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排序方式: 共有145条查询结果,搜索用时 187 毫秒
51.
Teodor Stoichev David Amouroux Rosa C. Rodriguez Martin‐Doimeadios Mathilde Monperrus Olivier F. X. Donard Dimiter L. Tsalev 《应用光谱学评论》2013,48(6):591-619
Abstract This review considers methods for mercury speciation with low limits of detection that can be applied to real aquatic environmental samples (waters, sediments, biological tissues). Special attention is given to the necessity of clean sampling procedures and the proper storage of the samples. In this review, different extraction techniques for sediments and biological tissues are considered. The performance of different separation techniques, like liquid chromatography and off‐line and on‐line gas chromatography, are compared for their environmental applications. 相似文献
52.
Magnus Karlsson Pernille R. Jensen Jens ?. Duus Sebastian Meier Mathilde H. Lerche 《Applied magnetic resonance》2012,43(1-2):223-236
Dissolution dynamic nuclear polarization (DNP) provides a broadly applicable and rather simple means of developing probes for the real-time molecular imaging of cellular functions in vivo. The development of novel dissolution DNP substrate formulations is only rewarding for substrates that yield detectable metabolism within few minutes. In addition, in vivo preparations usually require amorphous samples at molar substrate concentrations for an efficient and reproducible DNP step with sufficient material. The composition ranges of novel substrate preparations need to be established experimentally owing to the solute’s impact on vitrification behavior. Here, we describe simple rationales employed in the development of novel substrate preparations for dissolution DNP-magnetic resonance. Solution state substrate polarizations between 10 and 40?% have been obtained for ~40 metabolic substrates in highly concentrated preparations that yield physiologically tolerable solutions with sufficient T 1 for in vivo nuclear magnetic resonance. Substrate metabolism is observed for novel in vivo substrates such as 3-hydroxybutyrate and aspartate. 相似文献
53.
We report the observation of a higher-order frequency shift due to the trapping field in a (87)Sr optical lattice clock. We show that, at the magic wavelength of the lattice, where the first-order term cancels, the higher-order shift will not constitute a limitation to the fractional accuracy of the clock at a level of 10(-18). This result is achieved by operating the clock at very high trapping intensity up to 400 kW/cm(2) and by a specific study of the effect of the two two-photon transitions near the magic wavelength. 相似文献
54.
Frank Hülsemann Karsten Koehler Jürgen Wittsiepe Michael Wilhelm Annett Hilbig Mathilde Kersting 《Isotopes in environmental and health studies》2017,53(4):356-367
Natural stable isotope ratios (δ15N) of humans can be used for nutritional analyses and dietary reconstruction of modern and historic individuals and populations. Information about an individual’s metabolic state can be obtained by comparison of tissue and dietary δ15N. Different methods have been used to estimate dietary δ15N in the past; however, the validity of such predictions has not been compared to experimental values. For a total of 56 meals and 21 samples of 24-h diets, predicted and experimental δ15N values were compared. The δ15N values were predicted from self-recorded food intake and compared with experimental δ15N values. Predicted and experimental δ15N values were in good agreement for meals and preparations (r?=?0.89, p?.001) as well as for the 24-h diets (r?=?0.76, p?.001). Dietary δ15N was mainly determined by the amount of fish, whereas the contribution of meat to dietary δ15N values was less pronounced. Prediction of human dietary δ15N values using standardised food records and representative δ15N data sets yields reliable data for dietary δ15N intake. A differentiated analysis of the primary protein sources is necessary when relating the proportion of animal-derived protein in the diet by δ15N analysis. 相似文献
55.
Atmospheric solid analysis probe mass spectrometry vs electrospray tandem mass spectrometry of polydimethylsiloxanes in positive and negative ionization modes
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56.
Pentacarbonyltetrafluoroboratomanganese(I), (OC)5MnFBF3, reacts with 2-butyne at 15°C in dichloromethane to give tetracarbonyl(η4-tetramethylcyclobutadiene)manganese(I) tetrafluoroborate, [(OC)4Mn(C4Me4)]+ BF4? 相似文献
57.
