全文获取类型
收费全文 | 52024篇 |
免费 | 3584篇 |
国内免费 | 2778篇 |
专业分类
化学 | 22992篇 |
晶体学 | 208篇 |
力学 | 2737篇 |
综合类 | 1024篇 |
数学 | 19429篇 |
物理学 | 11996篇 |
出版年
2023年 | 404篇 |
2022年 | 658篇 |
2021年 | 1897篇 |
2020年 | 1093篇 |
2019年 | 1214篇 |
2018年 | 974篇 |
2017年 | 1171篇 |
2016年 | 1413篇 |
2015年 | 1253篇 |
2014年 | 1925篇 |
2013年 | 3274篇 |
2012年 | 2230篇 |
2011年 | 2440篇 |
2010年 | 2302篇 |
2009年 | 2856篇 |
2008年 | 2982篇 |
2007年 | 3244篇 |
2006年 | 2549篇 |
2005年 | 1842篇 |
2004年 | 1652篇 |
2003年 | 1701篇 |
2002年 | 4142篇 |
2001年 | 1749篇 |
2000年 | 1288篇 |
1999年 | 1024篇 |
1998年 | 1067篇 |
1997年 | 745篇 |
1996年 | 720篇 |
1995年 | 672篇 |
1994年 | 622篇 |
1993年 | 628篇 |
1992年 | 621篇 |
1991年 | 446篇 |
1990年 | 353篇 |
1989年 | 298篇 |
1988年 | 309篇 |
1987年 | 244篇 |
1986年 | 268篇 |
1985年 | 393篇 |
1984年 | 303篇 |
1983年 | 166篇 |
1982年 | 321篇 |
1981年 | 503篇 |
1980年 | 454篇 |
1979年 | 510篇 |
1978年 | 399篇 |
1977年 | 318篇 |
1976年 | 264篇 |
1974年 | 84篇 |
1973年 | 158篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
Sándor Balogh Ede Kolos Gaszton Gáll und Pál Kolonits 《Fresenius' Journal of Analytical Chemistry》1975,276(3):201-204
Zusammenfassung Es wurden die Möglichkeiten für die titrimetrische Bestimmung von dimerem Glykolaldehyd (Fp: 94–96° C) und dl-Glycerinaldehyd (Fp: 138–141° C) untersucht. Die systematischen Fehler der auf Grund verschiedener funktioneller Gruppen durchführbaren Messungen sind von der Stabilität ihrer cyclischen Halbacetale sowie von den ihrer Depolymerisation folgenden weiteren Gleichgewichten bestimmt. Das aus dem dl-Glycerinaldehyd gebildete Halbacetal besitzt höhere Stabilität als dasjenige aus dem Glykolaldehyd.Mit der Hydrogensulfit-Methode werden 95–96%, mit der Hydroxylamin-Methode dagegen 98–99% vom wahren Wert gefunden.In Wasser gelöst zersetzt sich der Glykolaldehyd verhältnismäßig rasch, und das Gleichgewicht kann durch beide Aldehydreaktionen quantitativ gegen die Monomerenform verschoben werden.Unsere Untersuchungen ergaben, daß die Hydroxylamin-Methode zur Bestimmung der in Form cyclischer Halbacetale kristallisierenden, dimeren Hydroxyaldehyde Vorteile gegenüber der Hydrogensulfit-Methode bietet.Frau Dipl.-Ing. Eva Varsányi-Kiss danken wir für die derivatographischen Aufnahmen und deren Auswertung. 相似文献
952.
Villanueva J Villegas V Querol E Avilés FX Serrano L 《Journal of mass spectrometry : JMS》2002,37(9):974-984
In the post-genomic era, several projects focused on the massive experimental resolution of the three-dimensional structures of all the proteins of different organisms have been initiated. Simultaneously, significant progress has been made in the ab initio prediction of protein three-dimensional structure. One of the keys to the success of such a prediction is the use of local information (i.e. secondary structure). Here we describe a new limited proteolysis methodology, based on the use of unspecific exoproteases coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), to map quickly secondary structure elements of a protein from both ends, the N- and C-termini. We show that the proteolytic patterns (mass spectra series) obtained can be interpreted in the light of the conformation and local stability of the analyzed proteins, a direct correlation being observed between the predicted and the experimentally derived protein secondary structure. Further, this methodology can be easily applied to check rapidly the folding state of a protein and characterize mutational effects on protein conformation and stability. Moreover, given global stability information, this methodology allows one to locate the protein regions of increased or decreased conformational stability. All of this can be done with a small fraction of the amount of protein required by most of the other methods for conformational analysis. Thus limited exoproteolysis, together with MALDI-TOF MS, can be a useful tool to achieve quickly the elucidation of protein structure and stability. 相似文献
953.
Flow-injection chemiluminescence determination of epinephrine in pharmaceutical preparations using raw apple juice as enzyme source 总被引:3,自引:0,他引:3
Raw apple juice exhibits the activity of polyphenol oxidase which can be employed for the determination of mono-, di- and polyhydric phenols. The chemiluminescence detection in the two-channel manifold was used to develop the new procedure for determination of epinephrine. Epinephrine can be determined by this method in pharmaceutical preparations in concentration ranges 1.0–10.0 and 10.0–25.0 mg l−1, with a detection limit of 0.2 mg l−1. Good selectivity against typical antioxidants and other coexisting substances was achieved. Relatively slow loss of the enzyme activity was observed during 1 week. The proposed method is very simple. Fresh juice solution from one apple can be prepared in 10 min and used for at least 8 h with excellent precision. Consumption of luminol solution was 0.15 ml min−1. The throughput was 20 samples per hour. 相似文献
954.
