全文获取类型
收费全文 | 4747篇 |
免费 | 118篇 |
专业分类
化学 | 2813篇 |
晶体学 | 37篇 |
力学 | 153篇 |
数学 | 717篇 |
物理学 | 1145篇 |
出版年
2021年 | 28篇 |
2020年 | 63篇 |
2019年 | 42篇 |
2018年 | 52篇 |
2017年 | 41篇 |
2016年 | 93篇 |
2015年 | 105篇 |
2014年 | 101篇 |
2013年 | 201篇 |
2012年 | 264篇 |
2011年 | 282篇 |
2010年 | 157篇 |
2009年 | 129篇 |
2008年 | 215篇 |
2007年 | 220篇 |
2006年 | 204篇 |
2005年 | 210篇 |
2004年 | 193篇 |
2003年 | 148篇 |
2002年 | 156篇 |
2001年 | 74篇 |
2000年 | 68篇 |
1999年 | 59篇 |
1998年 | 48篇 |
1997年 | 57篇 |
1996年 | 41篇 |
1995年 | 58篇 |
1994年 | 34篇 |
1993年 | 35篇 |
1992年 | 48篇 |
1991年 | 33篇 |
1990年 | 39篇 |
1989年 | 33篇 |
1988年 | 51篇 |
1987年 | 44篇 |
1985年 | 33篇 |
1984年 | 30篇 |
1983年 | 27篇 |
1982年 | 35篇 |
1981年 | 48篇 |
1980年 | 36篇 |
1979年 | 28篇 |
1978年 | 40篇 |
1977年 | 32篇 |
1976年 | 35篇 |
1975年 | 31篇 |
1974年 | 31篇 |
1973年 | 26篇 |
1933年 | 24篇 |
1908年 | 27篇 |
排序方式: 共有4865条查询结果,搜索用时 15 毫秒
81.
Simultaneous determination of ondansetron and tropisetron in human plasma using HPLC with UV detection 总被引:4,自引:0,他引:4
Bauer S Störmer E Kaiser R Tremblay PB Brockmöller J Roots I 《Biomedical chromatography : BMC》2002,16(3):187-190
A rapid and sensitive HPLC method for the simultaneous quantitation of ondansetron and tropisetron, two serotonin (5-HT) receptor antagonists frequently used in treatment and prevention of nausea and emesis, is described. The procedure involves liquid-liquid extraction of human plasma with dichloromethane coupled with reversed-phase HPLC and UV detection. The lower limits of quantification (LOQ) were 0.62 ng/mL for ondansetron and 1.25 ng/mL or tropisetron. Intra- and inter-assay coefficients of variation ranged from 1.5 to 7.5% and 5.3 to 13.7%, respectively. The sensitivity and precision were sufficient for determination of plasma concentrations after therapeutic administration of both drugs and the method can be used for the estimation of pharmacokinetic parameters. 相似文献
82.
Using a mass spectrometric sampling method, we have observed the decomposition of CH4 in an rf plasma usedfor diamond deposition. The gas samples were extracted through an orifice located downstream of the plasma zone and analyzed online. For the experiments a dilute mixture of H2 and CH4 containing 0.1–3% CH4 has been used. CH4 is converted to C2H2 and C2H4 quantitatively. Small amounts of heavier hydrocarbons are formed. A comparison of the experimental results with a recent kinetic model treating a purely thermal environment is made and the differences between our experiment and the model are explained. The role of acetylene as a species formed in an atmosphere rich in atomic hydrogen is proposed. The electron impact dissociation process is suggested as the rare-determining step in the plasma-chemical decomposition of methane. 相似文献
83.
