全文获取类型
收费全文 | 26177篇 |
免费 | 4549篇 |
国内免费 | 2289篇 |
专业分类
化学 | 17206篇 |
晶体学 | 544篇 |
力学 | 932篇 |
综合类 | 194篇 |
数学 | 755篇 |
物理学 | 13384篇 |
出版年
2024年 | 21篇 |
2023年 | 186篇 |
2022年 | 499篇 |
2021年 | 730篇 |
2020年 | 931篇 |
2019年 | 894篇 |
2018年 | 824篇 |
2017年 | 1026篇 |
2016年 | 1304篇 |
2015年 | 1206篇 |
2014年 | 1514篇 |
2013年 | 2691篇 |
2012年 | 1650篇 |
2011年 | 1681篇 |
2010年 | 1394篇 |
2009年 | 1499篇 |
2008年 | 1540篇 |
2007年 | 1549篇 |
2006年 | 1369篇 |
2005年 | 1197篇 |
2004年 | 1161篇 |
2003年 | 1002篇 |
2002年 | 1085篇 |
2001年 | 741篇 |
2000年 | 774篇 |
1999年 | 648篇 |
1998年 | 558篇 |
1997年 | 453篇 |
1996年 | 361篇 |
1995年 | 410篇 |
1994年 | 316篇 |
1993年 | 266篇 |
1992年 | 252篇 |
1991年 | 183篇 |
1990年 | 172篇 |
1989年 | 124篇 |
1988年 | 129篇 |
1987年 | 118篇 |
1986年 | 102篇 |
1985年 | 97篇 |
1984年 | 94篇 |
1983年 | 38篇 |
1982年 | 63篇 |
1981年 | 30篇 |
1980年 | 33篇 |
1979年 | 37篇 |
1978年 | 12篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The solid phase extraction as a solvent-free method for the analysis of oil and grease in waters was studied. The use of
a PTFE surface as a solid phase allows the retention of the volatile fraction of oil and grease, and further analysis of carbon–hydrogen
bonds using infrared spectroscopy can be done on the surface.
Various oils and grease samples were tested: n-hexadecane, n-tetradecane, n-nonadecane, n-docosane, isooctane, diesel oil
and gasoline. Temperatures from 25° to 90 °C and a range of heating times were checked for extraction.
Precision tests showed relative standard deviation values of around 10% in several samples of the same concentration. Calibration
lines of n-hexadecane showed high correlation coefficients from 0.9 to 30 mg/l. Recoveries for the various oils using different
calibration lines showed values from 90 to 110%.
The method described here is fast and clean, and allows reproducible measurements of oil and grease in water that do not require
the use of a solvent.
Received March 1, 2001; accepted August 15, 2001; published online July 15, 2002 相似文献
992.
Wojakowska A. Krzyżak E. Wojakowski A. 《Journal of Thermal Analysis and Calorimetry》2001,65(2):491-495
The phase diagram for the RbBr–CuBr system has been determined. In the system two intermediate compounds are formed: RbCu2Br3, melting congruently at 537 K and Rb3CuBr4, melting incongruently at 544 K. The coordinates of the two eutectic points are: 501 K, 54 mole% CuBr and 522 K, 74 mole%
CuBr.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
993.
994.
An aniline-based polymer was electrochemically prepared and applied as a new fiber coating for solid phase microextraction (SPME) of some priority phenols from water samples. The polyaniline (PANI) film was directly electrodeposited on the platinum wire surface in sulfuric acid solution using cyclic voltammetry (CV) technique. The efficiency of new coating was investigated using a laboratory-made SPME device and gas chromatography with flame ionization detection for the extraction of some phenols from the headspace of aqueous samples. The scanning electron microscopy (SEM) images showed the homogeneity and the porous surface structure of the film. The results obtained proved the ability of this polymer as a suitable SPME fiber coating for trapping the selected phenols. Influential parameters affecting the extraction process were optimized and an extraction time of 50 min at 50 °C gave maximum efficiency, when the aqueous sample was saturated with NaCl and adjusted at pH 2. This new coating can be prepared easily in a reproducible manner and it is rather inexpensive and stable against most of organic solvents. The PANI thickness can be precisely controlled by the number of CV cycles. At the optimum conditions, the R.S.D. for a double distilled water spiked with phenol and chlorophenols at ppb level were 4.8-17% (n = 3) and detection limits for the studied compounds were between 0.69 and 3.7 ng ml−1, except for phenol and 4-chlorophenol. The optimized method was successfully applied to some real-life water samples. 相似文献
995.