K. Rossmanith 《Monatshefte für Chemie / Chemical Monthly》1995,126(5):543-547
Summary The Separation Factors for Pr/La, Nd/Pr, and Sm/Nd, characterizing the crystallization of ammonium and magnesium double nitrates under practical conditions, were determined analytically and are discussed. They confirm that La and Pr can be separated better by fractional crystallization of ammonium double nitrates, whereas in the presence of Sm the magnesium double nitrates are preferable. 相似文献
955.
Addition and Substitution Reactions at Tetrafluoro- and Tetrachlorodiborane(4) From equimolar mixtures of B2F4 and MenN(SiMe3)3-n (n = 0–3) the mono-addition products 1–4 are formed at low temperatures. By elimination of Me3SiF the adduct 2 gives the dimeric monosubstituted diborane 8 , which slowly decomposes at room temperature to the aminoborane 6 and (BF)n. The course of the reactions was studied by means of 11B and 19F NMR spectroscopy and by measuring the vapor pressures. According to the 11B and 31P NMR spectra the reaction of B2Cl4 with PCl5 or [Me4N]Cl in liquid hydrogen chloride at 0°C does not yield [PCl4]2+[B2Cl6]2? or [Me4N]2+[B2Cl6]2? but gives [PCl4]+[BCl4]?, PCl3 and BCl3 or [Me4N]+[BCl4]? and BCl3 besides (BCl)n. 相似文献
956.
Summary Interpretive methods are accepted to give the best possible results for selectivity optimization in HPLC. However the methods are very complex, and most work so far has been detailed academic studies. This paper describes an evaluation of a complete integrated system incorporating peak labelling, modelling of retention behaviour and calculation of response surfaces, with particular emphasis on the retention modelling. The peak labelling section has been discussed previously.A piece-wise quadratic function is investigated for the modelling of retention times across an isoeluotropic plane to effect selectivity optimization in HPLC. This requires 10 data-points on the isoeluotropic plane. The predicted global optimum and local optima are evaluated by comparison of calculated and experimental retention data, for a nine component sample. Seven interstitial points, distributed across the whole plane between the data-points, are similarly evaluated for a related sample. The typical error (in retention time) is less than 2%, often 1%, and the maximum error is 4.2%. At the global optimum the error was found be less than 1.3% for all 9 peaks. 相似文献
957.
A selective Spectrophotometric method has been developed for the trace determination of copper after adsorption of its 1-phenyl-4,4,6-trimethyl(1H, 4H)-pyrimidine-2-thiol (PTPT) complex onto microcrystalline naphthalene. The complex is quantitatively adsorbed on naphthalene in the pH range 7.5–11.5, separated by filtration, dissolved in dimethylformamide (DMF) and determined spectrophotometrically at 400 nm. Beer's law is obeyed in the concentration range 2.5–37.5 g of copper in 10 ml of DMF. The molar absorptivity and Sandell's sensitivity are 1.30 × 1041 · mol–1 · cm–1 and 0.0048g cm–2, respectively. Ten replicate analyses of a solution containing 20.0 g of copper gave a mean absorbance of 0.410 with a relative standard deviation of 0.91 %. The interferences of various ions have been studied and the method has been validated by the determination of copper in various standard reference materials, beers, wines, human hair, goat liver and environmental samples.On leave from St. Stephen's College, Delhi 110 007, India 相似文献
958.
Gert Brandl Fritz Kastner Rainer Fritsch Herbert Zinner Albrecht Mannschreck 《Monatshefte für Chemie / Chemical Monthly》1992,123(11):1059-1069
Summary A procedure is described which serves to measure circular dichrograms () on line during stops of flow in liquid chromatography. Since the concentration of substrate in the spectrometer cell during the stop is not known, the differential absorption coefficients are calculated from the experimental differential absorbances A by means of UV absorption (i. e. photomultiplier voltage) data. Verifications of the procedure are obtained by its application to three substrates (Table 1), the () spectra of which were known. The present on-line technique is compared with a corresponding off-line method.The N,N-dimethylthiobenzamides1 and2 as well as the 9,10-phenanthrenequinone7 consist of interconvertible enantiomers because their planar states are destabilized by steric overcrowding of groups. The unknown dichrograms () of1, 2 and7 are obtained (Figs. 2 and 4) and discussed with reference to the helicities of these molecules.In memory of the late Professor Dr. Dr. h. c. Günther Snatzke. 相似文献
959.
A new synthetic route for the preparation of the betain-like compound (CH3N)6P4F8 from (CH3NPF3)2,N-methyl-hexamethyldisilazane andN,N-dimethyl-urea has been found. The steps of this multi-stage reaction could be rationalized to a far extent.
10. Mitteilung:Kubjacek M., Utvary K., Mh. Chem.112, 305 (1981). 相似文献
960.
In the system U–La–N a new phase of composition La2U2N5 was observed. The differaction pattern of this phase can be indexed with a tetragonal unit cell:a=8.43 Å,c=8.50 Å andc/a=1.008. The pseudocubic sub-cell withaca/2 is closely related to the CsCl-type.
Auszug aus der von der Technisch-Naturwissenschaftlichen Fakuktät der TU Wien approbierten Diplomarbait des Herrn Dipl.-Ing.J. Waldhart. 相似文献