Claverie JM Monchois V Audic S Poirot O Abergel C 《Combinatorial chemistry & high throughput screening》2002,5(7):511-522
We outline a joint academic/industrial (CNRS/AVENTIS) functional genomics project aiming at the discovery of new anti-bacterial gene targets. Starting from all publicly available bacterial genomes, a subset of the most evolutionary conserved protein-coding genes has been identified. We retained genes with clear homolog in E. coli and at least one gram-positive bacterium among B.subtilis, M. tuberculosis, L. lactis or S. pyogenes. This subset was further reduced to genes encoding non-membrane proteins of unknown or hypothetical functions. The 221 E. coli Open Reading Frames (ORFs) identified through this comprehensive bioinformatic analysis are now submitted to a systematic 3-D structure determination protocol including cloning, protein expression and purification, crystallisation and X-ray diffraction. Our strategy was designed to focus on promising wide-spectrum targets as well as original biochemical pathways. Bioinformatics is used throughout all phases of project, including the initial large-scale comparative genomics analyses, the purification/expression and crystallisation stages for the detection of helpful sequence-specific features (e.g. cofactor binding motifs, non-structured N- or C- term extremities, etc ), and finally for the interpretation of the structures in conjunction with multiple sequence alignments for the identification of key residues, interaction areas on molecular surfaces, and overall function predictions. 相似文献
84.
Florence Babonneau Christine Toutou Stéphanie Gavériaux 《Journal of Sol-Gel Science and Technology》1997,8(1-3):553-556
17O NMR experiments using enriched water were performed to followthe hydrolysis-condensation process of dimethyldiethoxysilanetetraethoxysilane and a 1/1 mixture of dimethyldiethoxysilane andtetraethoxysilane (H2O/OEt = 0.5; pH = 2). The spectrarecorded over several hours time period were simulated to followquantitatively the variations of residual water, hydroxyl groups(Si–17
H) and oxo bridges (Si–17
–Si). Presence of a resonance signal due oxo bridges between di- andtetrafunctional Si units clearly demonstrates that co-condensationreactions occur to a large extent between the two alkoxides, and that thesebonds are stable during the aging period. 相似文献
85.
Städler B Falconnet D Pfeiffer I Höök F Vörös J 《Langmuir : the ACS journal of surfaces and colloids》2004,20(26):11348-11354
We present a novel concept for the creation of lipid vesicle microarrays based on a patterning approach termed Molecular Assembly Patterning by Lift-off (MAPL). A homogeneous MAPL-based single-stranded DNA microarray was converted into a vesicle array by the use of vesicles tagged with complementary DNAs, permitting sequence-specific coupling of vesicles to predefined surface regions through complementary DNA hybridization. In the multistep process utilized to fulfill this achievement, active spots consisting of PLL-g-PEGbiotin with a resistant PLL-g-PEG background, as provided by the MAPL process, was converted into a DNA array by addition of complexes of biotin-terminated DNA and NeutrAvidin. This was then followed by addition of POPC vesicles tagged with complementary cholesterol-terminated DNA, thus providing specific coupling of vesicles to the surface through complementary DNA hybridization. Quartz crystal microbalance with dissipation (QCM-D) and optical waveguide lightmode spectroscopy monitoring were used to optimize the multistep surface modification process. It was found that the amount of adsorbed biotinDNA-NeutrAvidin complexes decreases with increasing molar ratio of biotinDNA to NeutrAvidin and decreasing ionic strength of the buffer solution. Modeling of the QCM-D data showed that the shape of the immobilized vesicles depends on the amount of available anchoring groups between the vesicles and the surface. Fluorescent microscopy images confirmed the possibility to create well-defined patterns of DNA-tagged, fluorescently labeled vesicles in the micrometer range. 相似文献
86.
The shift ΔBov of the ESR line due to the saturation of the NMR of hyperfine-coupled nuclei (Overhauser shift) was measured for single crystals of the organic conductor (FA2)±PF6?. ΔBov is proportional to AP, where A is the average hyperfine interaction between the conduction electrons and the nuclei in resonance and P is the dynamic nuclear polarization. The proton spin relaxation times were measured from the time dependence of the Overhauser shift, ΔBov(t), after rf pulses. 相似文献
87.
