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81.
Sara Herrero Martín Carmelo García PintoJosé Luis Pérez Pavón Bernardo Moreno Cordero 《Journal of chromatography. A》2010,1217(30):4883-4889
A method based on QuEChERS extraction is proposed for the determination of trihalomethanes (chloroform, bromodichloromethane, dibromocloromethane and bromoform) in soil samples. The new version of QuEChERS adapted to soil samples consists of liquid extraction with ethyl acetate, the addition of water to moisten the samples, salting-out partitioning of the water with anhydrous MgSO4, and direct injection of the organic extract, obtained after the centrifugation step, into the gas chromatograph. This simplified extraction procedure maintains the advantages of the original method and avoids some steps, making the final procedure simpler, faster, and cheaper, with the consequent reduction in errors associated with sample manipulation. The experimental conditions of the analytical method, based on fast gas chromatography (FGC) and micro-electron capture detection (μECD), were optimized. The column and oven program used allowed fast separation of the compounds in less than 4 min and the total analysis cycle time was as short as 10 min. The existence of a matrix effect was checked and the analytical conditions of the method were studied in a fortified garden soil sample. The highly sensitive and selective detector used afforded to detection limits in the order of ng/kg for the target compounds. To validate the proposed method two certified reference materials (CRMs) were analyzed. 相似文献
82.
Herdes C Lin Z Valente A Coutinho JA Vega LF 《Langmuir : the ACS journal of surfaces and colloids》2006,22(7):3097-3104
We present here Monte Carlo simulation and experimental results on the adsorption of nitrogen and water in aluminum methylphosphonate polymorph alpha (AlMePO-alpha). We have assumed a detailed atomic model for the material, using experimental information to construct the simulation cell. Nitrogen was modeled with two different approaches: as a simple Lennard-Jones (LJ) sphere with no charges, and as a diatomic molecule with charges explicitly included. Water was represented by the TIP4P model. Experimental adsorption isotherms were used to tune the proposed molecular model for the adsorbent. Simulated adsorption capacities were in agreement with the experimental results obtained for the studied systems. The influence of the surface model on the adsorption behavior was taken into account by considering different values of the surface methyl group size parameter. Our results corroborate the strong sensitivity of the simulation results to this parameter, as previously observed by Schumacher and co-workers. It is also observed that charged models are essential to accurately describe the low-pressure region of the adsorption isotherm, where the solid-fluid interaction rules the system behavior. However, a simple uncharged molecular model for nitrogen is able to describe the three loci arrangement at maximum loading. Experimental and simulation results presented here also confirm the low water affinity of AlMePO-alpha. These results enforce the application of this methodology to achieve quantitative predictions on similar systems, with the appropriate transferability of the molecular parameters. 相似文献
83.
Corsaro C Crupi V Majolino D Parker SF Venuti V Wanderlingh U 《The journal of physical chemistry. A》2006,110(3):1190-1195
The vibrational dynamics of water molecules encapsulated in synthetic Na-A and Mg-exchanged A zeolites were studied versus temperature by inelastic neutron scattering (INS) measurements (30-1200 cm(-1)) as a function of the induced ion-exchange percentage by using the indirect geometry tof spectrometer TOSCA at the ISIS pulse neutron facility (RAL, UK). The experimental INS spectra were compared with those of ice Ih to characterize the structural changes induced by confinement on the H2O hydrogen-bonded network. We observed, after increasing the Mg2+ content, a tendency of water molecules to restore the bulklike arrangements together with more hindered dynamics. These results are confirmed by the analysis of the evaluated one-phonon amplitude-weighted proton vibrational density of states aimed, in particular, to follow the evolution of the water molecules librational mode region. 相似文献
84.
We present an exposition of quadratic-residue codes through their embedding in codes over the quadratic subfield of the th cyclotomic field, the algebraic number field of th roots of unity. This representation allows the development of effective syndrome decoding algorithms that can fully exploit the code’s error-correcting capability. This is accomplished via Galois automorphisms of cyclotomic fields. For each fixed , the general results hold for all pairs , with a finite number of exceptions that depends on . A complete discussion of the set of quadratic-residue codes of length and dimension illustrates these results. This set includes the Golay code, the only perfect binary three-error-correcting code.A preliminary version of this paper was presented at the International Conference on Statistics, Combinatorics and Related Areas, October 3–5, 2003, University of Southern Maine, Portland, ME, USA. 相似文献
85.
