全文获取类型
收费全文 | 4047篇 |
免费 | 570篇 |
国内免费 | 307篇 |
专业分类
化学 | 1571篇 |
力学 | 238篇 |
综合类 | 21篇 |
数学 | 138篇 |
物理学 | 1160篇 |
无线电 | 1796篇 |
出版年
2024年 | 13篇 |
2023年 | 40篇 |
2022年 | 141篇 |
2021年 | 126篇 |
2020年 | 130篇 |
2019年 | 116篇 |
2018年 | 105篇 |
2017年 | 198篇 |
2016年 | 230篇 |
2015年 | 234篇 |
2014年 | 309篇 |
2013年 | 363篇 |
2012年 | 404篇 |
2011年 | 326篇 |
2010年 | 240篇 |
2009年 | 268篇 |
2008年 | 208篇 |
2007年 | 251篇 |
2006年 | 221篇 |
2005年 | 185篇 |
2004年 | 152篇 |
2003年 | 118篇 |
2002年 | 73篇 |
2001年 | 60篇 |
2000年 | 77篇 |
1999年 | 44篇 |
1998年 | 43篇 |
1997年 | 49篇 |
1996年 | 41篇 |
1995年 | 24篇 |
1994年 | 24篇 |
1993年 | 24篇 |
1992年 | 19篇 |
1991年 | 12篇 |
1990年 | 10篇 |
1989年 | 4篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有4924条查询结果,搜索用时 0 毫秒
21.
Priyadeep Bhutani Rekha U. Shivakumar H. N. Prabhakar K. Ranjanna Atish T. Paul 《Biomedical chromatography : BMC》2020,34(10):e4902
Garcinia cambogia is one of the most commonly used anti-obesity dietary supplements, and hydroxycitric acid (HCA) is a major constituent in the commercial preparations of Garcinia. High doses of HCA are often consumed without much awareness of its pharmacokinetic and toxicokinetic parameters, and therefore, a complete understanding of its effects is lacking. The first step in understanding these parameters is the availability of a reliable bioanalytical method. Here, we present the first report on a UPLC–MS/MS method for analysis of HCA in rat plasma after a simplified and cost-effective protein precipitation. Chromatographic separation of the analytes in the supernatant was achieved using hydrophilic interaction liquid chromatography, where mass parameters were optimized and a rapid 5-min quantitative assay was developed. The method was highly sensitive, accurate, precise and linear in the concentration range of 10.5–5000 ng/mL (validated according to the United States Food and Drug Administration guidelines). Further, the method was successfully used to describe the pharmacokinetic profile of HCA in rat plasma after the administration of pure HCA and commercial Garcinia preparations. 相似文献
22.
阐述基于Mie散射理论和激光技术而研制的云粒子探测仪的相关问题。利用m量级的小孔光阑模拟感应区域的散射光,并对系统的探测敏感区域面积进行测定;通过使用不同直径的标准粒子对系统进行标定,得到可靠的响应曲线,用于定量测量云粒子尺度谱及粒子数密度。在进行了一系列实验室内的实验之后,将仪器装载在飞机上进行穿云飞行测量实验,表明了该仪器在飞行过程中工作正常、稳定,并且能够即时地显示采样区内云粒子尺度谱分布和数浓度;通过分析探测得到的数据,并与云粒子谱分布进行比较,确认了探测数据有效可靠,反映了该仪器具有良好的测云能力。 相似文献
23.