Development and application of a simple routine method for the determination of selenium in serum by octopole reaction system ICPMS 总被引:1,自引:0,他引:1
The aim of the study was to develop an inductively coupled plasma mass spectrometry (ICPMS) method for robust and simple routine determination of selenium in serum. Polyatomic interferences on 76Se, 77Se, and 78Se were removed by applying an octopole reaction system ICPMS with the reaction cell pressurized with H2 gas. We developed a novel simple optimization routine for the H2 gas flow based on a signal-to-noise ratio (SNR) calculation of the selenium signal measured in a single selenium standard. The optimum H2 flow was 2.9 mL min–1. The selenium content in serum was determined after a 50-fold dilution with 0.16 M HNO3 and quantified by using addition calibration and gallium as an internal standard. The method detection limit was 0.10 g L–1 for 76Se and 78Se and 0.13 g L–1 for 77Se. Human serum samples from a case-control study investigating if selenium was associated with risk of colorectal adenoma were analyzed. The average selenium concentration for the control group (n=768) was 137.1 g L–1 and the range was 73.4–305.5 g L–1. The within-batch repeatability (a batch is ten samples) estimated from 182 replicate analyses was 6.3% coefficient of variation (CV), whereas the between-batch repeatability was 7.4% CV estimated from 361 replicates between batches. The method accuracy was evaluated by analysis of a human serum certified reference material (Seronorm Serum level II, Sero A/S, Norway). There was a fairly good agreement between the measured average of 145±3 g L–1 (n=36) and the certified value of 136±9 g L–1. In addition the method was successfully applied for analysis of zinc serum concentrations without further optimization. For the Seronorm certified reference material a value of 911±75 g L–1 (n=31) for zinc was obtained, which corresponds well to the certified zinc value of 920±60 g L–1. 相似文献
88.
Foltz C Stecker B Marconi G Bellemin-Laponnaz S Wadepohl H Gade LH 《Chemical communications (Cambridge, England)》2005,(40):5115-5117
Chiral C3-symmetric trisoxazolines are highly efficient stereodirecting ligands in enantioselective Cu(II) Lewis acid catalysis which is based on the concept of a stereoelectronic hemilability of the divalent copper; in direct comparison with the analogous bisoxazoline systems they are more efficient in the enantioselective alpha-amination as well as the enantioselective Mannich reaction of prochiral beta-ketoesters. 相似文献
89.
St Hilaire PM Willert M Juliano MA Juliano L Meldal M 《Journal of combinatorial chemistry》1999,1(6):509-523
To map the substrate specificity of cysteine proteases, two combinatorial peptide libraries were synthesized and screened using the archetypal protease, papain. The use of PEGA resin as the solid support for library synthesis facilitated the application of an on-resin fluorescence-quenched assay. Results from the screening of library 2 indicated a preference for Pro or Val in the S3 subsite and hydrophobic residues in S2; the most prevalent residue not being Phe but Val. The S1 subsite exhibited a dual specificity for both small, nonpolar residues, Ala or Gly, as well as larger, Gln, and charged residues, Arg. Small residues predominated in the S1'-S4' subsites. Active peptides from the libraries and variations thereof were resynthesized and their kinetics of hydrolysis by papain assessed in solution phase assays. Generally, there was a good correlation between the extent of substrate cleavage on solid phase and the kcat/KM's obtained in solution phase assays. Several good substrates for papain were obtained, the best substrates being Y(NO2)PMPPLCTSMK(Abz) (kcat/KM = 2109 (mM s)-1), Y(NO2)PYAVQSPQK(Abz) (kcat/KM = 1524 (mM s)-1), and Y(NO2)PVLRQQRSK(Abz) (kcat/KM = 1450 (mM s)-1). These results were interpreted in structural terms by the use of molecular dynamics (MD). These MD calculations indicated two different modes for the binding of substrates in the narrow enzyme cleft. 相似文献
90.
The dynamic processes of N(1s) core-hole excitation in gas-phase CH3CN molecule have been studied at both Hartree-Fock and hybrid density-functional theory levels. The vibrational structure is analyzed for fully optimized core-excited states. Frank-Condon factors are obtained using the linear coupling model for various potential surfaces. It is found that the vibrational profile of the N-K absorption can be largely described by a summation of two vibrational progressions: a structure-rich profile of nu(CN) stretching mode and a large envelope of congestioned vibrational levels related to the strong (-C-CN) terminal bending bond. Excellent agreement between theoretical and experimental spectra is obtained. 相似文献