In the presence of ethereal phenyllithium, the salt I undergoes rearrangement giving the expected tetrahydropyridine III in about 17% yield and four other products, N-(3,4,5-trimethoxybenzyl)methylamine (VI), 1,3,4-trimethyl-2-(6-methyl-2,3,4-trimethoxyphenyl)-1,2,5,6-tetrahydropyridine (IV), 1,3,3-trirnethyl-2-(3,4,5-trimethoxyphenyl)-4-rnethylenepiperidine (V) and 1,3,4-trimethyl-4-(3,4,5-trimethoxybenzyl)-1,4,5,6-tetrahydropyridine (VII), the latter being the 1,4-Stevens rearrangement product which cyclizes easily to β-2′,3′,4′-trimethoxy-2,5,9-trimethyl-7,8-benzomorphan (VIII). Their structures have been proved both by analytical and spectral data. A possible route for VIII and its stereochemical aspects are discussed. 相似文献
57.
We present EPR analysis of the reaction of ethyl iododifluoroacetate with 1-tetradecene in the presence of Zn + NiCl2 x 6H2O, confirming the mechanistic studies that provide evidence of a single electron transfer process. We have trapped for the first time the ethoxycarbonyldifluoromethyl radical with a variety of spin traps, such as phenyl tert-butyl nitrone (PBN), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 2-methyl-2-nitrosopropane (MNP), and 2-nitro-2-nitrosopropane (NNP), and the EPR spectra of the corresponding adducts have been recorded. In a second step the ethoxycarbonyldifluoromethyl radical adds to the olefin to furnish a second radical intermediate, which can be trapped with NNP. Evidence of this second radical was obtained by EPR only with electron-rich olefins, such as alpha-methylstyrene and 2,4,6-trimethylstyrene, and the new adducts were recorded and interpreted. In addition, we also report the EPR spectra of the corresponding adducts when other alkylating reagents are used, such as ethyl iodoacetate, n-perfluorohexyl iodide, methyl omega-iodohexadecanoate, and n-butyl iodide. 相似文献
58.
The generation of 2-indolylacyl radicals from the corresponding phenyl selenoesters, aldehydes, and alpha-keto carboxylic acids and the scope of their participation in intermolecular addition reactions to carbon-carbon double bonds have been studied. Whereas the phenyl selenoester method has provided easy access to a variety of 1,4-dicarbonyl compounds bearing the 2-acylindole moiety, the glyoxylic acid route has been employed for the preparation of 2-indolyl pyridyl ketones. 相似文献
59.
[reaction: see text] Racemic oxodiester 1 undergoes stereoselective cyclocondensation with (S)-tryptophanol, (S)-(3,4-dimethoxyphenyl)alaninol, or the corresponding amino acids, in a process involving a tandem dynamic kinetic resolution/desymmetrization of diastereotopic groups, to give bicyclic lactams, which are cyclized to substituted indolo[2,3-a]- and benzo[a]quinolizidines. 相似文献
60.
D. Liesen A. Warczak P. Armbruster F. Bosch C. Kozhuharov B. Liu 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1987,5(1):49-55
Impact parameter dependentK x-ray emission from both collision partners in208Pb+26 →120Sn and Xe collisions at 3,6 MeV/N projectile has been investigated. In solid target collisions the Sn-K emission yield shows impact parameter dependent structures. These structures are interpreted in terms of post-collisional capture processes. TheK x-ray emission from the heavier collision partner related to the 1s σ vacancy production, is found to be dependent on the primary projectile outer-shell configuration. 相似文献