全文获取类型
收费全文 | 1534篇 |
免费 | 20篇 |
国内免费 | 104篇 |
专业分类
化学 | 1005篇 |
晶体学 | 1篇 |
力学 | 1篇 |
综合类 | 4篇 |
数学 | 5篇 |
物理学 | 392篇 |
综合类 | 250篇 |
出版年
2024年 | 11篇 |
2023年 | 85篇 |
2022年 | 29篇 |
2021年 | 25篇 |
2020年 | 23篇 |
2019年 | 18篇 |
2018年 | 15篇 |
2017年 | 29篇 |
2016年 | 24篇 |
2015年 | 41篇 |
2014年 | 46篇 |
2013年 | 128篇 |
2012年 | 140篇 |
2011年 | 113篇 |
2010年 | 52篇 |
2009年 | 68篇 |
2008年 | 58篇 |
2007年 | 94篇 |
2006年 | 82篇 |
2005年 | 66篇 |
2004年 | 66篇 |
2003年 | 45篇 |
2002年 | 47篇 |
2001年 | 41篇 |
2000年 | 34篇 |
1999年 | 27篇 |
1998年 | 21篇 |
1997年 | 24篇 |
1996年 | 23篇 |
1995年 | 38篇 |
1994年 | 28篇 |
1993年 | 13篇 |
1992年 | 23篇 |
1991年 | 16篇 |
1990年 | 14篇 |
1989年 | 16篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有1658条查询结果,搜索用时 15 毫秒
111.
The quantitative determination of boldine alkaloid in boldo leaf extracts by employing cyclic voltammetry, at a liquid/liquid interface as well as the validation of this methodology against the reference method, high performance liquid chromatography (HPLC), are reported in the present paper. The voltammetric analysis was performed successfully and economically using two kinds of liquid/liquid interfaces: water/1,2-dicholoroethane and water/PVC (polyvinyl chloride)-gelled 1,2-dichloroethane. Linear calibration curves in the concentration range of 1.04 × 10−5 mol L−1 to 5.19 × 10−4 mol L−1 were obtained with a detection limit equal to (6.1 ± 0.7) × 10−5 mol L−1 and the quantitative determination of this alkaloid, in complex matrixes such as boldo leaf extracts, by the electrochemical technique proposed was found to be equal to the values obtained using the standard HPLC method. The validation analysis of this methodology against HPLC demonstrated that accuracy, linearity, limit of detection (LOD), limit of quantification (LOQ), specificity and precision are acceptable. The electroanalytical technique proposed is economical and selective, involves simple equipment and can be applied for the quantitative determination of boldine alkaloid in complex matrixes such as leaf extracts without special drug separation. Moreover, cyclic voltammetry (CV) experiments applied at the liquid/liquid interface under different experimental conditions allowed us to study the transfer mechanism of boldine, and determine a value of pKaw = 6.90 for protonated boldine, from the variation of voltammetric peak current with pH. 相似文献
112.
A number of 2-azetidinones were synthesized in good to excellent yields by a novel reaction between Schiff bases, substituted acetic acids and alkoxymethylene-N,N-dimethyliminium salts, the adduct formed from DMF and O-alkylating agents. The advantages of this new method are mild reaction conditions, low cost, avoiding the use of chlorinating agents and easy purification of the products. The best results were obtained when DMF and dimethyl sulfate were used at room temperature. 相似文献
113.
114.
Mohammad A. Khalilzadeh Issa Yavari Zinatossadat Hossaini Hasan Sadeghifar 《Monatshefte für Chemie / Chemical Monthly》2009,140(4):467-471
Abstract A novel method for oxazine ring formation is established using the reaction of ammonium thiocyanate and acid chlorides with
napthols in the presence of N-methylimidazole to afford [1,3]oxazine-4-thione derivatives in excellent yields.
Graphical abstract
相似文献
115.
