全文获取类型
收费全文 | 40115篇 |
免费 | 3936篇 |
国内免费 | 2355篇 |
专业分类
化学 | 34382篇 |
晶体学 | 526篇 |
力学 | 862篇 |
综合类 | 250篇 |
数学 | 683篇 |
物理学 | 9703篇 |
出版年
2024年 | 42篇 |
2023年 | 325篇 |
2022年 | 731篇 |
2021年 | 652篇 |
2020年 | 887篇 |
2019年 | 930篇 |
2018年 | 955篇 |
2017年 | 1244篇 |
2016年 | 1530篇 |
2015年 | 1462篇 |
2014年 | 1405篇 |
2013年 | 2930篇 |
2012年 | 2388篇 |
2011年 | 2260篇 |
2010年 | 1888篇 |
2009年 | 2169篇 |
2008年 | 1841篇 |
2007年 | 2084篇 |
2006年 | 1940篇 |
2005年 | 1829篇 |
2004年 | 1728篇 |
2003年 | 1438篇 |
2002年 | 1503篇 |
2001年 | 1055篇 |
2000年 | 1143篇 |
1999年 | 1007篇 |
1998年 | 863篇 |
1997年 | 832篇 |
1996年 | 759篇 |
1995年 | 814篇 |
1994年 | 650篇 |
1993年 | 547篇 |
1992年 | 454篇 |
1991年 | 449篇 |
1990年 | 408篇 |
1989年 | 349篇 |
1988年 | 454篇 |
1987年 | 419篇 |
1986年 | 340篇 |
1985年 | 316篇 |
1984年 | 313篇 |
1983年 | 165篇 |
1982年 | 273篇 |
1981年 | 201篇 |
1980年 | 176篇 |
1979年 | 166篇 |
1978年 | 32篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
J. Fekete M. Milen L. Hazai L. Poppe Cs. Szántay A. Kettrup I. Gebefügi 《Chromatographia》2003,57(3-4):147-153
Summary Reversed (RP-HPLC) and normal phase chromatographic (NP-HPLC) separations have been developed for diastereomers ofN-acyl-1-methyl-1,2,3,4-tetrahydo-β-carbolines which are acylated derivatives of simple natural β-carboline alkaloids. Separations
of derivatives having different acyl moieties in theO,O-diacyl-tartaric acid ester subtituent differed remarkably. Little or no resolution in either NP-HPLC or RP-HPLC could be
achieved with the diacetyl-tartrate derivative. Base-line separation by RP-HPLC but no separation by NP-HPLC was possible
with the bulkier and more apolar dipivaloyl derivative. Retention order of the bis-benzoylated diastereomers was reversed
and separation time increased dramatically by RP-HPLC. Good separation of the medium polarity, but rigid,N-camphanyl derivative by NP-HPLC has been achieved, whereas RP-HPLC could not be used for separation of these diastereomers.
Separability of different diastereomers was highly dependent on polarity and rigidity of the derivatizingN-acyl moieties. Conformational analysis by molecular mechanics and comparison of the lowest energy conformational states of
the diastereomers was applied to rationalise separation-retention behaviour of stereoisomers by RP-HPLC.
Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001 相似文献
92.
Antonella Leggio 《Tetrahedron》2007,63(34):8164-8173
A highly efficient and practical synthesis of peptides in solution phase has been developed. The procedure is based on the use of p-nitrobenzenesulfonyl (nosyl) group for the protection of the amino function of α-amino acids. Every step of the procedure, protection of the amino function by the nosyl group, formation of the peptide bond, and removal of the sulfonamide group, is characterized by high yields and excellent purity of the final products. The described strategy allows the preparation of short peptide sequences keeping the chiral integrity of amino acid precursors. Compatibility of nosyl group with the side-chain protecting groups used in Fmoc-based strategy is demonstrated. The method here presented is an alternative strategy that could provide advantages for future peptide synthesis. 相似文献
93.
Kenichiro Todoroki 《Analytica chimica acta》2006,555(1):14-19
A fluorimetric liquid chromatographic method for the determination of 5-hydroxyindoles based on the benzylamine derivatization process mediated through an online photocatalytic oxidation has been developed. In this study, we used a photocatalytic column comprising tefzel tubing packed with TiO2-coated glass beads, as a pre-column derivatization reactor. The fluorescence derivatization of 5-hydroxyindoles using benzylamine proceeded during their passage through the reaction column under near-UV irradiation. The 5-hydroxyindole derivatives were separated continuously on a reversed-phase liquid chromatography within 50 min, using 100 mM acetate buffer (pH 4.6)-acetonitrile (72:28, v/v; isocratic elution) containing 3 mM sodium octanesulfonate; the samples were detected fluorimetrically at 465 nm upon excitation at 350 nm. The detection limits (signal-to-noise ratio = 3) of the 5-hydroxyindoles were in the range from 160 to 360 fmol per 5 μL injection. We have applied this method, which requires minimal sample pre-treatment, to the determination of 5-hydroxyindole-3-acetic acid in human urine. 相似文献
94.
The role of achiral sorbent matrix in chiral recognition of amino acid enantiomers in ligand-exchange chromatography 总被引:2,自引:0,他引:2
Summary As an alternative to the known three-point interaction model describing recognition of optical isomers by a chiral resolving agent, a new concept has been developed stating that two interaction points between the resolving agent and the enantiomers are also sufficient for achieving chiral recognition of the latter, provided that the diastereomeric adducts formed by the resolving agent with the enantiomers additionally interact with a non-chiral chromatographic sorbent. This concept is based on the results of ligand-exchange chromatography of -amino acid enantiomers with copper(II) complexes of chiral bifunctional ligands as the resolving agents in chromatographic systems. 相似文献
95.
