全文获取类型
收费全文 | 4079篇 |
免费 | 212篇 |
国内免费 | 244篇 |
专业分类
化学 | 3032篇 |
晶体学 | 24篇 |
力学 | 319篇 |
综合类 | 11篇 |
数学 | 498篇 |
物理学 | 651篇 |
出版年
2024年 | 8篇 |
2023年 | 76篇 |
2022年 | 67篇 |
2021年 | 44篇 |
2020年 | 78篇 |
2019年 | 78篇 |
2018年 | 66篇 |
2017年 | 81篇 |
2016年 | 169篇 |
2015年 | 132篇 |
2014年 | 130篇 |
2013年 | 296篇 |
2012年 | 189篇 |
2011年 | 231篇 |
2010年 | 235篇 |
2009年 | 281篇 |
2008年 | 240篇 |
2007年 | 254篇 |
2006年 | 206篇 |
2005年 | 174篇 |
2004年 | 190篇 |
2003年 | 146篇 |
2002年 | 115篇 |
2001年 | 88篇 |
2000年 | 92篇 |
1999年 | 97篇 |
1998年 | 91篇 |
1997年 | 72篇 |
1996年 | 71篇 |
1995年 | 55篇 |
1994年 | 57篇 |
1993年 | 50篇 |
1992年 | 46篇 |
1991年 | 56篇 |
1990年 | 29篇 |
1989年 | 32篇 |
1988年 | 30篇 |
1987年 | 23篇 |
1986年 | 15篇 |
1985年 | 32篇 |
1984年 | 19篇 |
1983年 | 12篇 |
1982年 | 20篇 |
1981年 | 17篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 10篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1967年 | 2篇 |
排序方式: 共有4535条查询结果,搜索用时 15 毫秒
71.
Six isomeric C20 alkylthiophenes were synthesized in order to prove the occurrence of such compounds in various deep sea sediments. As expected from the analogy with the corresponding saturated hydrocarbons, 2- and 3-heptadecylthiophene have considerably longer relative retention times than the isomeric isoprenoid compounds, i.e. 2- and 3-(4,8,12-trimethyltridecyl)thiophene and 3- and 4-methyl-2-(3,7,11-trimethyldodecyl)thiophene. Covats retention indices for all compounds were determined by isothermal analysis at 190°C on two different silicone-coated capillary columns. The mass spectra of the monoalkylated thiophenes show a base peak at m/z 98 corresponding to a Mcafferty rearrangement product in the case of the 3-substituted isomers, whereas a simple benzylic cleavage dominates in the mass spectra of 2-alkylthiophenes. 4-Methyl-2-(3,7,11-trimethylodecy)thiophene is an exception from this simple rule because also shows the McLafferty rearrangement as dominant fragmentation. Two of the isoprenoid thiophenes are prominent compounds in various deep sea sediments. 相似文献
72.
73.
An ion trap mass spectrometer, equipped with a membrane/jet separator interface, is used for the direct detection of volatile and semi-volatile organic compounds in aqueous solutions. Aqueous sample is passed through a capillary membrane, the outside surface of which is continuously purged by helium. The permeate is pneumatically transported to the mass spectrometer via a jet separator which acts as an additional enrichment device. The performance and response characteristics of non-porous silicone and microporous polytetrafluoroethylene (PTFE) membranes are studied. The microporous membrane allows sufficient water to pass for it to be used as a reagent gas for chemical ionization. Both types of membranes provide detection limits in the parts per trillion (pptr) to parts per billion (ppb) range with a linear dynamic range of 3 orders of magnitude for some volatile organic compounds. Results show that there is no detectable matrix effect on response in the selected cases examined. The use of microporous membranes to analyze more polar compounds, such as 5-hydroxymethyl furfuraldehyde and lactic acid, is also demonstrated. The effects of other experimental parameters, such as membrane temperature and length, on sensitivity are also investigated. 相似文献
74.