《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(1):195-203
Seven discrete sugar‐pendant diamines were complexed to the {M(CO)3}+ (99mTc/Re) core: 1,3‐diamino‐2‐propyl β‐D ‐glucopyranoside ( L 1 ), 1,3‐diamino‐2‐propyl β‐D ‐xylopyranoside ( L 2 ), 1,3‐diamino‐2‐propyl α‐D ‐mannopyranoside ( L 3 ), 1,3‐diamino‐2‐propyl α‐D ‐galactopyranoside ( L 4 ), 1,3‐diamino‐2‐propyl β‐D ‐galactopyranoside ( L 5 ), 1,3‐diamino‐2‐propyl β‐(α‐D ‐glucopyranosyl‐(1,4)‐D ‐glucopyranoside) ( L 6 ), and bis(aminomethyl)bis[(β‐D ‐glucopyranosyloxy)methyl]methane ( L 7 ). The Re complexes [Re( L 1 – L 7 )(Br)(CO)3] were characterized by 1H and 13C 1D/2D NMR spectroscopy which confirmed the pendant nature of the carbohydrate moieties in solution. Additional characterization was provided by IR spectroscopy, elemental analysis, and mass spectrometry. Two analogues, [Re( L 2 )(CO)3Br] and [Re( L 3 )(CO)3Br], were characterized in the solid state by X‐ray crystallography and represent the first reported structures of Re organometallic carbohydrate compounds. Conductivity measurements in H2O established that the complexes exist as [Re( L 1 – L 7 )(H2O)(CO)3]Br in aqueous conditions. Radiolabelling of L 1 – L 7 with [99mTc(H2O)3(CO)3]+ afforded in high yield compounds of identical character to the Re analogues. The radiolabelled compounds were determined to exhibit high in vitro stability towards ligand exchange in the presence of an excess of either cysteine or histidine over a 24 h period. 相似文献
996.
固相萃取液相色谱-质谱/质谱联用测定河水中大环内酯类抗生素 总被引:19,自引:4,他引:19
应用固相萃取(SPE)及LC—MS/MS技术,建立了水中痕量大环内酯类抗生素即红霉素、脱水红霉素、罗红霉素的分析方法,优化了固相萃取、液相色谱-质谱/质谱等相关条件。水样经HLB固相萃取柱富集净化,以多反应检测方式(MRM)对待测物进行定性和定量分析。3种抗生素在10-2000ng/L范围内具有良好的线性。其定量下限为5ng/L(S/N〉10)。加标纯水和实际水样的回收率在71%-111%之间,相对标准偏差(RSD)在3.7%-8.6%之间。该方法灵敏度高、选择性好、准确度高,适合实际水样中痕量大环内酯类药物的检测。使用该方法测得珠江广州河段某水样中红霉素、脱水红霉素和罗红霉素质量浓度分别为164、291和134ng/L。 相似文献
997.
Andrés G. León Ana I. Olives M. Antonia Martín Benito del Castillo 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):577-583
The use of cyclodextrins (CDs) in HPLC as mobile phase additives provides a flexible alternative for the separation of chemically
related compounds because these separations can be performed on conventional columns and are economically advantageous over
the use of chiral stationary phases. The present paper describes the influence of the presence of β-cyclodextrin (β-CD) and
2-hydroxypropyl-β-cyclodextrin (HPβ-CD) on the separation of the β-carboline alkaloids norharmane, harmane and harmine. The
nature of the stationary phase (reverse phases C1 and C18) affects the chromatographic separations and also the stability of the inclusion complexes that are developed. The changes
in the proportion of the organic solvents at constant concentration of CDs (3 mM for β-CD and 15 mM for HPβ-CD) modify the
retention factors (k′) for all alkaloids studied. The nature of the organic solvent in the mobile phase also changes the chromatographic parameters.
