全文获取类型
收费全文 | 2083篇 |
免费 | 78篇 |
国内免费 | 7篇 |
专业分类
化学 | 1491篇 |
晶体学 | 26篇 |
力学 | 17篇 |
数学 | 304篇 |
物理学 | 330篇 |
出版年
2023年 | 14篇 |
2022年 | 37篇 |
2021年 | 83篇 |
2020年 | 67篇 |
2019年 | 51篇 |
2018年 | 48篇 |
2017年 | 40篇 |
2016年 | 89篇 |
2015年 | 76篇 |
2014年 | 80篇 |
2013年 | 128篇 |
2012年 | 128篇 |
2011年 | 139篇 |
2010年 | 103篇 |
2009年 | 97篇 |
2008年 | 135篇 |
2007年 | 110篇 |
2006年 | 90篇 |
2005年 | 81篇 |
2004年 | 74篇 |
2003年 | 59篇 |
2002年 | 65篇 |
2001年 | 36篇 |
2000年 | 38篇 |
1999年 | 24篇 |
1998年 | 18篇 |
1997年 | 13篇 |
1996年 | 23篇 |
1995年 | 18篇 |
1994年 | 24篇 |
1993年 | 20篇 |
1992年 | 26篇 |
1991年 | 22篇 |
1990年 | 18篇 |
1989年 | 12篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 11篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 4篇 |
1979年 | 5篇 |
1977年 | 2篇 |
1968年 | 1篇 |
1936年 | 2篇 |
1927年 | 2篇 |
1924年 | 2篇 |
1923年 | 1篇 |
排序方式: 共有2168条查询结果,搜索用时 31 毫秒
101.
Assessment of pharmacokinetic interaction between piracetam and l‐carnitine in healthy subjects
下载免费PDF全文
![点击此处可从《Biomedical chromatography : BMC》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Gustavo D. Mendes Gabriela Traldi Zaffalon Antonio Sérgio Silveira Juliana Cama Ramacciato Rogério Heládio Lopes Motta Thiago Gagliano‐Jucá Anibal Gil Lopes José Cássio de Almeida Magalhães Gilberto De Nucci 《Biomedical chromatography : BMC》2016,30(4):536-542
A rapid, sensitive and specific method for quantifying piracetam in human plasma using Piracetam d‐8 as the internal standard (IS) is described. The analyte and the IS were extracted from plasma by one‐step precipitation of protein using an acetonitrile (100%). The extracts were analyzed by high‐performance liquid chromatography coupled with electrospray tandem mass spectrometry (HPLC‐MS/MS). The method had a chromatographic run time of 3.8 min and a linear calibration curve over the range 0.5–50 µg/mL (r > 0.99). This LC‐MS‐MS procedure was used to assess the bioavailability of two piracetam formulations: piracetam + l‐carnitine (Piracar®; 270/330 mg tablet) and piracetam (Nootropil®; 800 mg tablet) in healthy volunteers of both sexes. The geometric means with corresponding 90% confidence interval (CI) for test/reference percentage ratios were 88.49% (90% CI = 81.19 – 96.46) for peak concentration/dose and 102.55% (90% CI = 100.62 – 104.51) for AUCinf/dose. The limit of quantitation of 0.5 µg/mL is well suited for pharmacokinetic studies in healthy volunteers. It was concluded that piracetam (Piracar®; 270/330 mg tablet) has a bioavailability equivalent to the piracetam (Nootropil®; 800 mg tablet) formulation with regard to both the rate and the extent of absorption. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
102.
Daniel Romero Nieto Mikhail G. Zolotukhin Lioudmila Fomina Serguei Fomine 《Journal of Physical Organic Chemistry》2010,23(9):878-884
Energies of mono‐ and multiprotonation for 1,2,3‐indanetrione and ninhydrin in triflic acid (TFSA) media were estimated at PBE0/aug‐cc‐pvtz//6‐31+G** level of theory. The reactivity of formed intermediates in the reaction of aromatic electrophilic substitution has been studied at the same level of theory. It appears that the basicity of carbonyl groups in 1,2,3‐indanetrione is extremely low due to mutual influences of carbonyl groups. Carbonyl 2 is the least basic but the most reactive in accordance with experiment. Calculations demonstrated that monoprotonated intermediates are the principal reactive species in the reaction of hydroxyalkylation of 1,2,3‐indanetrione in TFSA. A new isomerization mechanism of 2,2‐diaryl‐1,3‐indanediones to 3‐(diarylmethylene)isobenzofuranones in TFSA media has been proposed. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
103.
104.
105.
106.
Raiza Fernández Freddy Galarraga Zully Benzo Gonzalo Márquez M. Gabriela Requiz 《International journal of environmental analytical chemistry》2013,93(3):230-240
Biomonitoring of PAH air pollution using lichens was carried out. Sixteen PAHs were studied in 11 locations along the valley of Caracas (Venezuela). The results of this work indicate that 14 of the 16 analysed PAHs were highly accumulated into the lichen thalli of Pyxine coralligera Malme. PAH levels in the samples revealed that the several volatile PAHs (naphthalene, acenaphtylene, acenaphtene, and fluoranthene) have the highest levels in the majority of the studied locations. The fluoranthene/pyrene and phenantrene/antracene ratios suggested that the major sources of PAHs are anthropogenic, mainly associated with gasoline and diesel combustion (pyrolytic) and unburnt oil derivates (petrogenic). The total PAH concentrations obtained in the present study were in the range of 0.24 to 9.08?µg/g, similar to those reported by other works in European and Asian cities. 相似文献
107.
