全文获取类型
收费全文 | 308篇 |
免费 | 18篇 |
专业分类
化学 | 260篇 |
晶体学 | 1篇 |
力学 | 2篇 |
数学 | 38篇 |
物理学 | 25篇 |
出版年
2023年 | 7篇 |
2022年 | 10篇 |
2021年 | 44篇 |
2020年 | 12篇 |
2019年 | 7篇 |
2018年 | 9篇 |
2017年 | 15篇 |
2016年 | 33篇 |
2015年 | 15篇 |
2014年 | 10篇 |
2013年 | 28篇 |
2012年 | 27篇 |
2011年 | 16篇 |
2010年 | 14篇 |
2009年 | 14篇 |
2008年 | 21篇 |
2007年 | 8篇 |
2006年 | 11篇 |
2005年 | 9篇 |
2004年 | 5篇 |
2003年 | 5篇 |
2002年 | 2篇 |
2001年 | 2篇 |
2000年 | 1篇 |
1992年 | 1篇 |
排序方式: 共有326条查询结果,搜索用时 265 毫秒
91.
Joanna Pałdyna Beata Krasnodębska‐Ostręga Monika Sadowska Justyna Gołębiewska 《Electroanalysis》2013,25(8):1926-1932
A sensitive voltammetric method (DPASV) was developed for the determination of Tl(I) and Tl(III) in plant extracts. To limit the influence of the organic matrix on the measurements, UV irradiation and addition of Amberlite XAD‐7 resin was studied. The application of 0.5 g of the resin allowed defining thallium speciation in 10.0 mL of a solution containing 0.20 mL of Sinapis alba extract. The quantification limit of 0.5 ng mL?1 Tl(I) was found for only 10 min of preconcentration, and is low enough to allow dilution of the sample before thallium determination. The procedure was validated using the recovery study and intermethod comparison with HPLC ICP MS. 相似文献
92.
In this study, the glycerol solutions were fermented using Lactobacillus casei bacteria. The broths were pre-treated by microfiltration, followed by a further separation with nanofiltration. The latter process was carried out in two stages, using the NF270 and NF90 membranes, respectively. The concentrates thus obtained were enriched with citric acid (first stage) and then with lactic acid and glycerol (second stage). By means of SEM and AFM microscopy, as well as ATR-FTIR analysis, the intensity of membrane-fouling was studied. The colloidal fouling and bio-fouling caused a more than two-fold decrease in the permeate flux during microfiltration of the broth. This pre-treatment stage was effective, and a permeate turbidity of less than 0.2 NTU was obtained. However, the nanofiltration membranes exhibited a 30 % flux decline over the course of the process, mainly due to the organic fouling. 相似文献
93.
Justyna Kęczkowska 《Central European Journal of Physics》2011,9(2):330-337
The work presents the results of the scanning electron microscopy (SEM) and Raman spectrometry studies of carbonaceous nanostructures
containing nickel nanocrystallites. The films were obtained using a two-step method. In the first phase the Physical Vapour
Deposition (PVD) method was applied, whereas in the second Chemical Vapour Deposition (CVD) method was used. The paper presents
results for samples with various Ni content obtained with different parameters of the two-phase technological process. The
research confirms that the thin films obtained by PVD method contain Ni nanocrystallites distributed in a carbonaceous matrix.
The matrix is composed of various carbon allotropes (amorphous carbon, graphite, fullerene). The thin films made by CVD method
make a matrix when multiwalled, carbonaceous nanotubes are obtained. Depending on the technological process parameters of
each phase, we obtain multiwall nanotubes with a various degree of defects. 相似文献
94.
Daniel D?ugosz Malgorzata Pach Anna Zabrzeńska Monika Zegar Barbara J. Oleksyn Justyna Kalinowska-T?u?cik Katarzyna Ostrowska 《Monatshefte für Chemie / Chemical Monthly》2008,83(1):543-548
Bis(cycloamidine-2-yl)alkanes easily form bromide tribromide salts in the reduction-oxidation processes with bromine. Bis(tetrahydroimidazolium-2-yl)ethane
and bis(hexahydropyrimidinium-2-yl)-ethane bromide tribromides are such new convenient brominating agents for aromatic amides
in chemo- and regioselective electrophilic substitutions and α-bromination reactions. 相似文献
95.