The logarithm of the capacity factor (k′) is linearly increased with the proportion of water in the hydro-organic mobile phase (ethanolic or methanolic) but the
slopes obtained vary depending on the CD added to the mobile phase. The role of competitive equilibria, i.e., chromatographic
distribution and inclusion complexes formation is discussed.
This paper was presented at XIIIth International Cyclodextrin Symposium. Torino, Italy, May, 14–17, 2006 相似文献
998.
利用二维离子速度成像(Ion-Velocity Imaging)方法对二溴甲烷分子在234和267 nm附近的光解动力学行为进行了研究. 实验中得到了二溴甲烷光解产生的Br*(2P1/2)和Br(2P3/2)在不同波长下的角度和平动能分布. 在平动能分布中发现两个高斯分布, 推测其中主要是C—Br的快速解离, 而高能宽分布则来自于CH2Br自由基的二次解离过程. 通过角度分布得到了Br*与Br中来自直接解离和非绝热交叉跃迁两种来源的比例. 结果表明Br*原子主要来自于B1态的直接解离, 而Br则绝大部分是从B1态向A1的非绝热交叉跃迁得到, 并导致了两种解离通道能量分布的差别. 相似文献
999.
The analysis of samples contaminated by organic compounds is an important aspect of environmental monitoring. Because of the complex nature of these samples, isolating target organic compounds from their matrices is a major challenge. A new isolation technique, solid phase microextraction, or SPME, has recently been developed in our laboratory. This technique combines the extraction and concentration processes into one step; a fused silica fiber coated with a polymer is used to extract analytes and transfer them into a GC injector for thermal desorption and analysis. It is simple, rapid, inexpensive, completely solvent-free, and easily automated. To minimize matrix interferences in environmental samples, SPME can be used to extract analytes from the headspace above the sample. The combination of headspace sampling with SPME separates volatile and semi-volatile analytes from non-volatile compounds, thus greatly reducing the interferences from non-target compounds. This paper reports the use of headspace SPME to isolate volatile organic compounds from various matrices such as water, sand, clay, and sludge. By use of the technique, benzene, toluene, ethyl-benzene, and xylene isomers (commonly known as BTEX), and volatile chlorinated compounds can be efficiently isolated from various matrices with good precision and low limits of detection. This study has found that the sensitivity of the method can be greatly improved by the addition of salt to water samples, water to soil samples, or by heating. Headspace SPME can also be used to sample semi-volatile compounds, such as PAHs, from complex matrices. 相似文献
1000.
Cheremina O Bachmakov I Neubert A Brune K Fromm MF Hinz B 《Biomedical chromatography : BMC》2005,19(10):777-782
Oxycodone (14-hydroxy-7,8-dihydrocodeinone) is a potent opioid receptor agonist. In the present study, a liquid-liquid extraction-based reversed-phase HPLC method with UV detection was validated and applied for the analysis of oxycodone and its major metabolite, noroxycodone, in human plasma. The analytes were separated using a mobile phase, consisting of acetonitrile and phosphate buffer (8:92, v/v) at a flow rate of 1 mL/min, and UV detection at 205 nm. The retention times for oxycodone, noroxycodone and codein (internal standard) were 14.7, 13.8 and 10.2 min, respectively. The validated quantitation range of the method was 2-100 ng/mL for oxycodone and 10-100 ng/mL for noroxycodone. The developed procedure was applied to assess the pharmacokinetics of oxycodone and its metabolite following administration of a single 20 mg oral dose of oxycodone hydrochloride to one healthy male volunteer. 相似文献