全文获取类型
收费全文 | 532篇 |
免费 | 62篇 |
国内免费 | 30篇 |
专业分类
化学 | 563篇 |
晶体学 | 6篇 |
力学 | 2篇 |
综合类 | 1篇 |
物理学 | 52篇 |
出版年
2024年 | 1篇 |
2023年 | 4篇 |
2022年 | 22篇 |
2021年 | 31篇 |
2020年 | 36篇 |
2019年 | 13篇 |
2018年 | 7篇 |
2017年 | 9篇 |
2016年 | 19篇 |
2015年 | 26篇 |
2014年 | 28篇 |
2013年 | 47篇 |
2012年 | 23篇 |
2011年 | 29篇 |
2010年 | 19篇 |
2009年 | 22篇 |
2008年 | 30篇 |
2007年 | 26篇 |
2006年 | 20篇 |
2005年 | 19篇 |
2004年 | 32篇 |
2003年 | 27篇 |
2002年 | 30篇 |
2001年 | 16篇 |
2000年 | 16篇 |
1999年 | 10篇 |
1998年 | 9篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有624条查询结果,搜索用时 15 毫秒
1.
Rizwan Ahmad Niyaz Ahmad Fatimah AlOthman Haya Mohammad Fatimah AlZahrani 《Biomedical chromatography : BMC》2020,34(1):e4712
Coffee and tea are the most widely consumed beverages worldwide. However, the consumer may be unaware of the exact amount of methyl xanthine (MX, i.e. caffeine [C], theobromine [TB] and theophylline [TH]) consumed, as most of the products do not list the proper amounts. This may lead to serious risks including cardiovascular, kidney and stimulant effects. The aim of the study was to determine the MX amount in ready-to-use beverages (coffee and tea) collected from various outlets in the city of Al-Khobar, Saudi Arabia. Forty different samples of espresso, black coffee and red tea were collected. A fast, reliable and efficient UHPLC–DAD method was developed and validated for MX determination. Total lipids were extracted and fractionated in order to determine glycolipids, phospholipids and neutral lipids. The r2 value for the method was 0.980–0.988 in a linearity range of 0.5–200 ppm. The range for MX (C [0.02–2.39 mg/ml], TB [0.00–0.10 mg/ml] and TH [0.00–0.004 mg/ml]) and total lipids was 1–5 g. The amount of glycolipids (3.1 g) was higher among the lipid fractions followed by phospholipids (1.8 g) and neutral lipids (0.25 g). In general, espresso beverages (20–30 ml) contained high amounts of MX whereas black coffee beverages contained high amount of lipids. Most of the beverages expressed C, TB, TH, lipids or their fractions; however, the product with high amounts of MX and lipids at the same time was espresso (brands Chemistry and Wogard). Although the MX and lipid levels in these beverages well below the allowed limits, care must still be taken, especially when using the beverages with high serving volumes (200–250 ml) or coffee prepared via the filter method i.e. black coffee, using a high temperature for a longer time. 相似文献
2.
Two lipids with similar melting ranges but of different composition were analyzed using differential scanning calorimetry and X-ray diffraction. The lipids were processed via extrusion or were tempered at different temperatures; they were analyzed directly after extrusion and after storage at 40°C. Precirol ATO 5® showed high sensitivity to storage time and varied temperature exposure. Extrusion showed only marginal influences on the solid state. Melting peaks were narrower and shifted to higher temperatures in comparison to the untreated powder. Dynasan 114® was more robust, changes in the solid state could only be shown for samples treated above the melting range. Thus, Dynasan 114® is more appropriate for solid lipid extrusion of pharmaceutical products. 相似文献
3.
Kaloustian J Pauli A-M Lechene de la Porte P Lafont H Portugal H 《Journal of Thermal Analysis and Calorimetry》2003,71(2):341-351
Cholesterol constitutes the major component of most gallstones. It was identified and determined, in gallstones, issued from
eleven patients, by thermal analysis: differential scanning calorimetry (DSC), with the use of the melting temperature and
enthalpy, thermogravimetry (TG), with the mass loss of water. Anhydrous cholesterol (ChA) was characterized by two endothermic
peaks (polymorphic, melting) and cholesterol monohydrate (ChH) by two endothermic peaks (dehydration, melting), too. Cha needle
and Chh plate crystals were observed under polarizing light microscopy. The numerous stones obtained from nine patients were
cholesterol stones: the ChA was higher 45 and lower 96%. ChH was present in stones of three patients.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
5.