Justyna ?awecka 《Tetrahedron letters》2008,49(4):723-726
A new route to thiacrown ethers 5a-d and 6a-d incorporating a 2,2′-bipyridine subunit is elaborated using, (1) homo-coupling of 1,2,4-triazine sulfides 3a-d tethered to poly(ethylene glycol) chains with potassium cyanide and (2) Diels-Alder/retro Diels-Alder reaction with norbornadiene or 1-pyrrolidino-1-cyclopentene as the key steps. 相似文献
96.
A thin-layer chromatography (TLC)-densitometric method was developed to determine N-(hydroxymethyl)nicotinamide in tablets and basic solutions along with nicotinic acid. Analysis was performed on silica gel F254 plates using chloroform-ethanol (2 + 3, v/v) mobile phase. The densitometric observations were made at 260 nm. The results showed good precision and accuracy; relative standard deviation was 2.37%, and recovery ranged from 97.60 to 100.82%. The limit of detection was 0.1 microg/spot, while the linearity range was from 0.2 to 1.75 microg/spot. Applicability of the newly developed method was tested for determination of N-(hydroxymethyl)nicotinamide in the preparation Cholamid. Densitometric measurements were used to evaluate stability of N-(hydroxymethyl)nicotinamide in basic solutions. It was found that decomposition corresponded to first-order reaction kinetics. The computed kinetic and thermodynamic parameters at 30 degrees C were as follows: k = 0.00675/min, t0,5 = 1.71 h, t0,1 = 0.26 h, and Ea = 44.75 kJ/mol. 相似文献
97.
98.
In the present study, the preparation of methyl esters of chlorophenoxy acids with trimethylsilyldiazomethane (TMSD) is reported. TMSD is a mild and safe alternative to the highly toxic, thermally labile, and explosive diazomethane. Due to the novelty of this method, the characteristics of the reaction were investigated and the derivatization procedure was screened for statistically significant factors. The parameters identified as influential are the amount of methanol as well as reaction time and temperature. A careful optimization by a complete factorial response surface design was carried out to generate optimal reaction conditions with respect to maximum recovery and reaction yield. Special emphasis has been given to an accurate validation of the derivatization procedure. LODs were determined between 4.1 and 8.1 ng/mL for the enriched sample and linearity was proofed for more than five orders of magnitude. The applicability of the method was demonstrated by the analyses of quality control material including sample preparation with SPE. Polar herbicides investigated in the study belong to acetic acid (2,4-D, MCPA, and 2,4,5-T), butanoic acid (MCPB), and propanoic acid (mecoprop). An efficient, fast, and high-throughput method for the determination of polar herbicides including the potential for automation is proposed. 相似文献
99.
Kinga Wzgarda-Raj Justyna Dominikowska Natallia Husik Agnieszka J. Rybarczyk-Pirek 《Acta Crystallographica. Section C, Structural Chemistry》2023,79(9):374-380
X-ray diffraction studies reveal that pyrazine-2-thiol undergoes condensation to 2,2′-dithiobispyrazine [systematic name: 2-(pyrazin-2-yldisulfanyl)pyrazine], C8H6N4S2 ( I ), under aerial conditions. In the molecule of I , the pyrazine rings are arranged in an almost perpendicular manner, with an absolute value of the C—S—S—C torsion angle of −91.45 (6)°. A search in the Cambridge Structural Database confirmed that such a conformation is typical for disulfide compounds. Three different rotamers of disulfide I were studied using quantum theoretical studies. The rotamer of lowest energy was observed in the crystalline state in the structure stabilized by hydrogen-bond, chalcogen-bond and stacking interactions. Further quantum chemical computations confirm that 2,2′-dithiobispyrazine can react according to the SN2 mechanism. 相似文献
100.
Błazewicz A Fijałek Z Warowna-Grześkiewicz M Banasiuk J 《Journal of chromatography. A》2008,1204(1):114-118
A HPLC method with amperometric and coulometric detection for the determination of SZ1677 and its two derivatives SZ1676 and SZ1823 has been developed. This active substance is under development (clinical trial) and there are no analytical methods published for the determination of SZ1677 thus far. The limit of quantitation for SZ1677 was 25 and 100ngml(-1) by the coulometric and amperometric detection, respectively. The elaborate method for the simultaneous analysis of SZ1677 and its two derivatives proved to be fast, precise, accurate and sensitive. 相似文献