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91.
92.
Flash chromatography of the mixture obtained by reaction of L-rhamnose with methanol in the presence of cation-exchange resin, in addition to methyl α-L-rhamnofuranoside (3) and methyl rhamnopyranosides 1 and 2 gave methyl β-L-rhamnofuranoside (4) in 8% yield. 13C and 1H NMR spectra of 3 and 4 as well as their calculated conformation in solution are discussed. 相似文献
93.
Jitka Hubackova Marie Dvorackova Petr Svoboda Pavel Mokrejs Jan Kupec Iva Pozarova Pavol Alexy Peter Bugaj Michal Machovsky Marek Koutny 《Polymer Testing》2013
Research concentrated on the biodegradable capability of PCL blends with various types of starch in an anaerobic aqueous environment of mesophilic sludge from a municipal wastewater treatment plant. For blend preparation, use was made of a native starch Meritena from maize, another from Waxy – a genetically modified type of maize, as well as Gel Instant, a gelatinized starch, and an amaranth starch. Additional PCL/starch blends were prepared from the same starch types, but these were initially plasticized with glycerol. The biodegradability tests were supplemented with thermo gravimetric analysis (TGA), and differential scanning calorimetry (DSC); morphology was identified using scanning electron microscopy (SEM), plus mechanical properties were also tested. While mixtures of PCL with starches plasticized with glycerol exhibited improved mechanical properties and a higher degree of biodegradation in the anaerobic environment, mixtures of PCL with pure forms of starch were ascertained as rather resistant to the anaerobic aqueous environment. TGA and DSC analysis confirmed the removal of starch and glycerol from the PCL matrix. SEM then proved these results through the absence of starch grains in the samples following anaerobic biodegradation. 相似文献
94.
Jitka Cejkova Vadym Prokopec Alzbeta Kokaislova Frantisek Stepanek 《Applied Surface Science》2009,255(18):7864-7870
Surface Enhanced Raman Scattering (SERS) on copper substrates of various morphologies, prepared by electrochemical deposition on platinum targets, was investigated. The substrate preparation procedures differed by the coating bath compositions, applied current densities and the duration of individual steps. The surface morphology of the substrates was visualized by means of Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). SERS spectra of selected organic thiols were measured and the relation between SERS spectral intensity and the surface structure of SERS-active substrates was studied. It has been shown that good Raman surface enhancement can be achieved on the copper substrates prepared by electrochemical deposition from ammoniac baths. Copper substrates fabricated from acidic baths did not show efficient Raman surface enhancement. The results of microscopic measurements demonstrated that the average surface roughness value does not play a substantial role, whereas the shape of the surface nanostructures is a key parameter. 相似文献
95.
96.
Jitka Moravcová Zita Vanclová Jindra Čapková Karel Kefurt Jan Staněk 《Journal of carbohydrate chemistry》2013,32(7):1011-1028
Abstract The regioselective enzymic hydrolysis of methyl 2,3-di-O-acetyl-5-deoxy-α-D-xylofuranoside (1) and methyl 2,3-di-O-acetyl-5-deoxy-β-D-xylofuranoside (2) in the presence of pig liver esterase (PLE) was studied by GLC. Diacetate 2 gave exclusively methyl 3-O-acetyl-5-deoxy-β-D-xylofuranoside (6) while diacetate 1 produced both methyl 2-O-acetyl-5-deoxy-α-D-xylofuranoside (3) and methyl 3-O-acetyl-5-deoxy-α-D- xylofuranoside (4) in low yield. At high conversion, methyl 5-deoxy-α-D-xylofuranoside (7) was the only product. The first-order rate constants, Michaelis constants, and maximal velocities were determined for 1, 2, and the monoacetates 3 - 6. The results were interpreted on the basis of a recent active-site model for PLE. 相似文献
97.
