全文获取类型
收费全文 | 347篇 |
免费 | 3篇 |
专业分类
化学 | 210篇 |
晶体学 | 28篇 |
力学 | 7篇 |
数学 | 22篇 |
物理学 | 83篇 |
出版年
2023年 | 2篇 |
2022年 | 8篇 |
2021年 | 6篇 |
2020年 | 1篇 |
2019年 | 3篇 |
2018年 | 2篇 |
2017年 | 2篇 |
2016年 | 8篇 |
2015年 | 2篇 |
2014年 | 11篇 |
2013年 | 14篇 |
2012年 | 32篇 |
2011年 | 31篇 |
2010年 | 23篇 |
2009年 | 19篇 |
2008年 | 24篇 |
2007年 | 23篇 |
2006年 | 19篇 |
2005年 | 10篇 |
2004年 | 11篇 |
2003年 | 8篇 |
2002年 | 5篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 6篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 7篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 4篇 |
1974年 | 1篇 |
1883年 | 1篇 |
排序方式: 共有350条查询结果,搜索用时 15 毫秒
141.
KM Clauwaert Van Bocxlaer JF HJ Major JA Claereboudt WE Lambert Van den Eeckhout EM Van Peteghem CH De Leenheer AP 《Rapid communications in mass spectrometry : RCM》1999,13(14):1540-1545
This paper describes the investigation of the potential of a quadrupole orthogonal acceleration time-of-flight mass spectrometer (Q-TOF) equipped with an atmospheric pressure ionisation interface for quantitative measurements of small molecules separated by reversed phase liquid chromatography. To this end, the detection limits and linear dynamic range in particular were studied in an LC/MS/MS experiment using 3,4-methylenedioxymethamphetamine standards and 3,4-methylenedioxyethylamphetamine for internal standardisation. In a second phase, the experiment was repeated with real biological extracts (whole blood, serum, and vitreous humour). A calibration for 3,4-methylenedioxymethamphetamine and its metabolite 3,4-methylenedioxyamphetamine was prepared in each of these matrices again using 3,4-methylenedioxyethylamphetamine as internal standard. The resulting quantitative data were compared with those obtained by liquid chromatography with fluorescence detection for the same extracts. The Q-TOF results revealed excellent sensitivity and a linear dynamic range of nearly four decades (2-10 000 pg on-column, r(2) = 0.9998, 1/x weighting). Furthermore, all the calibration curves prepared in biological material were superimposable, LC/MS/MS and LC-fluorescence, and the quantitative results for actual samples compared very favourably. It was concluded that the Q-TOF achieves a linear dynamic range for quantitative LC/MS/MS work exceeding that of fluorescence detection and at much better absolute sensitivity. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
142.
Ariel C. de Oliveira Paulo R. Souza Bruno H. Vilsinski Manuel E. G. Winkler Marcos L. Bruschi Eduardo Radovanovic Edvani C. Muniz Wilker Caetano Artur J. M. Valente Alessandro F. Martins 《Molecules (Basel, Switzerland)》2021,26(15)
Gelatin (GE), amino-functionalized polyphenolic tannin derivative (TN), and graphene oxide (GO) were associated to yield thermo- and pH-responsive hydrogels for the first time. Durable hydrogel assemblies for drug delivery purposes were developed using the photosensitizer methylene blue (MB) as a drug model. The cooling GE/TN blends provide brittle physical assemblies. To overcome this disadvantage, different GO contents (between 0.31% and 1.02% wt/wt) were added to the GE/TN blend at 89.7/10.3 wt/wt. FTIR and RAMAN spectroscopy analyses characterized the materials, indicating GO presence in the hydrogels. Incorporation studies revealed a total MB (0.50 mg/mL) incorporation into the GE/TN-GO hydrogel matrices. Additionally, the proposed systems present a mechanical behavior similar to gel. The GO presence in the hydrogel matrices increased the elastic modulus from 516 to 1650 Pa. SEM revealed that hydrogels containing MB present higher porosity with interconnected pores. Dissolution and swelling degree studies revealed less stability of the GE/TN-GO-MB hydrogels in SGF medium (pH 1.2) than SIF (pH 6.8). The degradation increased in SIF with the GO content, making the polymeric matrices more hydrophilic. MB release studies revealed a process controlled by Fickian diffusion. Our results point out the pH-responsible behavior of mechanically reinforced GE/TN-GO-MB hydrogels for drug delivery systems purposes. 相似文献
143.
Films of cellulose acetate containing different concentrations of meso-octamethyl-porphyrinogen (calix[4]pyrrole) were prepared and characterized using UV-vis and FTIR spectroscopy, DSC and SEM. Incorporation of calix[4]pyrrole into cellulose acetate leads to a decrease in the degree of hydrophilicity of the polymeric matrix. However, a slight increase in the percentage of water uptake of the polymer is found with an increase of the initial amount of calix[4]pyrrole in the composite composition. This effect can be related to the plasticizing effect that the calix[4]pyrrole provokes in the cellulose acetate. A comparative study of transport parameters of oxygen and some non-associated electrolytes (sodium, copper(II) and nickel chlorides) was carried out. The diffusion coefficients of molecular oxygen through cellulose acetate films decrease with an increase of the concentration of calix[4]pyrrole in the composite films. The transport (diffusion and permeation) of the above mentioned electrolytes through a set of composite films shows a decrease of permeability and diffusion coefficients with an increase of calix[4]pyrrole concentration. Such behaviour is typical of systems where interactions between the polymer and diffusing species occur. However, from the analysis of the distribution coefficient, it was found that those interactions are only dependent on the calix[4]pyrrole content for 2:1 electrolytes. 相似文献
144.