[reaction: see text] A highly stereoselective synthesis of (-)-erythrodiene starting from 4-isopropylcyclohexanone is described. The key reactions are an asymmetric methoxycarbonylation of the starting ketone and a highly diastereoselective radical cascade involving addition of a phenylthiyl radical to a terminal alkyne followed by a 1,5-hydrogen transfer and a 5-exo-cyclization. 相似文献
58.
Regulus P Spessotto S Gateau M Cadet J Favier A Ravanat JL 《Rapid communications in mass spectrometry : RCM》2004,18(19):2223-2228
High-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry (HPLC/ESI-MS/MS) has been used to search for the formation of as yet unidentified radiation-induced DNA lesions. For that purpose, the characteristic fragmentation of most of 2'-deoxyribonucleosides that corresponds to the loss of the 2-deoxyribose moiety (loss of 116 mass units) has been utilized to specifically detect modified nucleosides. Aerated aqueous solutions of DNA were exposed to ionizing radiation, and subsequently DNA was digested to nucleosides with a cocktail of endo- and exonucleases. HPLC/ESI-MS/MS analysis of the resulting 2'-deoxyribonucleoside mixture allowed us to detect four novel DNA modifications. In a subsequent step, the sensitivity of the tandem mass spectrometer was used to search for the formation of the newly detected lesions in the DNA of gamma-irradiated cells. Thus, one of the four newly detected lesions was found to be significantly generated in cellular DNA upon exposure to ionizing radiation. In addition, the latter lesion was also shown to be present in untreated cells, indicating that the modified nucleoside could be formed endogenously. 相似文献
59.
Monperrus M Rodriguez Gonzalez P Amouroux D Garcia Alonso JI Donard OF 《Analytical and bioanalytical chemistry》2008,390(2):655-666
A new double-spiking approach, based on a multiple-spiking numerical methodology, has been developed and applied for the accurate
quantification of inorganic mercury (IHg) and methylmercury (MeHg) by GC–ICPMS in different environmental matrices such as
water, sediments and a wide range of biological tissues. For this purpose, two enriched mercury species (201MeHg and 199IHg) were added to the samples before sample preparation in order to quantify the extents of the methylation and demethylation
processes, and thereby correct the final species concentrations. A critical evaluation of the applicability of this methodology
was performed for each type of matrix, highlighting its main advantages and limitations when correcting for the conversion
reactions of the species throughout the whole sample preparation procedure. The double-spike isotope dilution (DSIDA) methodology
was evaluated by comparing it with conventional species specific isotope dilution (IDA) when analysing both certified reference
materials and environmental samples (water, biotissues and sediment). The results demonstrate that this methodology is able
to provide both accurate and precise results for IHg and MeHg when their relative concentrations are not too different (ratio
MeHg/IHg > 0.05), a condition that holds for most natural waters and biotissues. Significant limitations on the accurate and
precise determination of the demethylation factor are however observed, especially for real sediment samples in which the
relative concentrations of the species are substantially different (ratio MeHg/IHg < 0.05). A determination of the sources
of uncertainty in the methylation/demethylation factors has demonstrated that the accurate and precise measurement of the
isotope ratios in the species involved in the transformations is crucial when quantifying the extents of these reactions.
Although the double-spike methodology is established as a reference approach that permits the correction of most analytical
biases and the accurate quantification of Hg species, some limitations have been identified for the first time in this work. 相似文献
60.
Mathilde Lachia François Richard Raphael Bigler Amandine Kolleth-Krieger Michael Dieckmann Alexandre Lumbroso Ulfet Karadeniz Saron Catak Alain De Mesmaeker 《Tetrahedron letters》2018,59(20):1896-1901
γ-Lactams are important building blocks for the synthesis of biologically active molecules and can easily be accessed via Beckmann rearrangement of cyclobutanones. However, Beckmann fragmentation is often a competing reaction for these strained ketones. We found that performing the Beckmann rearrangement with Tamura’s reagent in the presence of aqueous HCl suppresses the undesired fragmentation reaction. This improved procedure was applied to a broad scope of substrates affording monocyclic, bicyclic, tricyclic or spirocyclic lactams.Our experimental results and DFT calculations suggest that the mechanism of the rearrangement probably involves a tetrahedral intermediate and doesn’t proceed via oxime fragmentation as in a classical Beckmann rearrangement. 相似文献