Cunsolo V Foti S La Rosa C Saletti R Canters GW Verbeet MP 《Journal of mass spectrometry : JMS》2003,38(5):502-509
An electrospray ionisation (ESI) mass spectrometric method for the determination of the equilibrium constant and free energy (DeltaG) of protein unfolding was used to monitor the denaturation process at different pH of three metallo-proteins, i.e. wild-type copper azurin, zinc azurin and wild-type amicyanin. The time course of the unfolding process was followed by dissolving the proteins under denaturing conditions (methanol-water (1 : 1, v/v)) at different pH (2.5, 3.0, 3.5) and recording ESI spectra at time intervals. The spectra showed two series of peaks, corresponding to the native holo-protein and the unfolded apo-protein. From the intensity ratio of these two series of peaks at increasing time and at equilibrium, the equilibrium constants for the unfolding process for the three proteins could be determined. From these equilibrium constants a DeltaG degrees derivation was attempted. The DeltaG degrees values obtained decrease with decrease in pH, in agreement with the expected reduction of conformational stability of proteins at lower pH. The results obtained confirm that ESI-MS can be used for monitoring of unfolding process and to derive quantitative thermodynamic data. 相似文献
86.
Ultrasound-assisted extraction of soy isoflavones 总被引:19,自引:0,他引:19
Efficiency in extracting four isoflavone derivatives (daidzin, glycitin, genistin and malonyl genistin) from freeze-dried ground soybeans was compared for mix-stirring extraction and ultrasound-assisted extraction, using different solvents and extraction temperatures with both. The efficiency of the extraction of soy isoflavones was improved by ultrasound but was dependent on the solvent employed. Optimization of the ratios of sample quantity to solvent volume and length of extraction time was also performed. Isoflavones can be quantitatively extracted from soybeans with 50% ethanol at 60 degrees C using ultrasound-assisted extraction in 20 min. 相似文献
87.
The technique of extraction with pressurized liquids is applied for extraction of catechin and epicatechin in tea leaves and in grape seeds. The extracts obtained are then analyzed by HPLC. A comparison has been made of the recoveries obtained employing extraction by magnetic stirring, ultrasound-assisted extraction, and extraction with pressurized liquids. In the three extraction systems, four different pure solvents were utilized, namely water, methanol, ethanol, and ethyl acetate. Methanol produced the best results. For this comparison, an initial step was to check the stability of catechins during the process of extraction using pressurized liquids at high temperature (100-200 degrees C). It has been confirmed that recoveries of these two compounds begin to fall, to below 95%, at 130 degrees C and above. Pressurized liquid extraction using methanol as solvent, produces results, in terms of recovery of catechin and of epicatechin, notably higher than any of the other conditions of extraction tested. The duration of the extraction cycle was also evaluated and re-extractions were performed to ensure the full recovery from the samples. It was found that, forcatechin, the R.S.D. of the method is 3.21%, and for epicatechin the R.S.D. was 2.96% (n = 5). The final optimized pressurized liquid extraction method allows for the determination of catechin and epicatechin in diverse types of samples with a rapid (10 min) and reproducible method. 相似文献
88.
Cadmium (7–80 ng ml?1) is extracted with diethyldithiocarbamate into chloroform from aqueous media, at pH 11–12 and the fluorescent complex is developed by addition of a calcein solution in dimethylformamide. The method is applied to the determination of cadmium in waste waters, high-purity metals and zinc ores. 相似文献
89.
Wang H Kozekov ID Harris TM Rizzo CJ 《Journal of the American Chemical Society》2003,125(19):5687-5700
trans-4-Hydroxynonenal (HNE) is a major peroxidation product of omega-6 polyunsaturated fatty acids. The reaction of HNE with DNA gives four diastereomeric 1,N(2)-gamma-hydroxypropano adducts of deoxyguanosine; background levels of these adducts have been detected in animal tissue. Stereospecific syntheses of these four adducts at the nucleoside level have been accomplished. In addition, a versatile strategy for their site-specific incorporation into oligonucleotides has been developed. These adducts are destabilizing as measured by melting temperature when compared to an unadducted strand. The thermal destablization of the adducted 12-mers ranged from 5 to 16 degrees C and is dependent on the absolute stereochemistry of the adduct. The HNE adducts were also examined for their ability to form interstrand DNA-DNA cross-links when incorporated into a CpG sequence. We find that only one of the HNE stereoisomers formed interstrand DNA-DNA cross-links. 相似文献
90.
Alessandro de Robertis Concetta de Stefano Rosario Scarcella Carmelo Rigano 《Thermochimica Acta》1984,80(2):197-208
The formation constants for the complexes Mg2+ -, Ca2+ -, Sr2+ - and Ba2+ - succinate (succ2−) have been determined by potentiometric measurements, in aqueous solution, at different temperatures and ionic strengths. The species [M(succ)]0 and [M(succ)H]+ were found for all systems. For the stability constant the ionic strength dependence has been found, and general parameters for the relation log β = f(I) have been obtained. From the temperature dependence of stability constants ΔH values have been deduced. The procedure adopted in calculating all the thermodynamic parameters for the systems under study, where weak complexes are formed, is discussed. The stability of the complexes follows the order Mg < Ca Sr ≈ Ba. 相似文献