全文获取类型
收费全文 | 1978篇 |
免费 | 53篇 |
国内免费 | 101篇 |
专业分类
化学 | 1908篇 |
晶体学 | 1篇 |
力学 | 6篇 |
综合类 | 8篇 |
数学 | 63篇 |
物理学 | 146篇 |
出版年
2023年 | 12篇 |
2022年 | 67篇 |
2021年 | 65篇 |
2020年 | 48篇 |
2019年 | 59篇 |
2018年 | 32篇 |
2017年 | 35篇 |
2016年 | 69篇 |
2015年 | 75篇 |
2014年 | 58篇 |
2013年 | 92篇 |
2012年 | 203篇 |
2011年 | 126篇 |
2010年 | 108篇 |
2009年 | 128篇 |
2008年 | 115篇 |
2007年 | 159篇 |
2006年 | 95篇 |
2005年 | 98篇 |
2004年 | 94篇 |
2003年 | 50篇 |
2002年 | 44篇 |
2001年 | 19篇 |
2000年 | 16篇 |
1999年 | 19篇 |
1998年 | 22篇 |
1997年 | 16篇 |
1996年 | 33篇 |
1995年 | 31篇 |
1994年 | 21篇 |
1993年 | 27篇 |
1992年 | 13篇 |
1991年 | 20篇 |
1990年 | 13篇 |
1989年 | 10篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1979年 | 2篇 |
排序方式: 共有2132条查询结果,搜索用时 31 毫秒
81.
A sensitive catalytic method is developed for the spectrophotometric determination of oxalic acid. The procedure is based on the effect of oxalate on the oxidation of Victoria blue B by dichromate in dilute sulfuric acid medium. The reaction is quantitatively estimated by measuring the decrease in absorbance of Victoria blue B at the maximum wavelength of 610nm after quenching the reaction with tap water. The factors effecting the sensitivity and reproducibility of the reaction were studied. The method is not interfered with by foreign species generally associated with oxalate and oxalic acid. The described method is simple, specific, inexpensive and suitable for oxalic acid concentrations of between 0.06 and 9.0µgmL–1. It was validated with satisfactory results by determining oxalic acid content in water extracts from plant materials such as spinach and Lathyrus sativus. 相似文献
82.
This paper describes the validation of an isocratic HPLC method for the assay of voriconazole in tablets. The method employs
a Merck LiChrospher? 100 RP-8 (125 × 4.6 mm I.D., 5 μm particle size) column, with a mobile phase of methanol : triethylamine solutions 0.6 %,
pH 6.0 (50:50, v/v) and UV detection at 255 nm. A linear response (r > 0.9999) was observed in the range of 20.0–100.0 μg mL−1. The method showed good recoveries (average 100.4%) and the relative standard deviation intra and inter-day were ≤ 1.0 %.
Validation parameters as specificity and robustness were also determined. The method can be used for both quality control
assay of voriconazole in tablets and for stability studies as the method separates voriconazole from its degradation products
and tablet excipients. 相似文献
83.
An on-line coupled continuous flow liquid membrane extraction (CFLME) and C18 precolumn system was developed for sample preconcentration in liquid chromatography determination. After preconcentration by CFLME, which is based on the combination of continuous flow liquid–liquid extraction and supported liquid membrane, bisphenol A (BPA) was enriched in 960 μl of 1 mol l−1 NaOH used as acceptor. This acceptor was on-line neutralized and transported onto the C18 precolumn where analytes were absorbed and focused. Then the focused analytes were injected onto a C18 analytical column for separation and detected at 220 nm with a diode array detector. CFLME related parameters such as flow rates, pH of donor and acceptor, and enrichment time were optimized. The proposed method presents a detection limit of 0.03 μg l−1 (S/N=3) when 60 ml samples was enriched with an enrichment time of 30 min. Compared with C18 based column-switching procedure, this proposed procedure presents similar sample throughput and lower detection limits. The proposed method was successfully applied to determine BPA in tap water, river water, and municipal sewage effluent samples. 相似文献
84.
