全文获取类型
收费全文 | 5602篇 |
免费 | 699篇 |
国内免费 | 433篇 |
专业分类
化学 | 4580篇 |
晶体学 | 60篇 |
力学 | 125篇 |
综合类 | 25篇 |
数学 | 312篇 |
物理学 | 1632篇 |
出版年
2024年 | 12篇 |
2023年 | 41篇 |
2022年 | 143篇 |
2021年 | 101篇 |
2020年 | 206篇 |
2019年 | 136篇 |
2018年 | 170篇 |
2017年 | 174篇 |
2016年 | 276篇 |
2015年 | 256篇 |
2014年 | 362篇 |
2013年 | 486篇 |
2012年 | 380篇 |
2011年 | 386篇 |
2010年 | 332篇 |
2009年 | 371篇 |
2008年 | 332篇 |
2007年 | 336篇 |
2006年 | 326篇 |
2005年 | 272篇 |
2004年 | 268篇 |
2003年 | 194篇 |
2002年 | 141篇 |
2001年 | 116篇 |
2000年 | 83篇 |
1999年 | 118篇 |
1998年 | 78篇 |
1997年 | 95篇 |
1996年 | 65篇 |
1995年 | 63篇 |
1994年 | 60篇 |
1993年 | 36篇 |
1992年 | 35篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 20篇 |
1988年 | 33篇 |
1987年 | 31篇 |
1986年 | 19篇 |
1985年 | 24篇 |
1984年 | 26篇 |
1983年 | 10篇 |
1982年 | 24篇 |
1981年 | 10篇 |
1980年 | 14篇 |
1979年 | 13篇 |
1978年 | 3篇 |
1977年 | 2篇 |
排序方式: 共有6734条查询结果,搜索用时 46 毫秒
101.
An automated on-line method for the determination of the isoflavones, daidzein and genistein, was developed using in-tube solid-phase microextraction coupled to high-performance liquid chromatography (in-tube SPME-HPLC). In-tube SPME is a new extraction technique for organic compounds in aqueous samples, in which analytes are extracted from the sample directly into an open tubular capillary by repeated draw/eject cycles of sample solution. Daidzein, genistein and their glucosides tested in this study were clearly separated within 8 min by HPLC using an XDB-C8 column with diode array detection. In order to optimize the extraction of these compounds, several in-tube SPME parameters were examined. The glucosides daidzin and genistin were analyzed as aglycones after hydrolysis because the glucosides were not concentrated by in-tube SPME. The optimum extraction conditions for daidzein and genistein were obtained with 20 draw/eject cycles of 40 microl of sample using a Supel-Q porous layer open tubular capillary column. The extracted compounds were easily desorbed from the capillary by mobile phase flow, and carryover was not observed. Using the in-tube SPME-HPLC method, the calibration curves of these compounds were linear in the range 5-200 ng/ml, with a correlation coefficient above 0.9999 (n = 18), and the detection limits (S/N = 3) were 0.4-0.5 ng/ml. This method was successfully applied to the analysis of soybean foods without interference peaks. The recoveries of aglycones and glucosides spiked into food samples were above 97%. 相似文献
102.
本文对优化液相色谱分离条件的重叠分辨率法进行了改进与扩展:首先利用全范围的二元线性梯度淋洗进行溶剂强度的优化,再通过重叠分辨率法进行了溶剂选择性的优化,并且建立了计算机系统,使此方法程序化。利用此方法建立了分离N-二茂铁甲酰基-N'-芳基硫脲类衍生物的最佳流动相体系。 相似文献
103.
The mass resolution of a time-of-flight (TOF) mass spectrometer is directly proportional to its total flight pathlength. Multi-turn or multi-passage ion optical geometries are necessary to obtain fight distances of sufficient length within reasonable size limitations. We have investigated ion optics for a multi-turn TOF mass spectrometer with electrostatic sectors. The concept of 'perfect' focusing conditions is introduced. Furthermore, a new type of multi-turn TOF mass spectrometer, the MULTUM Linear plus, was developed. It consists of four cylindrical electric sectors and 28 electric quadrupole lenses. It has a vacuum chamber 60 x 70 x 20 cm in size. Mass resolution is demonstrated to increase according to the number of ion cycles. A mass resolution of 350 000 (m/z = 28, FWHM) was achieved after 501.5 cycles. The MULTUM Linear plus analyzer is not simple, however; 28 electric quadrupole lenses are used. In order to reduce the number of ion optical parts, an improved multi-turn TOF mass spectrometer, the MULTUM II, consisting of only four toroidal electric sectors, was also developed. The possibility of tandem mass spectrometric applications using multi-turn TOF mass spectrometers is also discussed. 相似文献
104.
105.