Delatour T 《Analytical and bioanalytical chemistry》2004,380(3):515-523
Liquid chromatography–tandem mass spectrometry (LC-MS/MS) is a versatile coupling system which combines both selectivity and sensitivity and certainty. Hence, it is generally considered as the most reliable technique to quantify chemical compounds in complex matrices. In the present paper, we evaluate the performance of LC-MS/MS methods for the quantification of 3-nitrotyrosine in human urine in order to point out its dependence on the design of the quantification method, and emphasize the role of matrix effects in the performance. We compare external and internal calibrations, isotope dilution and isotopomer-based exact matching. The role of both sample preparation and multiple transitions monitoring is particularly addressed. 相似文献
75.
Abdulqawi NumanNeil D Danielson 《Analytica chimica acta》2002,460(1):49-60
The application of on-line photochemistry with flow injection (FI) and liquid chromatography (LC) in conjunction with atmospheric pressure electrospray mass spectrometry (LC-APESI-MS) for the identification of similar indole derivatives is reported here. The photo-transformation of the indole compounds is strongly affected by the substituent groups on the aromatic and heterocyclic rings. Upon photolysis for 2.5 min, the mass spectrum of tryptamine (Try) which has no OH substituent on the aromatic ring does not differ greatly from that obtained without photolysis. However, after photolysis of serotonin (Ser) which has one OH group on C5 of the aromatic ring, the mass spectrum indicates the formation of dimers and higher molecular weight ions. The fragmentation pattern of 5-hydroxytryptophol (Phol) without photolysis resembles that of Ser with a base peak of m/z 160. Upon photolysis using MeOH-H2O (10/90), Phol is found to form a base peak at m/z 375 (100%) and a major peak at m/z 214 (66%) in addition to other ions with lower abundance. Melatonin (Mel) and tryptophan (Phan) upon photolysis are found to form high molecular weight ions with a relative low abundance. The mass spectrum of indole-3-acetic acid (Inaa) with on-line photolysis also shows different ions that are not formed without photolysis. 相似文献
76.
77.
A. El Sayed S. S. Abd El Rehim H. Mansour 《Monatshefte für Chemie / Chemical Monthly》1991,122(12):1019-1027
Summary The electrochemical behaviour of pure indium in KOH solutions (1–4M) was studied at different temperatures (25–70°C) by potentiostatic techniques. Two anodic peaks corresponding to the formation of In(OH)3 and In2O3 were observed. The heights of the two peaks increased with the increase of alkali concentration. An increase of temperature increased the peak currents and shifted their corresponding potentials to more negative values. The variation of the peak currents and peak potentials with scan rate suggested that the anodic dissolution of indium was a diffusion controlled process. In cyclic voltammetry, the reverse scan consistently showed one peak which was attributed to the reduction of anodic oxidation products into indium. X-ray diffraction analysis confirmed the presence of In(OH)3 at the first anodic peak, In(OH)3 and In2O3 at the second anodic peak and In2O3 in the permanent passive region.
Zum elektrochemischen Verhalten einer Indium-Elektrode in konzentrierter Kalilauge bei verschiedenen Temperaturen
Zusammenfassung Es wurde das Verhalten von reinem Indium in 1 – 4M KOH-Lösungen bei Temperaturen zwischen 25 und 70°C mittels potentiostatischer Methoden untersucht. Zwei anodische Peaks, entsprechend der Bildung von In(OH)3 und In2O3, traten auf. Die Höhe der beiden Peaks wurde mit zunehmender Alkalikonzentration gesteigert. Eine Temperaturerhöhung verstärkte die Peakströme und verschob die entsprechenden Potentiale zu negativeren Werten. Die Abhängigkeit der Peakströme und Peakpotentiale von der Scangeschwindigkeit legte den Schluß nahe, daß die anodische Lösung von Indium in einem diffusionskontrollierten Prozeß stattfindet. Bei der cyclischen Voltammetrie zeigte der reverse Scan einheitlich einen Peak, der der Reduktion der anodischen Oxidationsprodukte zu Indium zugeschrieben wurde. Röntgendiffraktionsanalyse bestätigte die Präsenz von In(OH)3 beim ersten anodischen Peak, In(OH)3 beim zweiten Peak und In2O3 im permanent passiven Bereich.相似文献
78.
