全文获取类型
收费全文 | 12972篇 |
免费 | 1638篇 |
国内免费 | 541篇 |
专业分类
化学 | 3803篇 |
晶体学 | 48篇 |
力学 | 2290篇 |
综合类 | 196篇 |
数学 | 5109篇 |
物理学 | 3705篇 |
出版年
2024年 | 20篇 |
2023年 | 185篇 |
2022年 | 253篇 |
2021年 | 306篇 |
2020年 | 267篇 |
2019年 | 256篇 |
2018年 | 261篇 |
2017年 | 456篇 |
2016年 | 520篇 |
2015年 | 500篇 |
2014年 | 612篇 |
2013年 | 889篇 |
2012年 | 725篇 |
2011年 | 733篇 |
2010年 | 625篇 |
2009年 | 768篇 |
2008年 | 792篇 |
2007年 | 844篇 |
2006年 | 671篇 |
2005年 | 530篇 |
2004年 | 596篇 |
2003年 | 542篇 |
2002年 | 454篇 |
2001年 | 330篇 |
2000年 | 354篇 |
1999年 | 328篇 |
1998年 | 290篇 |
1997年 | 220篇 |
1996年 | 226篇 |
1995年 | 200篇 |
1994年 | 152篇 |
1993年 | 142篇 |
1992年 | 104篇 |
1991年 | 138篇 |
1990年 | 89篇 |
1989年 | 66篇 |
1988年 | 75篇 |
1987年 | 72篇 |
1986年 | 59篇 |
1985年 | 89篇 |
1984年 | 54篇 |
1983年 | 39篇 |
1982年 | 50篇 |
1981年 | 35篇 |
1980年 | 46篇 |
1979年 | 46篇 |
1978年 | 29篇 |
1977年 | 35篇 |
1976年 | 36篇 |
1974年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 546 毫秒
991.
Malte Brasholz Brian A. Johnson Anastasios Polyzos Simon Saubern John H. Ryan 《Tetrahedron》2010,66(33):6445-1893
A domino cyclization reaction of the bis-unsaturated ketone 3 with hydroxylamine proceeds with good yield and high stereoselectivity, in a flow reactor system. The tricyclic spiropiperidine products 5 and 2 obtained are valuable building blocks for the synthesis of analogues of the histrionicotoxin alkaloids. 相似文献
992.
A pH‐sensitive detector for flow‐through potentiometry based on a graphite/quinhydrone/silicone composite electrode was applied to determine calcium and magnesium in aqueous solutions. The determinations are performed by FIA titrations with EDTA as the carrier solution. The method was applied to the analysis of mineral water, drinking water, and river water. The results were compared with those obtained by batch complexometric titration. The proposed new method is suitable for the simple, rapid and automated determination of water hardness in small sample volumes of aqueous solutions. The detection limit for both ions is about 1×10?5 mol L?1, the standard deviation is less than 2%. The sampling frequency is 60 determinations per hour. 相似文献
993.
Mei‐Hsin Chiu Annamalai Senthil Kumar Sundaram Sornambikai Jyh‐Myng Zen Ying Shih 《Electroanalysis》2010,22(20):2421-2427
A screen‐printed silver strip with a built‐in three‐in‐one electrode (SPAgE) configuration of Ag‐working, Ag‐counter and Ag/AgxO (silver oxides) pseudoreference electrodes has been developed for sensitive and selective electrochemical flow injection analysis (FIA) of aluminum chlorohydrate (ACH) present in antiperspirants, through the free Cl? ion liberated from ACH in aqueous medium, as a redox signal at Ag‐working electrode in pH 6 phosphate buffer solution (PBS). The solution phase and instrumental parameters were systematically optimized. The calibration graph was linear in the window 1–200 ppm concentration of ACH and the lowest detection limit (S/N=3) was 295 ppb with a slope of 0.0989 μA/ppm and regression coefficient of 0.998. Calculated relative standard deviation (RSD) values for the detection of 5 and 50 ppm ACH by this method are 2.21 % and 2.16 %, respectively. Four different antiperspirant deodorants real samples with and without ACH content were successfully analyzed and the detected values obtained were found to be in good agreement with the product labeled values. 相似文献
994.