L. S. Cruz D. Felinto J.G. Aguirre Gómez M. Martinelli P. Valente A. Lezama P. Nussenzveig 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,41(3):531-539
High degrees of intensity correlation between two independent lasers were
observed after propagation through a rubidium vapor cell in which they generate
Electromagnetically Induced Transparency (EIT). As the optical field intensities are
increased, the correlation changes sign (becoming anti-correlation). The experiment was
performed in a room temperature rubidium cell, using two diode lasers tuned to the
85Rb D2 line (λ= 780 nm). The cross-correlation spectral function for the
pump and probe fields is numerically obtained by modeling the temporal dynamics of both
field phases as diffusing processes. We explored the dependence of the atomic response on
the atom-field Rabi frequencies, optical detuning and Doppler width. The results show
that resonant phase-noise to amplitude-noise conversion is at the origin of the observed
signal and the change in sign for the correlation coefficient can be explained as a
consequence of the competition between EIT and Raman resonance processes. 相似文献
145.
C. Bacci G. De Zorzi G. Penso B. Stella R. Baldini-Celio G. Capon R. Del Fabbro M. Grilli A.F. Grillo E. Iarocci C. Mencuccini G.P. Murtas M. Spinetti V. Valente 《Physics letters. [Part B]》1977,71(1):227-230
A new measurement of the cross section for the reaction e+e? → e+e?γ has been performed at the ADONE storage ring, using a wide angle electromagnetic detector together with tagging counters for detecting small-angle electrons. The results are in agreement with the QED predictons. New limits for the mass and coupling constant (e1, eγ) of a heavy electron, e1, have been established. 相似文献
146.
We consider two linearly coupled masses, where one mass can have inelastic impacts with a fixed, rigid stop. This leads to the study of a two degree of freedom, piecewise linear, frictionless, unforced, constrained mechanical system. The system is governed by three types of dynamics: coupled harmonic oscillation, simple harmonic motion and discrete rebounds. Energy is dissipated discontinuously in discrete amounts, through impacts with the stop. We prove the existence of a non-zero measure set of orbits that lead to infinite impacts with the stop in a finite time. We show how to modify the mathematical model so that forward existence and uniqueness of solutions for all time is guaranteed. Existence of hybrid periodic orbits is shown. A geometrical interpretation of the dynamics based on action coordinates is used to visualize numerical simulation results for the asymptotic dynamics. 相似文献
147.
G. Battistoni C. Bloise A. F. Grillo A. Marini F. Ronga V. Valente 《Il Nuovo Cimento C》1988,11(2):175-184
Summary We present results from MICRO, a muon telescope with good angular resolution, which has collected more than 31·106 cosmic muons. Upper limits are given for the flux coming from point sources and for the periodic component from Cygnus X3. 相似文献
148.
Antonio L. Pires Valente Fabio Augusto Eduardo Carasek da Rocha 《Journal of separation science》1995,18(5):315-317
Quantitative chromatographic analysis is liable to errors due to peak asymmetry because the uncertainty in the detected position of the end of the peak tail decreases the reliability of the computed peak area. This dependence may be a severe drawback whenever peaks of different areas must be compared, as in the case of calibration curves. A new approach to overcome the uncertainties of area calculation due to peak asymmetry is reported in this paper. The approach consists of calculating only the area included between the start and the maximum of the chromatographic peak. Simulated and experimental chromatographic data were used in this study. Both the peak start-to-peak maximum area (SMA) and the start-to-end or total area (TA) were calculated and the quantitative results were compared. Within the scope of this work it is concluded that the SMA yields calibration curves that are more linear and have intercepts closer to zero than the calibration curves obtained using the TA. 相似文献
149.
I. M. Valente M. J. Minski P. J. Peterson 《Journal of Radioanalytical and Nuclear Chemistry》1982,71(1-2):115-127
Detection limits are presented for the platinum metals in vegetation for different neutron activation analysis conditions,
namely, short irradiation, cyclic and long irradiations both thermally and epithermally. These detection limits for instrumental
neutron activation analysis are compared with those using preconcentration technique—dry ashing and fire assay and post irradiation
separation of the platinum metals using Srafion NMRR in exchange resin. The results are evaluated for plant material and it
is concluded that dry ashing followed by cyclic, epithermal irradiation for rhodium and palladium and long, thermal irradiations
for osmium, platinum, iridium and ruthenium enable these elements to be detected in plants at background levels. 相似文献
150.
The target of our study was the determination of urinary t,t muconic acid concentration, as a biomarker of benzene intake in a population. Urinary samples of 101 subjects, males and females were obtained to whom. Information regarding modification and confounding factors were recorded by questionnaire. The samples were analysed by HPLC, after extraction of trans,trans muconic acid on strong anionic exchange cartridges. In these analytical conditions the detection limit with good care is 10 micrograms/l. A concentration range from 10 to 1300 micrograms/l was found. High levels found in the population could explained by confounding factors present. The obtained results were correlated with sex, age and smoking or non smoking. There was a significant difference in mean concentrations between heavy smokers (160 micrograms/l) and non smokers (110 micrograms/l), that showed tobacco as an interference factor. The study showed that it is strictly necessary carry out a thorough anamnesis before sampling with the objective of finding eventual interference factors such as smoking, diet and drugs taken. 相似文献