稀土元素对人肝癌细胞SMMC-7721增殖的影响 总被引:7,自引:3,他引:7
用MTT法研究了14种稀土元素(La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb和Lu)对人肝癌细胞SMMC-7721增殖的影响。他们对肝癌细胞的生长作用可分为3类。其中La^3 、Ce^3 和Eu^3 对肝癌细胞的增殖有剂量依赖性正效应,能够在一定浓度范围内刺激细胞生长;Sm^3 ,Gd^3 ,Ho^3 ,Er^3 ,Yb^3 对肝癌细胞生长的刺激作用没有剂量依赖性特征;而Pr^3 ,Nd^3 ,Tb^3 ,Dy^3 ,Tm^3 和Lu^3 则表现出对肝癌细胞的增殖具有不用程度的抑制。推测14种稀土元素作用方式的不同与他们的原子结构有一定的关系,它们对肝癌细胞的相对增殖率随着原子序数的增加呈现出一定的规律性。 相似文献
85.
I. L. Davies M. W. Raynor D. J. Urwin K. D. Bartle M. Tolay E. Ekinci H. E. Schwartz 《Journal of separation science》1988,11(11):792-799
This work describes the analysis of a pyrolysis product of a lignite sample obtained from the Turkish Goynuk reserve. The aliphatic, aromatic and polar compounds present in the tar are separated and identified by various chromatographic techniques: Capillary gas chromatography/mass spectrometry (GC/MS), on-line high performance microbore liquid chromatography/capillary gas chromatography (LC/GC) and capillary supercritical fluid chromatography (SFC). The suitability of each technique for this particular application is discussed, and semi-quantitative results are presented for the major components detected. 相似文献
86.
《Arabian Journal of Chemistry》2022,15(7):103933
In this study, a fingerprint-activity relationship modeling between chemical fingerprints and antirheumatic activity was established, and multivariate statistical analysis was used to evaluate the quality of Taxilli Herba (TH) from different hosts. Characteristic fingerprints of 20 batches of TH samples were generated by high-performance liquid chromatography coupled with triple quadrupole-time of flight tandem mass spectrometry (HPLC-Triple TOF-MS/MS), and the similarity analysis was calculated based on thirteen common characteristic peaks by hierarchical clustering analysis (HCA). Subsequently, nine efficacy markers were discovered by combining fingerprints and antirheumatic activity through grey correlation analysis (GCA) and bivariate correlation analysis (BCA). Meanwhile, the content of 5 constituents in 9 markers was determined by high-performance liquid chromatography coupled with triple quadrupole-linear ion trap tandem mass spectrometry (HPLC-QTRAP-MS/MS). The comprehensive quality of TH was assessed using multivariate statistical analysis, including principal components analysis (PCA) and technique for order preference by similarity to ideal solution (TOPSIS). The results showed that a high dose of TH extract could markedly ameliorate arthritis damage compared to other doses, with flavonoids playing an important role in the antirheumatic activity. The comprehensive quality of samples from Morus alba L. (SS) was superior to those from Liquidambar formosana Hance (FXS). The present study will demonstrate the markers associated with efficacy, and provide an applicable strategy for more comprehensive quality control and evaluation of TH. 相似文献
87.
In response to the impact of the Covid-19 epidemic on organic chemistry laboratory teaching, this article analyzes and discusses the current limitation of traditional organic chemistry laboratory teaching. Taking the "preparation of n-bromobutane" as an example, a new "online-offline and virtual-actual combination" teaching mode with the combination of "Tencent Meeting, MLabs, laboratory and WeChat group" is introduced. The practice of the new mode of teaching includes the following steps: pre-class online guiding by teacher, student preview and practice based on the virtual simulation platform, students and teacher discussion and operation in laboratory, and post-classcomprehensive assessment and Q&A", and good teaching effects have been received. It is significant in cultivating and improving the independent innovation and practical ability, and provides reference for the organic chemistry laboratory teaching reform and also lays a foundation for the construction of "outstanding course" of organic chemistry. 相似文献
88.
