全文获取类型
收费全文 | 1215篇 |
免费 | 40篇 |
专业分类
化学 | 738篇 |
晶体学 | 7篇 |
力学 | 26篇 |
数学 | 216篇 |
物理学 | 268篇 |
出版年
2023年 | 10篇 |
2022年 | 13篇 |
2021年 | 12篇 |
2020年 | 17篇 |
2019年 | 32篇 |
2018年 | 34篇 |
2017年 | 20篇 |
2016年 | 43篇 |
2015年 | 48篇 |
2014年 | 39篇 |
2013年 | 75篇 |
2012年 | 88篇 |
2011年 | 79篇 |
2010年 | 52篇 |
2009年 | 59篇 |
2008年 | 70篇 |
2007年 | 74篇 |
2006年 | 59篇 |
2005年 | 58篇 |
2004年 | 32篇 |
2003年 | 21篇 |
2002年 | 31篇 |
2001年 | 26篇 |
2000年 | 14篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 16篇 |
1993年 | 21篇 |
1992年 | 15篇 |
1990年 | 6篇 |
1988年 | 9篇 |
1985年 | 6篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1967年 | 4篇 |
1966年 | 7篇 |
1965年 | 5篇 |
1964年 | 9篇 |
1963年 | 6篇 |
排序方式: 共有1255条查询结果,搜索用时 15 毫秒
851.
Combining bar adsorptive microextraction with capillary electrophoresis—Application for the determination of phenolic acids in food matrices 下载免费PDF全文
Nuno da Rosa Neng Rute C. P. Sequeiros José Manuel Florêncio Nogueira 《Electrophoresis》2014,35(17):2488-2494
In this contribution, bar adsorptive microextraction coated with a mixed‐mode anion exchange/RP followed by liquid desorption was combined for the first time with a capillary electrophoresis‐diode array detection system (BAμE(MAX)‐LD/CE‐DAD), for the determination of phenolic acids in food matrices, using chlorogenic, ferulic, cumaric, and caffeic acids as model compounds. Assays performed in aqueous media spiked at the 0.8 mg/L level yielded average recoveries up to 40% for all four phenolic acids, under optimized experimental conditions. The analytical performance showed also good precision (RSD < 15%), convenient LODs (18.0–85.0 μg/L) and linear dynamic ranges (0.8–8.0 mg/L) with convenient determination coefficients (r2 > 0.9900). By using the standard addition method, the application to food matrices such as green tea, red fruit juice, and honey allowed very good performances for the determination of minor amounts of phenolic acids. The proposed methodology proved to be a suitable alternative for the analysis of polar to ionic compounds, showing to be easy to implement, reliable, sensitive, and requiring a low sample volume to determine phenolic acids in food samples. 相似文献
852.
José Manuel Salgado Luís Abrunhosa Armando Venâncio José Manuel Domínguez Isabel Belo 《Applied biochemistry and biotechnology》2014,172(4):1832-1845
Two-phase olive mill waste (TPOMW) is presently the major waste produced by the olive mill industry. This waste has potential to be used as substrate for solid state fermentation (SSF) despite of its high concentration of phenolic compounds and low nitrogen content. In this work, it is demonstrated that mixtures of TPOMW with winery wastes support the production of lipase by Aspergillus spp. By agar plate screening, Aspergillus niger MUM 03.58, Aspergillus ibericus MUM 03.49, and Aspergillus uvarum MUM 08.01 were chosen for lipase production by SSF. Plackett–Burman experimental design was employed to evaluate the effect of substrate composition and time on lipase production. The highest amounts of lipase were produced by A. ibericus on a mixture of TPOMW, urea, and exhausted grape mark (EGM). Urea was found to be the most influent factor for the lipase production. Further optimization of lipase production by A. ibericus using a full factorial design (32) conducted to optimal conditions of substrate composition (0.073 g urea/g and 25 % of EGM) achieve 18.67 U/g of lipolytic activity. 相似文献
853.
André Luís Branco de Barros Luciene das Graças Mota Carolina de Aguiar Ferreira Natássia Caroline Resende Corrêa Alfredo Miranda de Góes Mônica Cristina Oliveira Valbert Nascimento Cardoso 《Journal of Radioanalytical and Nuclear Chemistry》2013,295(3):2083-2090
Bombesin is a tetradecapeptide that binds specifically to gastrin releasing peptide receptors in humans. Several forms of cancer, including lung, prostate, breast, and colon express receptors for bombesin-like peptides. Radiolabeled bombesin analogs with a high affinity for these receptors might therefore be used for scintigraphic imaging of these tumor types. A truncated bombesin derivative (HYNIC-βAla-Bombesin(7–14)) was radiolabeled with technetium-99m using EDDA and tricine as coligands. In vitro stability was evaluated in presence of plasma and excess of cysteine. The receptor-binding affinity assays was evaluated in MDA-MB-231 cancer cell line. In addition, in vivo biodistribution was performed in nude mice bearing breast tumor. In vitro assay showed a good affinity for the MDA-MB-231 cell line, showing 20.0 % of internalization at 4 h post-administration. 99mTc-HYNIC-βAla-Bombesin(7–14) biodistribution revealed a rapid clearance and a significant renal excretion. In addition, tumor uptake was higher than non-excretory organs, such as the spleen, the liver, and muscles. Tumor-to-muscle and tumor-to-blood ratios for 99mTc-HYNIC-βAla-Bombesin(7–14) showed high values at 4 h post-injection (5.34 and 4.55, respectively). Furthermore, blocked studies using cold bombesin peptide were performed, which demonstrated an important decrease in tumor uptake, indicating a tumor specificity for 99mTc-HYNIC-βAla-Bombesin(7–14). The 99mTc-HYNIC-βAla-Bombesin(7–14) displayed suitable radiochemical characteristics, and adequate affinity to breast tumor cells (MDA-MB-231). Therefore, this analog can be considered as a candidate for the identification of bombesin-positive tumors. 相似文献
854.