Krasnov V. P. Levit G. L. Korolyova M. A. Bukrina I. M. Sadretdinova L. Sh. Andreeva I. N. Charushin V. N. Chupakhin O. N. 《Russian Chemical Bulletin》2004,53(6):1253-1256
The influence of the reaction conditions (solvent, base) on the diastereoselectivity of acylation of (±)-3-methyl-2,3-dihydro-4H-1,4-benzoxazine with (S)-naproxen and N-tosyl-(S)-proline chlorides was studied. The highest diastereoselectivity was observed for the reaction carried out in benzene in the presence of aliphatic tertiary amines. 相似文献
106.
Kenji Maeda Ross A. Miller Ali Shafiee Joseph D. Armstrong III 《Tetrahedron letters》2005,46(9):1545-1549
Highly diastereoselective (>20:1) bromo-lactonization of N-sulfonyl-2-allyl-2-phenylglycine methyl ester (11) was observed. Successive treatment of the chiral lactone with MeONa gave the desired (2S,4R)-4-hydroxy-2-phenylproline derivative in high yield without erosion of the diastereoselectivity. The starting chiral non-racemic compound (5) was prepared from the racemic 2-phenylglycine using a classical kinetic resolution (crystallization), an asymmetric phase transfer alkylation, and an enzyme-catalyzed kinetic resolution. 相似文献
107.
Marti E. Kaisersberger E. Emmerich W.-D. 《Journal of Thermal Analysis and Calorimetry》2004,77(3):905-934
The definitions of the temperature resolution, the so-called resolution of DSC instruments given in literature are discussed.
A new definition of the resolution for DSC instruments is presented and outlined. The main characteristic introduced in this
new definition is a minimum between two caloric events as a prerequisite of an existing resolution. Possible candidates of
test substances have been evaluated. The oligomer n-hexatriacontane is revealing an interesting phenomenon, namely a lambda
transition which is in the peak temperature only 2 K lower than the melting temperature of 76°C. The substance was selected
as an ideal test substance for the quantification of the resolution of DSC instruments. The lambda transition is a second
order process which may reach under certain conditions the saturation of the occurring molecular dislocation within 0.2 K,
and after saturation the heat flow rate drops sharply down. Investigations concerning the main characteristics of n-hexatriacontane
in respect to the temperatures of transition (lambda transition and melting), to the involved enthalpies, and to the resolution
factors were performed as functions of the sample mass and the heating rate. The importance of relevant evaluation procedures
increasing the resolution factors of DSC curves are discussed and these procedures are integrated into the testing of the
resolution. The necessity for widening the experimental scope from instruments to evaluation procedures is forced by the existence
of instruments with built-in signal treatments based on electronic devices and software procedures. A comparison with literature
data is outlined for all of the mentioned characteristic values of n-hexatriacontane.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
108.
Quantitative analysis of docetaxel in human plasma using liquid chromatography coupled with tandem mass spectrometry 总被引:2,自引:0,他引:2
Kuppens IE van Maanen MJ Rosing H Schellens JH Beijnen JH 《Biomedical chromatography : BMC》2005,19(5):355-361
An assay for the quantitative determination of docetaxel in human plasma is described. Docetaxel was extracted from the matrix using liquid-liquid extraction with ter-butylmethylether, followed by high-performance liquid chromatographic analysis using an alkaline eluent. Paclitaxel was used as internal standard. Positive ionization electrospray tandem mass spectrometry was performed for selective and sensitive detection. The method was validated according to the FDA guidelines on bioanalytical method validation. The validated range for docetaxel was from 0.25--1000 ng/mL using 200 microL plasma aliquots. The method requires only a limited volume (200 microL) of human plasma and the method can be applied in studies requiring a low lower limit of quantitation of 0.25 ng/mL. The assay was applied successfully in several clinical and pharmacological studies with docetaxel. 相似文献
109.
压力流驱动电色谱分析方法及其若干色谱行为 总被引:1,自引:0,他引:1
压力流驱动电色谱分析方法是一种新型色谱技术。它组合了HPLC和电泳两种技术,为分离提供了更多可控制的因素。介绍了压力流驱动电色谱的装置和毛细管电色谱柱的装置方法,从理论上讨论了影响电色谱分离的因素并用实验数据加以证明。最后尝试性地将电色谱法用于测定香蕉样品中萨索啉(Salsolinol)等的含量。 相似文献
110.
PengWANG ShuRenJIANG ZhiQiangZHOU 《中国化学快报》2004,15(12):1457-1460
Cellulose-tris(3,5-dimethylphenylcarbamate)(CDMPC) were synthesized as highperformance liquid chromatography (HPLC) chiral stationary phase(CSP). The successful resolution of four chiral pesticides on the CSP was realized and the influence of the temperature on the separation was studied. 相似文献