Adriano Francisco Barbosa Arnaldo César Pereira Lauro Tatsuo Kubota César Ricardo Teixeira Tarley 《Talanta》2007,71(4):1512-1519
The present paper proposes the application of multiwall carbon nanotubes (MWCNTs) as a solid sorbent for lead preconcentration using a flow system coupled to flame atomic absorption spectrometry. The method comprises the preconcentration of Pb (II) ions at a buffered solution (pH 4.7) onto 30 mg of MWCNTs previously oxidized with concentrated HNO3. The elution step is carried out with 1.0 mol L−1 HNO3. The effect of the experimental parameters, including sample pH, sampling flow rate, buffer and eluent concentrations were investigated by means of a 24 full factorial design, while for the final optimization a Doehlert design was employed. Under the best experimental conditions the preconcentration system provided detection and quantification limits of 2.6 and 8.6 μg L−1, respectively. A wide linear range varying from 8.6 up to 775 μg L−1 (r > 0.999) and the respective precision (relative standard deviation) of 7.7 and 1.4% for the 15 and 200 μg L−1 levels were obtained. The characteristics obtained for the performance of the flow preconcentration system were a preconcentration factor of 44.2, preconcentration efficiency of 11 min−1, consumptive index of 0.45 mL and sampling frequency estimated as 14 h−1. Preconcentration studies of Pb (II) ions in the presence of the majority foreign ions tested did not show interference, attesting the good performance of MWCNTs. The accuracy of the method was assessed from analysis of water samples (tap, mineral, physiological serum and synthetic seawater) and common medicinal herbs submitted to the acid decomposition (garlic and Ginkgo Biloba). The satisfactory recovery values obtained without using analyte addition method confirms the feasibility of this method for Pb (II) ions determination in different type of samples. 相似文献
89.
Simple and sensitive spectrophotometric methods for the assay of retinol have been presented. The first method was based on the reaction of retinol with iodine to give a molecular charge-transfer complex, the retinol acting as n-donor and iodine as -electron acceptor. The second method depends on the formation of a highly coloured stable radical anion between retinol and 7,7,8,8-tetracyanoquinodimethane (TCNQ as a -electron acceptor. The molecular ratios of the reactants in the complexes have been established and the experimental conditions leading to maximum charge-transfer bands were also studied. Beer's law is obeyed over the retinol concentration range 2.5–26 µg/ml. The proposed procedures have been applied successfully to the analysis of drug formulation. The average recovery and average standard deviation was 99.99 ±1.13% with retinol-iodine and 100.001 ± 1.31% with retinol-TCNQ. A kinetic study was performed by heating retinol at 50°C for different periods of time, the result obtained by plotting log c against time indicates that thermal decomposition of retinol is of first order. The results obtained by both methods were in good agreement with those obtained by the official method. The developed procedures were found to be simple, accurate and precise and can be used for the determination of retinol in presence of its degradation products. 相似文献
90.
Summary An improved method of alanine-amino transferase determination is proposed. The reaction is carried out with alanine and 2-oxoglutarate as substrates and analysis is by HPLC on a reversed-phase chromatographic system using a C18 column and tetrabutylammonium phosphate in phosphate buffer (pH 7.0)-acetonitrile as mobile phase. The enzyme activity was determined by directly following the formation of pyruvic acid without employing any secondary reaction, which is necessary in the spectrophotometric method. The detection limit of pyruvic acid is 10 pmole l–1 and the standard deviation for the enzymatic activity of standard solutions is 5.4%. Furthermore under the chromatographic conditions selected it is possible to detect the presence of some intermediate species.Work supported by National Research Council of Italy. Presented in part at the First International Symposium on Applications of HPLC in Enzyme Chemistry, Verona, September 1990. 相似文献