Nicoleta Gabriela Hădărugă Daniel Ioan Hădărugă Heinz-Dieter Isengard 《Journal of inclusion phenomena and macrocyclic chemistry》2013,75(3-4):297-302
Cyclodextrins are some of the most used carriers for bioactive compounds (as host–guest complex) and many factors influence the association–dissociation of this complex, some of them being related to hydrophobicity. In the solid state, cyclodextrins contain two types of water molecules: “surface” water molecules (especially close to the crystal surface) and “strong-bonded” water molecules (especially from the cyclodextrin cavity), but the classification is hard to do, and the concentration of these water molecules are relatively difficult to estimate by simple methods. In the present study we used the volumetric Karl Fischer titration to estimate these types of water molecules in cyclodextrins by means of the rate of water reaction (related to diffusion from cyclodextrin crystals). “Surface” water molecules are titrated with rates between 1.8–2.8 mM/s for α-cyclodextrin, while for β-cyclodextrin these rates are little bit higher (2.9–3.4 mM/s). The rates corresponding to “strong-bonded” water molecules are approximately tens fold lower (0.05–0.3 mM/s for α-cyclodextrin and 0.15–0.33 mM/s for β-cyclodextrin). The approximate ratio between “surface” and “strong-bonded” water molecules could also be estimated by this simple and rapid method. 相似文献
108.
Amanda S. Santana Diego B. Carvalho Nadla S. Cassemiro Luiz H. Viana Gabriela R. Hurtado Marcos S. Amaral Najla M. Kassab Palimécio G. Guerrero Jr. Sandro L. Barbosa Miguel J. Dabdoub Adriano C.M. Baroni 《Tetrahedron letters》2014
A simple synthesis of 3-iodothiophenes was demonstrated using a wide range of (Z)-thioenynes. The key step in the iodocyclofunctionalization was the selective reduction of the triple bond in (Z)-thioenynes by the addition of iodine as an electrophilic agent. The 3-iodothiophenes were obtained in good to excellent yields of 61–92%. The 3-iodothiophenes were used as substrates in Sonogashira cross-coupling reactions to obtain thiophene acetylenes. 相似文献
109.
Multiresidue method for the fast determination of pesticides in nutraceutical products (Camellia sinensis) by GC coupled to triple quadrupole MS
下载免费PDF全文
![点击此处可从《Journal of separation science》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Gerardo Martínez‐Domínguez Patricia Plaza‐Bolaños Roberto Romero‐González Antonia Garrido Frenich 《Journal of separation science》2014,37(6):665-674
A method based on QuEChERS (quick, easy, cheap, effective, rugged, and safe) has been developed and validated for the determination and quantification of more than 140 pesticides in nutraceutical products obtained from green tea (Camellia sinensis). Extraction was performed with acidified acetonitrile (acetic acid 1%, v/v) and a clean‐up step using primary secondary amine (50 mg), graphitized black carbon (100 mg) and magnesium sulfate (200 mg) was needed. Pesticide determination was achieved utilizing GC coupled to triple quadrupole MS/MS using the selective‐reaction monitoring mode. The total run time was 23 min. Pesticides were quantified using matrix‐matched calibration. Recoveries ranged from 70 to 120% and relative SD was lower than 25% at 10, 50, and 100 μg/kg. LOQs were lower than 10 μg/kg. 148 pesticides were validated. The validated method was applied to commercial nutraceutical products, detecting 4,4‐dichlorobenzophenone (28 μg/kg), o,p′‐dicofol (38 μg/kg) and p,p‐dicofol (44 μg/kg) in a few samples. 相似文献
110.
Manoela K. Riekes Gislaine Kuminek Gabriela S. Rauber Silvia L. Cuffini Hellen K. Stulzer 《Central European Journal of Chemistry》2014,12(5):549-556
The polymorphs of nimodipine, Modification I (Mod I), the metastable racemate, and Modification II (Mod II), the stable conglomerate, were evaluated by means of the intrinsic dissolution procedure. For this purpose, a hydro alcoholic solution (ethanol:water, 50:50, v/v) was selected as the dissolution medium, maintained at 37±0.5°C. Different rotation speeds were tested (50, 75 and 100 rpm) and the lower one was chosen for the test validation. Although the sample initially characterized as polymorph Mod I presented higher intrinsic dissolution rates in all the conditions tested, no statistical differences were noticed between the two polymorphs. This result can be attributed to the partial solution-mediated phase transformation from Mod I to Mod II, detected through X-ray powder diffraction and differential scanning calorimetry. Also, reliable intrinsic dissolution rate data were acquired for the polymorph Mod II. The dissolution method was validated, being considered stable, specific, linear, sensible, accurate and precise. 相似文献