Martina Zatloukalová Vladimír K?en Radek Ga?ák Martin Kubala Patrick Trouillas Jitka Ulrichová Jan Vacek 《Bioelectrochemistry (Amsterdam, Netherlands)》2011,82(2):117-124
Flavonolignans, silybin and its derivatives (2,3-dehydrosilybin, 7-O-methylsilybin, 20-O-methylsilybin) and isosilybin were studied using ex situ (adsorptive transfer, AdT) cyclic and square wave voltammetry (SWV). The two oxidation steps were described for flavonolignans at potentials Ep1 + 0.5 V and Ep2 + 0.85 V depending on experimental conditions. An additional oxidation peak at Ep3 + 0.35 V was observed only for 2,3-dehydrosilybin. The anodic currents of flavonolignans are related to their electron transfer processes (oxidation of hydroxyl groups), which was supported by density functional theory (DFT) and B3P86 theory level. Our electrochemical results confirmed that 2,3-dehydrosilybin is a relatively strong antioxidant, which is strictly associated with oxidation at Ep3. The oxidation processes and antioxidant parameters of flavonolignans can be affected by transition metal complexation via hydroxyl groups. We found that silybin and 2,3-dehydrosilybin are able to chelate transition metals, especially Cu2+. The formation of silybin/Cu complexes was studied by AdT SWV and the observation was also confirmed using fluorescence spectroscopy. The electrochemical investigation of DNA interactions and damage caused in the presence of silybin/Cu complex and hydrogen peroxide is described. We present evidence that flavonolignans are involved not only in antioxidant abilities but also in the prooxidation effects under in vitro conditions. 相似文献
98.
Comparison of digestion methods for determination of total phosphorus in river sediments 总被引:1,自引:0,他引:1
In this study, four digestion methods used to determine total phosphorus in river sediments, including Na2CO3 fusion, the H2SO4 and H2SO4 + H2O2 methods and the SMT protocol were investigated. Interference effects of iron, calcium and organic matter in river sediments, and the substances contained in the digestion agents on the photometric determination of the phosphates were analysed. The digestion methods were tested on ten river sediment samples. Statistical analysis of the results showed significant differences between sample treatments relating to the mean total phosphorus concentration. 相似文献
99.
Lenka Česlová Michal HolčapekMartin Fidler Jitka DrštičkováMiroslav Lísa 《Journal of chromatography. A》2009,1216(43):7249-7257
Hops contain a wide range of polyphenolic compounds with antioxidant properties divided in various chemical classes. These compounds are detected in hop extracts and also in beer as its main product. Based on the careful optimization of column type, column packing, mobile phase composition and gradient steepness, two high-performance liquid chromatography–mass spectrometry (HPLC/MS) methods have been developed. The first method using Purospher Star RP-8e column and the gradient of aqueous acetonitrile containing 0.3% formic acid is optimized for the separation of low polar polyphenolic compounds, while the second one with Zorbax SB-CN column is used for more polar hops and beer components. In this work, more compounds are detected in comparison to previous reports. In total, 49 low polar and 37 polar compounds are detected in studied samples and their molecular weights are determined based on atmospheric pressure chemical ionization (APCI) mass spectra. Some compounds are identified based on the interpretation of their full scan and tandem APCI mass spectra, retention behavior and UV spectra, while the full structure elucidation of other species still requires further research. The quantitation of xanthohumol related prenylflavonoids and bitter acids is done with two detection techniques (APCI-MS and UV detection) providing comparable results. Both compound classes (i.e., prenylflavonoids and bitter acids) are separated and quantitated in a single HPLC run, where numerous other polyphenolics are detected as well. 相似文献
100.
Martina Zatloukalová Teodor Adrian Enache Vladimír Křen Jitka Ulrichová Jan Vacek Ana Maria Oliveira‐Brett 《Electroanalysis》2013,25(7):1621-1627
The galloyl substitution effect on the antioxidant potential of quercetin‐3‐O‐gallate (QG) and silybin‐3‐O‐gallate (SBG), and the oxidation of QG and SBG were studied by cyclic, differential and square‐wave voltammetry using a glassy carbon electrode, and compared with their structural components, quercetin (Q), silybin (SB), gallic acid and gallic acid methyl ester. Their multi‐step pH‐dependent anodic behaviour, first oxidation followed by oxidation of the hydroxyl groups at ring A, is similar to Q and SB. The galloyl substitution significantly improved the antioxidant potential of SB compared to Q, and brought useful knowledge about the antioxidant activity of Q and SB monogalloyl esters. 相似文献