全文获取类型
收费全文 | 10124篇 |
免费 | 1145篇 |
国内免费 | 674篇 |
专业分类
化学 | 7473篇 |
晶体学 | 34篇 |
力学 | 284篇 |
综合类 | 114篇 |
数学 | 408篇 |
物理学 | 3630篇 |
出版年
2024年 | 30篇 |
2023年 | 155篇 |
2022年 | 440篇 |
2021年 | 444篇 |
2020年 | 422篇 |
2019年 | 360篇 |
2018年 | 292篇 |
2017年 | 453篇 |
2016年 | 517篇 |
2015年 | 493篇 |
2014年 | 692篇 |
2013年 | 779篇 |
2012年 | 691篇 |
2011年 | 576篇 |
2010年 | 512篇 |
2009年 | 604篇 |
2008年 | 545篇 |
2007年 | 557篇 |
2006年 | 469篇 |
2005年 | 478篇 |
2004年 | 425篇 |
2003年 | 287篇 |
2002年 | 201篇 |
2001年 | 135篇 |
2000年 | 140篇 |
1999年 | 144篇 |
1998年 | 134篇 |
1997年 | 126篇 |
1996年 | 104篇 |
1995年 | 111篇 |
1994年 | 66篇 |
1993年 | 73篇 |
1992年 | 63篇 |
1991年 | 42篇 |
1990年 | 60篇 |
1989年 | 49篇 |
1988年 | 53篇 |
1987年 | 44篇 |
1986年 | 29篇 |
1985年 | 30篇 |
1984年 | 21篇 |
1983年 | 17篇 |
1982年 | 30篇 |
1981年 | 16篇 |
1980年 | 7篇 |
1979年 | 13篇 |
1978年 | 5篇 |
1971年 | 2篇 |
1969年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Summary The highly complex matrix of diesel particulate extracts was analyzed for nitrated polycyclic aromatic hydrocarbons (nitro-PAH)
using fused-silica capillary-column gas chromatography along with a thermionic nitrogen-phosphorus detector (TID) and high-performance
liquid chromatography followed by on-line catalytic reduction of the nitro-PAH to amino-PAH and subsequent fluorescence detection.
Positive isomer identification and quantitation of nitro-PAH are from retention times of authentic standards and their mass
spectra. The ease of nitro-PAH formation by nitration of PAH raises the question regarding the origin of these species, whether
they are produced as “native” products during the engine combustion process and/or in the exhaust, or instead, formed as the
result of chemical conversion to produce artifacts during the sampling procedure. This problem is assessed examing 1-nitropyrene-concentration
in particulates of three light-duty diesel engines for different sampling times. 1-Nitropyrene concentrations show only a
moderate increase with sampling time under average sampling conditions.
Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984 相似文献
92.
Barnes JH Schilling GD Stone SF Sperline RP Denton MB Young ET Barinaga CJ Koppenaal DW Hieftje GM 《Analytical and bioanalytical chemistry》2004,380(2):227-234
The use of a separation step, such as liquid chromatography, prior to inductively coupled plasma mass spectrometry (ICP–MS) has become a common tool for highly selective and sensitive analyses. This type of coupling has several benefits including the ability to perform speciation analysis or to remove isobaric interferences. Several limitations of conventional instruments result from the necessity to scan or pulse the mass spectrometer to obtain a complete mass spectrum. When the instrument is operated in such a non-continuous manner, duty cycle is reduced, resulting in poorer absolute limits of detection. Additionally, with scanning instruments, spectral skew can be introduced into the measurement, limiting quantitation accuracy. To address these shortcomings, a high-performance liquid chromatograph has been coupled to an ICP–MS capable of continuous sample introduction and simultaneous multimass detection. These features have been realized with a novel detector array, the focal plane camera. Instrument performance has been tested for both speciation analysis and for the elimination of isobaric interferences. Absolute limits of detection in the sub picogram to tens of picograms regime are obtainable, while the added mass dimension introduced by simultaneous detection dramatically increases chromatographic peak capacity. 相似文献
93.
