全文获取类型
收费全文 | 1503篇 |
免费 | 362篇 |
国内免费 | 31篇 |
专业分类
化学 | 467篇 |
晶体学 | 3篇 |
力学 | 28篇 |
综合类 | 4篇 |
数学 | 98篇 |
物理学 | 1296篇 |
出版年
2024年 | 1篇 |
2023年 | 8篇 |
2022年 | 17篇 |
2021年 | 17篇 |
2020年 | 23篇 |
2019年 | 21篇 |
2018年 | 30篇 |
2017年 | 39篇 |
2016年 | 92篇 |
2015年 | 78篇 |
2014年 | 138篇 |
2013年 | 103篇 |
2012年 | 138篇 |
2011年 | 162篇 |
2010年 | 117篇 |
2009年 | 122篇 |
2008年 | 106篇 |
2007年 | 108篇 |
2006年 | 82篇 |
2005年 | 57篇 |
2004年 | 60篇 |
2003年 | 42篇 |
2002年 | 44篇 |
2001年 | 36篇 |
2000年 | 36篇 |
1999年 | 35篇 |
1998年 | 26篇 |
1997年 | 27篇 |
1996年 | 23篇 |
1995年 | 20篇 |
1994年 | 16篇 |
1993年 | 16篇 |
1992年 | 15篇 |
1991年 | 7篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有1896条查询结果,搜索用时 15 毫秒
121.
Pourya Biparva 《Analytica chimica acta》2010,674(2):206-210
A rapid and effective preconcentration method for extraction of rhodamine 6G was developed by using a dispersive liquid-liquid microextraction (DLLME) prior to UV-vis spectrophotometry. In this extraction method, a suitable mixture of acetone (disperser solvent) and chloroform (extractant solvent) was injected rapidly into a conical test tube containing aqueous solution of rhodamine 6G. Therefore, a cloudy solution was formed. After centrifugation of the cloudy solution, sedimented phase was evaporated, reconstituted with methanol and measured by UV-vis spectrophotometry. Different operating variables such as type and volume of extractant solvent, type and volume of disperser solvent, pH of the sample solution, salt concentration and extraction time were investigated. The optimized conditions (extractant solvent: 300 μL of chloroform, disperser solvent: 3 mL of acetone, pH: 8 and without salt addition) resulted in a linear calibration graph in the range of 5-900 ng mL−1 of rhodamine 6G in initial solution with R2 = 0.9988 (n = 5). The Limits of detection and quantification were 2.39 and 7.97 ng mL−1, respectively. The relative standard deviation for 50 and 250 ng mL−1 of rhodamine 6G in water were 2.88% and 1.47% (n = 5), respectively. Finally, the DLLME method was applied for determination of rhodamine 6G in different industrial waste waters. 相似文献
122.
This paper presents a new sample preparation procedure for determination of selected acidic pharmaceuticals (ibuprofen, naproxen, ketoprofen, and diclofenac) in sewage sludge using microwave assisted solvent extraction, dispersive matrix extraction (DME) followed by the conventionally applied solid phase extraction (SPE), derivatization, and gas chromatography-mass spectrometry. The recoveries calculated from analytical data of spiked sludge samples changed in the range of 80-105% ± 15% for the four pharmaceuticals in mixed and activated sludge depending on the efficiency of the clean-up procedure. The measured concentration values of ibuprofen and naproxen were identical in the mixed and the activated sludge samples. However, ketoprofen and diclofenac showed about twice as high concentration in activated sludge than in the mixed one independently of the applied extraction method. The typical concentration ranges of ibuprofen, naproxen, ketoprofen and diclofenac in sewage sludge were 10-30 ng/g, 30-50 ng/g, 50-130 ng/g, and 50-140 ng/g respectively. 相似文献
123.
124.