Bright red crystals of [Mn(H2O)6][BiI4]2 · 2H2O are obtained from a solution of MnI2, BiI3, and I2 in absolute ethanol, which is exposed to humid air. Reversible dehydratization sets in at about 50 °C. Added water decomposes the hydrate by irreversible precipitation of BiOI. The optical bandgap is about 1.9(1) eV. X‐ray diffraction on a single‐crystal revealed a monoclinic lattice (space group P21/c) with a = 760.39(4) pm, b = 1315.6(1) pm, c = 1398.37(7) pm, and β = 97.438(4)°. In the crystal structure zigzag chains of edge‐sharing [BiI2/1I4/2]– octahedra and linear strings of H2O‐bridged [Mn(H2O)6]2+ octahedra run parallel [100]. 相似文献
995.
Wanessa R. Melchert 《Microchemical Journal》2010,96(1):77-81
A green and highly sensitive analytical procedure was developed for the determination of free chlorine in natural waters, based on the reaction with N,N-diethyl-p-phenylenediamine (DPD). The flow system was designed with solenoid micro-pumps in order to improve mixing conditions by pulsed flows and to minimize reagent consumption as well as waste generation. A 100-cm optical path flow cell based on a liquid core waveguide was employed to increase sensitivity. A linear response was observed within the range 10.0 to 100.0 µg L− 1, with the detection limit, coefficient of variation and sampling rate estimated as 6.8 µg L− 1 (99.7% confidence level), 0.9% (n = 20) and 60 determinations per hour, respectively. The consumption of the most toxic reagent (DPD) was reduced 20,000-fold and 30-fold in comparison to the batch method and flow injection with continuous reagent addition, respectively. The results for natural and tap water samples agreed with those obtained by the reference batch spectrophotometric procedure at the 95% confidence level. 相似文献
996.
Diogo L. Rocha 《Microchemical Journal》2010,94(1):53-59
The determination of uric acid in urine shows clinical importance, once it can be related to human organism dysfunctions, such as gout. An analytical procedure employing a multicommuted flow system was developed for the determination of uric acid in urine samples. Cu(II) ions are reduced by uric acid to Cu(I) that can be quantified by spectrophotometry in the presence of 2,2′-biquinoline 4,4′-dicarboxylic acid (BCA). The analytical response was linear between 10 and 100 μmol L− 1 uric acid with a detection limit of 3.0 μmol L− 1 (99.7% confidence level). Coefficient of variation of 1.2% and sampling rate of 150 determinations per hour were achieved. Per determination, 32 μg of CuSO4 and 200 μg of BCA were consumed, generating 2.0 mL of waste. Recoveries from 91 to 112% were estimated and the results for 7 urine samples agreed with those obtained by the commercially available enzymatic kit for determination of uric acid. The procedure required 100-fold dilution of urine samples, minimizing sample consumption and interfering effects. In order to avoid the manual dilution step, on-line sample dilution was achieved by a simple system reconfiguration attaining a sampling rate of 95 h− 1. 相似文献
997.
Two rapid and sensitive automated methods for the determination of nitrite and nitrate in soil samples 总被引:1,自引:0,他引:1
Spectrophotometric flow injection methods were developed for the individual determination of nitrite or nitrate, and for the simultaneous determination of nitrite and nitrate, in soil samples. Nitrite was determined directly using a modified version of the Griess-Ilosvay diazo-coupling reaction, measuring at 543 nm the absorbance of the azo-dye complex formed. Simultaneous nitrite and nitrate determinations were based on on-line nitrate reduction in a micro column containing copperised cadmium. A single chromogenic reagent containing all the necessary reactants was used in both methods. For determinations, the chemical and instrumental variables were optimised by univariate analysis and simplex chemometric method. The optimised conditions gave a linear calibration range between 0.05 and 1.6 µg m L− 1 for N-NO2− and between 0.05 and 7.0 µg m L− 1 for N-NO3−. The detection limits for nitrite and nitrate were 22 µg L− 1 and 44 µg L− 1 respectively. The proposed methods allowed up to 35-40 samples per hour to be analysed with good precision. The simultaneous method was successfully used for the determination of nitrite and nitrate in soil samples (the results obtained were validated against those obtained by reference methods). The proposed methods are simpler and faster than conventional methods and could be routinely used in environmental monitoring laboratories. 相似文献
998.