Rodríguez-Cea A de la Campa MR Sanz-Medel A 《Analytical and bioanalytical chemistry》2005,381(2):388-393
Cytochromes P-450 are members of a superfamily of hemoproteins involved in the oxidative metabolism of various physiological and xenobiotic compounds in eukaryotes and prokaryotes. The multiplicity of this group of enzymes has been widely studied by chromatographic techniques, mainly high-performance liquid chromatography (HPLC). Because these enzymes are membrane-bound proteins, sample preparation for chromatographic separation of P-450 enzymes requires a solubilization step. The sample-preparation procedures are critical, because detergents affect not only the efficiency of protein solubilization but also their further chromatographic resolution. Trout liver microsomes have been taken here as a model sample to investigate iron speciation in cytochrome P-450. Trouts were treated intraperitoneally with -naphthoflavone, a potent inducer of some P-450 enzymes, and a microsomal suspension containing 7.4±0.1 nmol mL–1 P-450 enzymes was obtained by ultracentrifugation. Lubrol PX was selected as detergent for solubilization, resulting in about 90% solubilization recovery. The solubilized cytochromes P-450 were further separated by AE–FPLC, with UV detection, or coupled to ICP–MS with an octapole reaction system, ICP–(ORS)MS (monitoring Fe signals at masses 54, 56, and 57). A sampling procedure and chromatographic conditions are developed and were successfully applied to iron speciation in trout liver P-450 enzymes. ICP–(ORS)MS detection of P-450 enzymes is Fe-specific and so will give accurate information on the prosthetic group of the protein, which can constitute an advantageous alternative to classical methods for detection of these hemoproteins. 相似文献
96.
A multidimensional GC-system containing two capillary columns of different polarities, operated within a double oven instrument, was used for the optimized separation of complex isomeric mixtures of various types of chlorinated aromatic compounds such as PCB, PCDD, or PCDF. Electron capture was applied parallel to flame ionization detection to obtain sensitive signals of the PCB or PCDD and the n-alkane standards required for identification by Kovats indices, respectively. Electron capture detection within a valvelessly operated MDGC system forced changes in the usual instrumental set-up, the live-switching coupling piece being located within the second oven of the double oven instrument. This oven is operated isothermally to ensure optimal and interference-free detection, for stable flow conditions within the coupling piece and for the determination of the isothermal retention indices (Kovats), which are much more suitable for identification by retention than the so-called “linear” indices. 相似文献
97.
A one-step solid-phase extraction procedure, based on a new silica gel adsorbent modified with cholesterol groups, has been
investigated for measurement of cotinine in urine. Cotinine is the main metabolite of nicotine in the human body and is analyzed
as a biomarker for assessment of direct or passive exposure to tobacco smoke. New cholesterol-modified adsorbents have been
obtained by chemical modification of silica gel of different porosity with cholesterol ligands. Although recovery by this
extraction procedure were optimum over a relatively broad range of sample pH (3.1–8.0), analytical conditions such as sample
loading, washing and elution conditions, concentration of cotinine to be extracted, and the type of adsorbent used for extraction
were found to affect the efficiency of the procedure and had to be controlled for optimum recovery. When these conditions
were controlled, recovery of cotinine from spiked human urine was reproducible and depended on compound ionization. Quantitative
analysis of cotinine was performed by reversed-phase high-performance liquid chromatography with UV detection.
Presented at: Conference of the Hyphenation of Liquid Chromatography–Nuclear Magnetic Resonance Spectroscopy, Liquid Chromatography–Mass
Spectrometry and Related Topics, Tuebingen, Germany, March 25–29, 2006. 相似文献
98.
An accurate estimation of pKa values in methanol-water binary mixtures is very important for several separation techniques such as liquid chromatography and capillary electrophoresis that use these solvent mixtures. In this study, the pKa values of 11 polyphenolic acids have been determined in methanol-water binary mixtures (10%, 20% and 30% (v/v)) by potentiometry, liquid chromatography (LC) and LC-DAD methodology.The results show a similar trend for the pKa values of all the studied compounds, as they increase with increasing concentration of organic modifier, which allows a linear relationship between pKa values and mole fraction of methanol to be obtained. The pKa values obtained in aqueous medium have been compared with those given in the literature, and also with the values predicted by the SPARC on-line pKa calculator. The data obtained have been used to test the feasibility of an estimation of dissociation constants in a methanol-water medium from the relationship between pKa values and the organic cosolvent fraction in the mixtures. 相似文献
99.
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
100.
Summary In high pressure gas chromatography efficient columns can be packed by the slurry packing technique. However, polysiloxane stationary phases like OV 101 and OV 3 dissolve in the packing liquids. In order to make them suitable for slurry packing these stationary phases have to be made insoluble. This immobilization is achieved by bonding the stationary phases to the surface by means of a high temperature treatment. Successive layers are applied by recoating and are immobilized by a cross-link reaction. A range of loading percentages from 4 upto 20 by weight could be prepared in this way. On the evaluated columns compounds like butanol and dioxane elute as rather symmetrical peaks with the correct values for the retention index, indicating the absence of adsorption effects. Efficiency is good and bleeding rate is low. The columns could be successfully used in the analysis of turpentines. 相似文献