Marek Trojanowicz Joanna Szpunar-Łobińska Zdzislaw Michalski 《Mikrochimica acta》1991,103(3-4):159-169
An original computer-controlled solid-state photometer has been developed and its potential for simultaneous multicomponent flow-injection analysis of binary and ternary mixtures is demonstrated. The device is simple in construction. Its principle of operation is based on rapid sequential measurements of the absorbance of the complexes formed by the analytes with chromogenic reagents at the wavelengths corresponding to the emission maxima of three light emitting diodes (563, 580 and 638 nm). 相似文献
94.
Summary A packed flow cell was used for fluorometric detection in micro high-performance liquid chromatography. The flow cell consisted of fused-silica tubing packed with the same material as the separation column. A focusing effect of the stationary phase on the signal intensity was observed, leading to an improvement of the mass detection limit, as achieved by on-column detection. 相似文献
95.
Summary Four most frequently used presentation forms of the ECD response are described. Equations describing the dependence of the
sensitivity, linearity and minimum detectable concentration of the ECD working under constant frequency mode of operation
on the pulse period are derived from Wenthworth’s kinetic model for all four forms mentioned above. The forms presented are
compared on the basis of equations derived. Results of the calculations are given and some predictions are experimentally
tested for sulphur hexafluoride. Good agreement was found between the predicted and observed relationships.
Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984 相似文献
96.
This paper describes the screening for metabolites of butoprozine, a new anti-anginal drug, in dog bile by means of reverse-phase HPLC. Although it does involve a simple clean-up step to remove a substantial amount of endogenous bile compounds, this screening method nevertheless avoids extraction of metabolites and thus allows all metabolites to be introduced into the chromatographic system. A single run of 100 minutes from 100% water to 100% methanol in a linear gradient effects adequate separation of the great majority of metabolites without interference from remaining endogenous compounds. Two methods of differentiating between metabolite peaks and endogenous peaks have been worked out. The first one makes use of 14C-labeled butoprozine by measuring the amount of radioactivity in the column effluent while simultaneously recording the UV absorbance. The second method compares continuous gradient chromatograms of bile recorded before and after butoprozine administration under very similar conditions. The latter method can be applied to both radioactive and non-radioactive materials. 相似文献
97.
A two-step sequence involving an Ugi four-component reaction and an intramolecular nucleophilic aromatic substitution (SNAr) has been developed for the rapid access to biaryl-ether containing macrocycles. In the course of this study, we documented that ammonium chloride can promote the Ugi-4CR in non-polar aprotic solvent (toluene) without the interference of an alternative Passerini reaction. Solid phase synthesis of macrocycles by this two-step sequence was also developed using polymer (Wang resin) supported α-(4′-fluoro-3′-nitro)phenethyl isocyanoacetate as one of the inputs. 相似文献
98.
99.
Analysis of methyl- and ethylmercury (MM and EM) halides in biological and environmental samples is generally performed by gas chromatography with electron capture detection. Tedious sample work-up protocols and poor chromatographic response (using packed columns) have, however, shown the need for the development of new methods in this field. This paper reports a sensitive method, free from these deficiencies, for the determination of methyl- and ethylmercury. The organomercury compounds (MM and EM) are first released from the sample matrix, by the combined action of acidic potassium bromide and cupric ions, and then extracted into dichloromethane. The initial extracts are subjected to thiosulfate clean-up and the organomercury species are isolated as their chloride derivatives by addition of cupric chloride, and subsequent extraction into a small volume of organic solvent. Capillary GC coupled with atomic fluorescence detection provided excellent separation efficiencies for methyl- and ethylmercury and proved to be a very selective and sensitive technique. The absolute detection limit for both MM and EM was found to be 0.2 pg. 相似文献
100.
M. R. Gregg 《Chromatographia》1985,20(2):129-133
Summary Electrode surface modification by electrochemical pretreatment of glassy carbon electrodes was shown to enhance significantly the sensitivity of the electrodes for the detection of timolol and oxprenolol, but reduce slightly the sensitivity to prenalterol. This method may permit the detection of exprenolol and timolol with increased sensitivity, or may allow their detection at lower applied potentials than is presently possible. Electrode surface modification may prove to be a valuable aid to the detection of compounds that are considered to be outside the practical limits of electrochemical detection. 相似文献