The chemical composition of neutral lipids from seeds of Cercis siliquastrum, Sapium sebiferum, and Koelreuteria paniculata were studied. Characteristic features of their individual classes were established.
__________
Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 318–319, July–August, 2007 相似文献
6.
Labeling experiments using several deuterated lipids were pursued to study the biosynthesis of macrocyclic isoprenoidal lipids of thermophilic methanogenic archaea, Methanothermobacter thermautotrophicus. The isopropylidene terminal of geranylgeranyl group of monomeric precursor appeared to be important for the CC bond formation at the hydrophobic end in the macrocyclic lipids. A mechanism involving a radical trigger at the allylic methyl group is proposed for this CC bond formation. 相似文献
7.
Formation and Structure of [Al_(13)(μ_3-OH)_6(μ_2-OH)_6(μ_2-OH)_(12)(H_2O)_(24)]Cl_(15)·13H_2O 总被引:1,自引:0,他引:1
1 INTRODUCTION At present, the polyaluminium compounds are mainly studied by single-crystal X-ray diffraction method to obtain the components, structures and exis- tence forms of aluminium in hydrolysis system, and then hydrolysis courses and mechanisms of each hydrolytic polyaluminium cation could be further dis- closed[1~5]. Generally, the single crystals suitable for X-ray diffraction are obtained from the crystallization of polyaluminium cations into sulfates or selenates. Following … 相似文献
8.
Sergii Afonin Alexander Frey Sybille Bayerl Dahlia Fischer Parvesh Wadhwani Sevil Weinkauf Anne S Ulrich 《Chemphyschem》2006,7(10):2134-2142
Cell-penetrating peptides (CPPs) are short polycationic sequences that can translocate into cells without disintegrating the plasma membrane. CPPs are useful tools for delivering cargo, but their molecular mechanism of crossing the lipid bilayer remains unclear. Here we study the interaction of the HIV-derived CPP TAT (48-60) with model membranes by solid-state NMR spectroscopy and electron microscopy. The peptide induces a pronounced isotropic (31)P NMR signal in zwitterionic DMPC, but not in anionic DMPG bilayers. Octaarginine and to a lesser extent octalysine have the same effect, in contrast to other cationic amphiphilic membrane-active peptides. The observed non-lamellar lipid morphology is attributed to specific interactions of polycationic peptides with phosphocholine head groups, rather than to electrostatic interactions. Freeze-fracture electron microscopy indicates that TAT(48-60) induces the formation of rodlike, presumably inverted micelles in DMPC, which may represent intermediates during the translocation across eukaryotic membranes. 相似文献
9.
Positively charged lipids of the 1,3-oxathiolane series were synthesized by interaction of 2-pentadecyl-5-tosyloxymethyl- or -5-iodomethyl-1,3-oxathiolane with 2-(N,N-dimcthyl-amino)ethanol.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2347–2349, September, 1996. 相似文献
10.
J. Kaloustian Paulette Lechene De La Porte T. El-Moselhy H. Lafont H. Portugal 《Journal of Thermal Analysis and Calorimetry》2005,82(2):331-338
Summary Cholesterol constitutes the major component of most gallstones. It was identified and determined in gallstones by thermal
analysis technique (DSC and TG-DTA), mainly by the use of the melting temperature (Tonset=145°C and Tmax=149°C) and by DTG peak decomposition (Tmax=364°C). Cholesterol anhydrous (ChA), which showed endothermic polymorphic peak, Tmax=40°C, without mass loss, was differentiated from cholesterol monohydrate (ChH), which showed a broad endothermic peak, Tmax=59°C, attributed to loss of water of crystallization (theoretical 4.45%). Morphological studies of gallstones were performed
by optical microscopy and scanning electron microscopy (SEM). The stones consisted of a pigmented core with a variably-sized
irregular central cavity, surrounded by a radially arranged deposits of plate-like ChH. The outer part of the stones showed
ChA crystal arborescences. X-ray microanalysis gave a typical spectrum rich in C and O, and in some instances the presence
of P, which was attributed to the presence of phospholipids. CaCO3 was easily characterized by TG with the use of DTG decomposition peak at 674°C. 相似文献