Dongbo Wang Laura M. Hamm Robert J. Bodnar Patricia M. Dove 《Journal of Raman spectroscopy : JRS》2012,43(4):543-548
A series of Mg‐bearing synthetic amorphous calcium carbonates (ACC) were characterized by Raman spectroscopy. The spectra showed a systematic increase in the carbonate ν1 peak position from the control samples that contained 0.0 mol % MgCO3 to samples that contained up to 43 mol % MgCO3. The relationship is best described by the function: mol % MgCO3 = (ν1 – 1079.66) / 0.2017. The Mg content is equally well‐predicted by a correlation with the instrumentally corrected ν1 full width at half maximum that is quantified by: mol % MgCO3 = (ν1 – 23.26) / 0.1969. An analysis of the Raman data collected for ACC combined with insights from crystalline materials suggests that compositional dependencies arise from changes in the local metal–oxygen bonds as Mg substitutes for Ca. The calibrations described here provide a rapid and nondestructive means of determining the Mg content of ACC, with additional advantages of minimal sample preparation and a high degree of lateral spatial resolution (approximately 1 µm). This method may be appropriate for investigations of heterogeneous samples such as biominerals. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
24.
25.
Indrani Mitra Partha Pratim Roy Supratik Kar Probir Kumar Ojha Kunal Roy 《Journal of Chemometrics》2010,24(1):22-33
Validation is a crucial aspect for quantitative structure–activity relationship (QSAR) model development. External validation is considered, in general, as the most conclusive proof of predictive capacity of a QSAR model. In the absence of truly external data set, external validation is usually performed on test set compounds, which are members of the original data set but not used in model development exercise. In the case of small data sets, QSAR researchers experience problem in model development due to the fact that the developed models may be less reliable on account of the small number of training set compounds and such models may also show poor external predictability because the models may not have captured all necessary features required for the particular structure–activity relationships. The present paper attempts to show that ‘true r(LOO)’ statistic calculated based on the model derived from the undivided data set with application of variable selection strategy at each cycle of leave‐one‐out (LOO) validation may reflect external validation characteristics of the developed model thus obviating the requirement of splitting of the data set into training and test sets. This approach may be helpful in the case of small data sets as it uses all available data for model development and validation thus making the resulting model more reliable. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
26.
González-Sáiz JM Esteban-Díez I Sánchez-Gallardo C Pizarro C 《Analytical and bioanalytical chemistry》2008,391(8):2937-2947
Wastes and by-products of the onion-processing industry pose an increasing disposal and environmental problem and represent
a loss of valuable sources of nutrients. The present study focused on the production of vinegar from worthless onions as a
potential valorisation route which could provide a viable solution to multiple disposal and environmental problems, simultaneously
offering the possibility of converting waste materials into a useful food-grade product and of exploiting the unique properties
and health benefits of onions. This study deals specifically with the second and definitive step of the onion vinegar production
process: the efficient production of vinegar from onion waste by transforming onion ethanol, previously produced by alcoholic
fermentation, into acetic acid via acetic fermentation. Near-infrared spectroscopy (NIRS), coupled with multivariate calibration
methods, has been used to monitor the concentrations of both substrates and products in acetic fermentation. Separate partial
least squares (PLS) regression models, correlating NIR spectral data of fermentation samples with each kinetic parameter studied,
were developed. Wavelength selection was also performed applying the iterative predictor weighting–PLS (IPW-PLS) method in
order to only consider significant spectral features in each model development to improve the quality of the final models
constructed. Biomass, substrate (ethanol) and product (acetic acid) concentration were predicted in the acetic fermentation
of onion alcohol with high accuracy using IPW-PLS models with a root-mean-square error of the residuals in external prediction
(RMSEP) lower than 2.5% for both ethanol and acetic acid, and an RMSEP of 6.1% for total biomass concentration (a very satisfactory
result considering the relatively low precision and accuracy associated with the reference method used for determining the
latter). Thus, the simple and reliable calibration models proposed in this study suggest that they could be implemented in
routine applications to monitor and predict the key species involved in the acetic fermentation of onion alcohol, allowing
the onion vinegar production process to be controlled in real time. 相似文献
27.