A small amount (≤ 10−6 mol fraction) of four alkaline earth metals, tin and yttrium were introduced into five, premixed, fuel-rich, H2–O2–N2 flames at atmospheric pressure in the temperature range 1820–2400 K. Aqueous salt solutions of the metals were sprayed into the premixed flame gas as an aerosol using an atomizer technique. Ions in a flame were observed by sampling flame gas through a nozzle into a mass spectrometer. The concentrations of the major neutral metallic species present in the flame were calculated from thermodynamic data currently available. The principal metallic ions observed were AOH+ (A = Mg, Ca, Sr, Ba, Sn) and A(OH)2+ (A = Y), formed initially by proton transfer to AO and OAOH from H3O+, a natural flame ion. Except for Mg, the ions were also produced by chemi-ionization processes. By adjusting the concentration(s) of the salt solution in the atomizer, it was found that a pair of ions could be brought into equilibrium within the time scale of the flame; the pairs included H3O+ with a metal ion or two metallic ions. Because water is a major product of combustion, a very large difference in proton affinity PA0(AO) − PA0(H2O) ≤ 490 kJ mol−1 (117 kcal mol−1) could be attempted for the proton transfer equilibrium. Using PA0(H2O) = 691.0 kJ mol−1 (165.2 kcal mol−1) as a reference base to anchor the proton affinity scale, ion ratio measurements led to proton affinity PA0 values of 766, 912, 1004, 1184, 1201, and 1222 kJ mol−1 (183, 218, 240, 283, 287, and 292 kcal mol−1) corrected to 298 K for OYOH, SnO, MgO, CaO, SrO, and BaO, respectively; of these, only the value for OYOH has not been reported previously. If it is assumed that the neutral thermodynamic data are correct (although some appear to be in error), the uncertainties in the PA results reported here are ± 21 kJ mol−1 (5 kcal mol−1). The realization that these equilibria can be achieved in flames provides a new approach to consolidate and build the high end of the proton affinity ladder, primarily of metallic species which are not accessible at lower temperatures. 相似文献
79.
气相二茂铁双电荷离子的动能谱研究 总被引:1,自引:0,他引:1
The unimolecular charge separation reactions of the doubly charged ions FeC_(10)H_(10)~(2+)、FeC_(10)H_9~(2+)、FeC_(10)H_8~(2+) produced in the ion source by electron impact from ferrocene have been studied using Mass analyzed Ion Kinetic Energy Spectrometry (MIKES) technique. From the values of the kinetic energy releases (T), the intercharge distances (R) of the exploding doubly charged ions in their transition structures have been estimated and some structural informations about the transition states can be obtained. The collision induced reactions of the FeC_(10)H_(10)~(2+) ion with Ar have been studied using MIKES, we postulate a new type of continuing reaction which may be "collisional charge separation induced dissociation". 相似文献
80.
The composition of the essential oil of Bifora radians, an aldehyde-producing weed, has been investigated by capillary gas chromatography, coupled gas chromatography – mass spectrometry, on-line catalytic hydrogenation and coupled gas chromatography – infrared spectrometry. The nineteen compounds identified included eighteen aldehydes: seven alkanals (C6, C9, C10, C11, C12, C13, and C14), ten alkenals, including five (E)-2-alkenals (C12, C13, C14, C15, and C16), and one (E,E)-2,4-alkadienal (C13). Typical Bifora odors were attributed to three major (E)-2-alkenals, C12, C13, and C14. 相似文献