酸性络蓝K双波长信号系数光度法测定钙镁 总被引:1,自引:0,他引:1
基于在氢氧化钠溶液中,钙、镁均与酸性络蓝K(ACBK)形成暗红色配合物,提出了测定钙和镁的双波长信号系数光度法.该法不仅消除了钙、镁之间的吸收干扰,且消除了一定量的共存离子的干扰.利用褪色法与生色法的吸光度之和作为信号吸光度,使方法的灵敏度大为提高.测定钙、镁总量时,分别用钙、镁吸收曲线正峰与负峰上的等吸收波长470.5 nm(λPa)及559.5 nm(λNa)处测定吸光度APa和ANa,求得总量的信号系数fs为2.439.于λPa470.5 nm处测得任一浓度钙、镁试液的吸光度APa′,其信号吸光度As=fsAλPa′.测定镁时,以试剂ACBK褪色对钙配合物生色的完全相消干扰的波长539 nm(Pa)作为测定波长,负峰波长583 nm为Na.按相同方法测得镁的信号系数fs为4.148. 相似文献
116.
117.
Acid/base modifiers are sometimes used as additives in normal phase elution on columns packed with CHIRALPAK® AD®. These modifiers affect enantioseparations in ways that are not yet fully understood for the lack of systematic studies. Shifts of the selectivity of certain pairs of enantiomers upon exposure of the column to these modifiers is amply documented. Furthermore, once the modifier has been removed from the mobile phase, the modified selectivity remains, which has been named the Memory Effect. After a column has been exposed to an eluent stream containing acidic/basic modifiers, this particular column no longer separates certain enantiomeric pairs with the same selectivity as a modifier naive column. This makes the transfer of developed methods from one to other CHIRALPAK AD columns difficult to predict, if the selectivity needs to be similar between the two columns. We selected four enantiomeric pairs for a systematic study of this Memory Effect. The selectivity of 4-chlorophenylalanine ethyl ester improves after a solution of ethanesulfonic acid (ESA) is percolated through the column. The selectivity of Propranolol and Tröger's base increases after a solution of Diiospropylamine is percolated through the column. The selectivity of Propranolol and Tröger's base enantiomers is inversely affected by percolation of the acid solution. The 4-chlorophenylalanine ethyl ester enantiomers is inversely affected by percolation of the base solution. In contrast, the selectivity of trans-stilbene oxide (TSO) is not affected by either modifier. Analytical studies of the stationary phase suggest that slow protonation/deprotonation of water molecules attached to the carbamate moiety may be responsible for the acid/base Memory Effect. To further the understanding of the effect of water on the Memory Effect, mobile phases – spiked with water (0.01–0.43%) – were used to measure changes in the Memory Effect. Finally, we showed that the influence of water on the Memory Effect can be minimized by percolating through the column a sufficiently concentrated solution of the appropriate base while using dried mobile phases. 相似文献
118.
Gloria Brusotti Tatiana Adrienne Ngueyem Raffaela Biesuz Gabriele Caccialanza 《Journal of separation science》2010,33(11):1692-1697
Euphorbiaceae barks are known to contain an appreciable amount of polyphenolic compounds responsible for several biological activities. Preliminary extraction from Bridelia grandis stem bark afforded high content of polyphenols, determined by spectrophotometric methods such as Folin–Ciocalteu (for total phenols, TP) and n‐butanol‐HCl (for condensed tannins, CT). A preliminary Plackett–Burman screening design was used to identify the key factors that influence the TP and CT extraction. Between all the variables known to influence the extraction from vegetable matrixes, six were selected; maceration was chosen as traditional extraction methodology. To investigate the effect of solvents and extraction method, methanol, acetone 70% (v/v in water), centrifugation and ultrasound were chosen. A full factorial design 23 was applied to optimize the extraction procedure. The responses were obtained analyzing the extracts for their TP and CT contents determined by the above‐mentioned spectrophotometric methods. The results confirm that, within the explored domain, the optimum solvent is methanol and the optimum method is one‐cycle centrifugation. Finally, it was also compared with the effect of maceration on the considered responses. It has never given results better than centrifugation, whereas in the case of CT it represents an advantage to employ a three‐cycle centrifugation instead of one. 相似文献
119.
120.