Economou A 《Analytica chimica acta》2010,683(1):38-51
This paper presents an overview of the field of electrochemical stripping analysis in flow systems covering developments in the last 12 years (since 1998). The review discusses the flow schemes utilized in stripping analysis, techniques for on-line sample pre-treatment, the main pre-concentration and stripping/detection modes, the most important flow-through cell configurations used and the different types of working electrodes. Finally, applications in inorganic and organic analysis are discussed. Special emphasis is given to different novel approaches developed over the last few years that hold some promise for the future such as the use of the lab-on-a valve (LOV) configuration, microfluidic manifolds, flow-probes for remote sensing, environmentally friendly electrode materials and hyphenation with spectroscopic techniques. 相似文献
999.
Iridium oxide nanoparticles are grown on a glassy carbon electrode by electrodepositing method. The electrochemical behavior and electrocatalytic activity of modified electrode towards reduction of iodate and periodate are studied. The reductions of both ions occur at the unusual positive peak potential of 0.7 V vs. reference electrode. The modified electrode is employed successfully for iodate and periodates detection using cyclic voltammetry, hydrodynamic amperometry and flow injection analysis (FIA). In the performed experiments, flow injection amperometric determination of iodate and periodate yielded calibration curves with the following characteristics: linear dynamic range up to 100 and 80 μM, sensitivity of 140.9 and 150.6 nA μM−1 and detection limits of 5 and 36 nM, respectively. The repeatability of the modified electrode for 21 injections of 1.5 μM of iodate solution is 1.5%. The interference effects of NO2−, NO3−, ClO3−, BrO3−, ClO4−, SO42−, Cu2+, Zn2+, Mn2+, Mg2+, Cd2+, Ca2+, Na+, K+, NH4+ and K+, CH3COO− and glucose were negligible at the concentration ratio of more than 1000. The obtained attractive analytical performance together with high selectivity and simplicity of the proposed method provide an effective and e novel modified electrode to develop an iodate and periodate sensor. Sensitivity, selectivity, the liner concentration range and the detection limit of the developed sensor are all much better than all known similar sensors in the literature for iodate and periodate determination. 相似文献
1000.
Edyta Nalewajko-Sieliwoniuk Iwona Tarasewicz Anatol Koj?o 《Analytica chimica acta》2010,668(1):19-1418
This study established a flow injection (FI) methodology for the determination of the total phenolic content in plant-derived beverages based on soluble manganese(IV) chemiluminescence (CL) detection. It was found that mixing polyphenols with acidic soluble manganese(IV) in the presence of formaldehyde evoked chemiluminescence. Based on this finding, a new FI-CL method was developed for the estimation of the total content of phenolic compounds (expressed as milligrams of gallic acid equivalent per litre of drink) in a variety of wine, tea and fruit juice samples. The proposed method is sensitive with a detection limit of 0.02 ng mL−1 (gallic acid), offers a wide linear dynamic range (0.5-400 ng mL−1) and high sample throughput (247 samples h−1). The relative standard deviation for 15 measurements was 3.8% for 2 ng mL−1 and 0.45% for 10 ng mL−1 of gallic acid. Analysis of 36 different samples showed that the results obtained by the proposed FI-CL method correlate highly with those obtained by spectrophotometric methods commonly used for the evaluation of the total phenolic/antioxidant level. However, the FI-CL method was found to be far simpler, more rapid and selective, with almost no interference from non-phenolic components of the samples examined. 相似文献