A simple and fast method of low-density extraction solvent-based solvent terminated dispersive liquid–liquid microextraction (ST-DLLME) was developed for the highly sensitive determination of carbamate pesticides in the water samples by gas chromatography-tandem mass spectrometry (GC-MSMS). After dispersing, the obtained emulsion cleared into two phases quickly when an aliquot of acetonitrile was introduced as a chemical demulsifier into the aqueous bulk. Therefore, the developed procedure does not need centrifugation to achieve phase separation. It was convenient for the usage of low-density extraction solvents in DLLME. Under the optimized conditions, the limits of detection for all target carbamate pesticides were in range of 0.001–0.50 ng mL−1 and the precisions were in the range of 2.3–6.8% (RSDs, 2 ng mL−1, n = 5). The proposed method has been successfully applied to the analysis of real water samples and good spiked recoveries over the range of 94.5–104% were obtained. 相似文献
125.
Agnieszka Zgoła-Grześkowiak 《Journal of chromatography. A》2010,1217(11):1761-1766
Dispersive liquid–liquid microextraction (DLLME) coupled with high-performance liquid chromatography with fluorescence detector was applied for the determination of alkylphenols and their short-chained ethoxylates in water samples. Development of DLLME procedure included optimisation of some important parameters such as kind and volume of extracting and dispersing solvents. Under optimised conditions 50 μL of trichloroethylene in 1.5 mL of acetone were rapidly injected into 5 mL of a water sample. After centrifuging the organic phase containing the analytes was taken for evaporation with a gentle nitrogen purge and reconstituted to 50 μL of acetonitrile. The aliquot of this solution was analysed with the use of HPLC. For octylphenol (OP) and octylphenol ethoxylates (OPEOs) linearity was satisfactory in the range 8–1000 μg L−1 and for nonylphenol (NP) and nonylphenol ethoxylates (NPEOs) linearity was in the range from 50 to about 3000 μg L−1. Limit of quantitation was 0.1 μg L−1 for OP and OPEOs and 0.3 μg L−1 for NP and NPEOs. Satisfactory recoveries between 66 and 79% were obtained for environmental samples. The results showed that DLLME is a simple, rapid and sensitive analytical method for the preconcentration of trace amounts of alkylphenols and their ethoxylates in environmental water samples. 相似文献
126.
Hongyuan Yan Baomi Liu Jingjing Du Gengliang Yang Kyung Ho Row 《Journal of chromatography. A》2010,1217(32):5152-5157
A simple ultrasound-assisted dispersive liquid–liquid microextraction method combined with liquid chromatography was developed for the preconcentration and determination of six pyrethroids in river water samples. The procedure was based on a ternary solvent system to formatting tiny droplets of extractant in sample solution by dissolving appropriate amounts of water-immiscible extractant (tetrachloromethane) in watermiscible dispersive solvent (acetone). Various parameters that affected the extraction efficiency (such as type and volume of extraction and dispersive solvent, extraction time, ultrasonic time, and centrifuging time) were evaluated. Under the optimum condition, good linearity was obtained in a range of 0.00059–1.52 mg L−1 for all analytes with the correlation coefficient (r2) > 0.999. Intra-assay and inter-assay precision evaluated as the relative standard deviation (RSD) were less than 3.4 and 8.9%. The recoveries of six pyrethroids at three spiked levels were in the range of 86.2–109.3% with RSD of less than 8.7%. The enrichment factors for the six pyrethroids were ranged from 767 to 1033 folds. 相似文献
127.
Chu-Chi Chang 《Analytica chimica acta》2010,662(1):39-65
In this study, the steroid hormone levels in river and tap water samples were determined by using a novel dispersive liquid-liquid microextraction method based on the solidification of a floating organic drop (DLLME-SFO). Several parameters were optimized, including the type and volume of the extraction and dispersive solvents, extraction time, and salt effect. DLLME-SFO is a fast, cheap, and easy-to-use method for detecting trace levels of samples. Most importantly, this method uses less-toxic solvent. The correlation coefficient of the calibration curve was higher than 0.9991. The linear range was from 5 to 1000 μg L−1. The spiked environmental water samples were analyzed using DLLME-SFO. The relative recoveries ranged from 87% to 116% for river water (which was spiked with 4 μg L−1 for E1, 3 μg L−1 for E2, 4 μg L−1 for EE2 and 9 μg L−1 for E3) and 89% to 102% for tap water (which was spiked with 6 μg L−1 for E1, 5 μg L−1 for E2, 6 μg L−1 for EE2 and 10 μg L−1 for E3). The detection limits of the method ranged from 0.8 to 2.7 μg L−1 for spiked river water and 1.4 to 3.1 μg L−1 for spiked tap water. The methods precision ranged from 8% to 14% for spiked river water and 7% to 14% for spiked tap water. 相似文献
128.