Maes A Garré B Desmet N van der Meulen K Nauwynck H De Backer P Croubels S 《Biomedical chromatography : BMC》2009,23(2):132-140
Two methods are presented for the determination of 'respectively' the plasma protein unbound and total concentration of acyclovir in horse plasma and body fluids: first, a liquid-liquid extraction was performed on plasma, combined with HPLC-fluorescence detection for the total plasma concentration; second a more sensitive method using high-performance liquid chromatography combined with heated electrospray ionization tandem mass spectrometry (LC-HESI-MS/MS) was described for plasma and for body fluids analysis. To obtain the unbound concentration of acyclovir in plasma, a simple deproteinization step using a Microcon filter was performed. Ganciclovir was used as an internal standard. Analysis was carried out on an Inertsil 5 ODS-3 column for the HPLC-fluorescence method. For the LC-HESI-MS/MS method a PLRP-S column was used. The limit of quantification (LOQ) for the total concentration was set at 50 and 2 ng mL(-1) for the HPLC-fluorescence method and the LC-HESI-MS/MS method, respectively. The limit of quantification for the unbound concentration was set at 5 ng mL(-1) and at 2 ng mL(-1) for body fluids. The methods were successfully used to perform pharmacokinetic and clinical studies in horses after intravenous and oral dosage of acyclovir and its prodrug valacyclovir. 相似文献
28.
29.
Geetika Wadhwa Kowthavarapu Venkata Krishna Rajeev Taliyan Neeraj Tandon Satyapal Singh Yadav Dipankar Banerjee Avinash Narwaria Chandrakant Katiyar Sunil Kumar Dubey 《Biomedical chromatography : BMC》2022,36(2):e5275
Trigonelline (TR), 4-hydroxyisoleucine (4-HI), and diosgenin (DG) are the main bioactives of the purified standardized extract of the popular plant Trigonella foenum-graecum L. (TFG), and it has been proven effective for the treatment of various diseases. However, to the best of our knowledge, no study has investigated the pharmacokinetic parameters of purified standardized T. foenum-graecum extract in normal and diabetic Wistar rats. The present study has developed and validated a rapid, reliable, and sensitive simultaneous ultra-performance liquid chromatography MS method to estimate these bioactives. The chromatographic separation was achieved using methanol, acetonitrile, and 0.1% formic acid with the ideal gradient flow system on a BEH Shield RP 18 column. A positive electrospray ionization mode was selected to estimate m/z values of TR (138.14 > 94.63), 4-HI (148.19 > 74.08), and DG (415.54 > 271.33). The method was robust and reproducible over the linearity range of 60–5000, 6–5000, and 15–5000 ng/mL for TR, 4-HI, and DG, respectively. Using this novel validated method, we investigated the pharmacokinetic parameters of bioactives using Phoenix WinNonlin version 8.0 (Certera) in normal and diabetic rats. The assay was successfully applied for the estimation of pharmacokinetic parameters using noncompartmental analysis. This investigation shows that the absorption rate increased, whereas distribution and elimination processes slowed down in diabetic rats compared with normal rats. 相似文献
30.
Susan C. Scott Nicole M. Anders Ping He Avelina Hemingway Steven D. Gore Christine L. Hann Michelle A. Rudek 《Biomedical chromatography : BMC》2022,36(3):e5289
The Bcl-2 family small molecule inhibitor navitoclax is being clinically evaluated to treat multiple cancers including lymphoid malignancies and small cell lung cancer. A sensitive and reliable method was developed to quantitate navitoclax in human plasma using liquid chromatography with tandem mass spectrometry with which to perform detailed pharmacokinetic studies. Sample preparation involved protein precipitation using acetonitrile. Separation of navitoclax and the internal standard, navitoclax-d8, was achieved with a Waters Acquity UPLC BEH C18 column using isocratic flow over a 3 min total analytical run time. A SCIEX 4500 triple quadrupole mass spectrometer operated in positive electrospray ionization mode was used for the detection of navitoclax. The assay range was 5–5,000 ng/ml and proved to be accurate (89.5–104.9%) and precise (CV ≤ 11%). Long-term frozen plasma stability for navitoclax at −70°C was at least 43 months. The method was applied for the measurement of total plasma concentration of navitoclax in a patient receiving a 250 mg daily oral dose. 相似文献