855.
856.
Marcia C. Martins Castoldi Le?ncio D. T. Camara Rafael S. Monteiro Alex M. Constantino Luciana Camacho José Walkimar M. de Carneiro Donato A. G. Aranda 《Reaction Kinetics and Catalysis Letters》2007,91(2):341-352
Quantum chemical calculations were performed to obtain information on the molecular properties of relevant species involved
in catalytic glucose hydrogenation for the production of sorbitol. 相似文献
857.
We prove the Hodge–Riemann bilinear relations, the hard Lefschetz theorem and the Lefschetz decomposition for compact K?hler
manifolds in the mixed situation.
Dedicated to Professor Henri Skoda on the occasion of his 60th birthday
Received: February 2005; Revision: March 2005; Accepted: April 2005 相似文献
858.
D. F. Brito E. F. B. Silva D. P. Rodrigues M. C. N. Machado M. A. Silva V. Simões M. W. N. C. Carvalho L. E. B. Soledade Iêda M. G. Santos A. G. Souza 《Journal of Thermal Analysis and Calorimetry》2007,87(3):853-857
A silico alumino phosphate with AFI structure (SAPO-5) was prepared
in a two-phase medium and characterized by XRD, followed by the addition of
TEA+. The kinetics of the TEA+/SAPO-5
thermal decomposition reaction was studied by isothermal and dynamic thermogravimetry.
Two kinetic models, D3 and D4
based on diffusion processes were found as best to fit the isothermal data.
On the other hand, the best fit for the dynamic data is the F1
first order reaction model. According to the apparent activation energy values,
the use of the dynamic method indicates a higher temperature dependence than
the isothermal method. 相似文献
859.
Inês C. R. Barbosa Maximiliane Kley Rudi Schäufele Karl Auerswald Wolf Schröder Flurin Filli Stefan Hertwig Hans Schnyder 《Rapid communications in mass spectrometry : RCM》2009,23(15):2347-2356
The horn of ungulate grazers offers a valuable isotopic record of their diet and environment. However, there have been no reports of the spatio‐temporal variation of the isotopic composition of horns. We investigated patterns of carbon (δ13C) and nitrogen (δ15N) isotopic composition along and perpendicular to the horn axis in Capra ibex and Rupicapra rupicapra rupicapra to assess the effects of animal age, within‐year (seasonal) and inter‐annual variation, natural contamination and sampling position on horn isotope composition. Horns of male C. ibex (n = 23) and R. r. rupicapra (n = 1) were sampled longitudinally on the front (only R. r. rupicapra) and back side and on the surface and sub‐surface. The sides of the R. r. rupicapra horn did not differ in δ13C. In both species, the horn surface had a 0.15‰ lower δ13C and a higher carbon‐to‐nitrogen (C/N) ratio than the sub‐surface. Washing the horn with water and organic solvents removed material that caused these differences. With age, the δ15N of C. ibex horns increased (+0.1‰ year?1), C/N ratio increased, and 13C discrimination relative to atmospheric CO2 (13Δ) increased slightly (+0.03‰ year?1). Geostatistical analysis of one C. ibex horn revealed systematic patterns of inter‐annual and seasonal 13C changes, but 15N changed only seasonally. The work demonstrates that isotopic signals in horns are influenced by natural contamination (δ13C), age effects (13Δ and δ15N), and seasonal (δ13C and δ15N) and inter‐annual variation (δ13C). The methods presented allow us to distinguish between these effects and thus allow the use of horns as isotopic archives of the ecology of these species and their habitat. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
860.
Alcohol ethoxylates (AEs) are nonionic surfactants. They are industrially important compounds that have historically been difficult to analyze, with the best results to date achieved through derivatization (e.g., silylation) followed by analysis by gas chromatography/mass spectrometry (GC/MS). Recently, mass spectrometric techniques such as field desorption (FD), time-of-flight secondary ion mass spectrometry (TOF-SIMS), fast atom bombardment (FAB), electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) have been employed to analyze surfynol(R) 4xx. In an effort to produce low-cost alkyl-capped AEs and anionic detergents from AEs, a fast and reliable measure of the product yields and conversions from AEs is required in research. We found that the product yields and conversions from reactions of AEs, obtained by the employment of atmospheric pressure chemical ionization (APCI), were in good agreement with those obtained from proton nuclear magnetic resonance spectroscopy ((1)H-NMR). Therefore, APCI can be used as a validated tool for studying AE reactions. Mixtures that contain either silylated or unsilylated ethoxylates and/or carboxylates yield the same APCI mass spectra. Copyright -Copyright 1999 John Wiley & Sons, Ltd. 相似文献