Campillo N Viñas P Cacho JI Peñalver R Hernández-Córdoba M 《Journal of chromatography. A》2010,1217(47):7323-7330
Dispersive liquid-liquid microextraction (DLLME) coupled with gas chromatography-mass spectrometry (GC-MS) was evaluated for the simultaneous determination of five chlorophenols and seven haloanisoles in wines and cork stoppers. Parameters, such as the nature and volume of the extracting and disperser solvents, extraction time, salt addition, centrifugation time and sample volume or mass, affecting the DLLME were carefully optimized to extract and preconcentrate chlorophenols, in the form of their acetylated derivatives, and haloanisoles. In this extraction method, 1mL of acetone (disperser solvent) containing 30μL of carbon tetrachloride (extraction solvent) was rapidly injected by a syringe into 5mL of sample solution containing 200μL of acetic anhydride (derivatizing reagent) and 0.5mL of phosphate buffer solution, thereby forming a cloudy solution. After extraction, phase separation was performed by centrifugation, and a volume of 4μL of the sedimented phase was analyzed by GC-MS. The wine samples were directly used for the DLLME extraction (red wines required a 1:1 dilution with water). For cork samples, the target analytes were first extracted with pentane, the solvent was evaporated and the residue reconstituted with acetone before DLLME. The use of an internal standard (2,4-dibromoanisole) notably improved the repeatability of the procedure. Under the optimized conditions, detection limits ranged from 0.004 to 0.108ngmL(-1) in wine samples (24-220pgg(-1) in corks), depending on the compound and the sample analyzed. The enrichment factors for haloanisoles were in the 380-700-fold range. 相似文献
129.
Bin Guo Zhiqiang Huang Meiling Wang Xiaoying Wang Ying Zhang Bo Chen Yongjun Li Hongfei Yan Shouzhuo Yao 《Journal of chromatography. A》2010,1217(29):4796-4807
A fast and flexible multi-residue procedure for effective extraction of benzimidazole fungicides and the related transformation products in various raw agricultural commodities is developed for direct and simultaneous analysis by liquid chromatography–tandem mass spectrometry (LC–MS/MS). The new sample preparation method, introduced to allow direct extraction of the labile fungicides as the intact forms in complex matrices, is achieved using a conservative homogenizing extraction and multifunction adsorption cleanup (CHEMAC), which basically involves salting-out partitioning/extraction with acetate-buffered acetonitrile at low-temperature and sequential rapid solid-phase dispersive cleanup with a ternary sorbent mixture. The CHEMAC procedure was optimized and further modified by incorporating several pretreatment variables influencing sample stability and process efficiency, such as pH, temperature, salt and sorbent utilized. By using CHEMAC, a noteworthy improvement in extraction recoveries was obtained for the problematic fungicides, while no significant differential matrix effects were detected on the LC–MS/MS analysis in all 9 matrices. The in-source fragmentation of benomyl occurred but caused no cross-talk interference in multi-component analysis. Thus the CHEMAC-based LC–MS/MS strategy can serve as an attractive approach to satisfactory overall process efficiencies (70–92%) with acceptable repeatability (relative standard deviations below 16%) for all the analyte–matrix combinations. Mean accuracies were obtained within the range of 70–110% at fortified levels of 1–500 ng/g, with intra-day and inter-day variations less than 15 and 20%, respectively. The successful practical application of the proposed method to real samples has also been demonstrated. 相似文献
130.