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531.
Typically following collection biological samples are kept in a freezer for periods ranging from a few days to several months before analysis. Experience has shown that in LC-MS-based metabonomics research the best analytical practice is to store samples as these are collected, complete the sample set and analyse it in a single run. However, this approach is prudent only if the samples stored in the refrigerator or in the freezer are stable. Another important issue is the stability of the samples following the freeze-thaw process. To investigate these matters urine samples were collected from 6 male volunteers and analysed by LC-MS and ultra-performance liquid chromatography (UPLC)-MS [in both positive and negative electrospray ionization (ESI)] on the day of collection or at intervals of up to 6 months storage at -20 degrees C and -80 degrees C. Other sets of these samples underwent a series of up to nine freeze-thaw cycles. The stability of samples kept at 4 degrees C in an autosampler for up to 6 days was also assessed, with clear differences appearing after 48h. Data was analysed using multivariate statistical analysis (principal component analysis). The results show that sample storage at both -20 and -80 degrees C appeared to ensure sample stability. Similarly up to nine freeze thaw cycles were without any apparent effect on the profile. 相似文献
532.
A short and efficient synthesis of the antitumor diterpenoid (+)-zerumin B has been accomplished starting from (+)-sclareolide. At the heart of the synthetic strategy lies the regioselective formation of the alpha-substituted gamma-hydroxybutenolide moiety of zerumin B. This was achieved by means of a [1,4] O-->C triisopropylsilyl migration followed by singlet oxygen (1 O 2) oxidation of the resulting 2-triisopropylsilyl-3-(alpha-hydroxy)alkylfuran. 相似文献
533.
Transition metal dichalcogenides are rising candidates for the replacement of Pt catalysts in water splitting. In this theoretical study we focus on the hydrogen evolution reaction part of this process and on how hydrogen (H) interacts with MoS2 nanostructures, free‐standing or positioned on a graphene substrate. Density functional theory calculations confirm the stability of such nanostructures and our results for H on several configurations, from 2D infinite monolayers to quasi‐1D MoS2 ribbons and quasi‐0D MoS2 flakes, are presented. We calculate the adsorption energy of H atoms on various sites of the MoS2 nanostructures, notably at Mo and S active edges. Comparing free‐standing and MoS2/graphene hybrid systems we find that the effect of the support on the adsorption of H on MoS2 nanostructures is quite significant when the substrate induces strain. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) 相似文献
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535.
The stereochemistry of the allylic oxidation (ene reaction) mediated by singlet oxygen ((1)O2), using the optically active alkene (S,S)-cis-1,4-diphenyl-2-butene-1,4-d2 , in MeOH and aprotic solvents was investigated. Our findings indicate that the title reaction is a highly stereospecific suprafacial process, independent of solvent polarity. The observation of an isotope effect, which matches the stereogenic ratio exactly, rules out biradical or open dipolar intermediates. 相似文献
536.
Tsimachidis D Cesla P Hájek T Theodoridis G Jandera P 《Journal of separation science》2008,31(6-7):1130-1136
A new capillary electrophoretic method for the chiral separation of four major Cinchona alkaloids (quinine/quinidine and cinchonine/cinchonidine) was developed using heptakis-(2,6-di-O-methyl)-beta-cyclodextrin as the chiral selector. The inner walls of the separation capillary were modified with a thin polyacrylamide layer, which substantially reduced the electroosmotic flow and improved the chiral resolution and the reproducibility of the migration time of the analytes. Various operation parameters were optimised, including the pH, the capillary temperature, the concentration of the background electrolyte, and the concentration of the chiral selector. Baseline separation of the two diastereomer pairs was achieved in 12 minutes in ammonium acetate background electrolyte pH 5.0 with addition of cyclodextrin in a concentration of 3 mM or higher. 相似文献
537.
Exact Results and Bounds for the Joint Tail and Moments of the Recurrence Times in a Renewal Process
Losidis Sotirios Politis Konstadinos Psarrakos Georgios 《Methodology and Computing in Applied Probability》2021,23(4):1489-1505
Methodology and Computing in Applied Probability - The best known result about the joint distribution of the backward and forward recurrence times in a renewal process concerns the asymptotic... 相似文献
538.
D Kalaitzakis T Montagnon I Alexopoulou G Vassilikogiannakis 《Angewandte Chemie (International ed. in English)》2012,51(35):8868-8871
All in one: Meyers' bicyclic lactams were synthesized in high yield from furans using a new and powerful method that involves a one-pot singlet-oxygen-initiated reaction cascade (see scheme; TFA=trifluoroacetic acid). This method has broad synthetic potential because of the ease of access to a wide range of furans with a variety of substituents. 相似文献
539.
540.
Gika H Theodoridis G Mattivi F Vrhovsek U Pappa-Louisi A 《Journal of separation science》2012,35(3):376-383
The analysis of amino acids presents significant challenges to contemporary analytical separations. The present paper investigates the possibility of retention prediction in hydrophilic interaction chromatography (HILIC) gradient elution based on the analytical solution of the fundamental equation of the multilinear gradient elution derived for reversed‐phase systems. A simple linear dependence of the logarithm of the solute retention (ln k) upon the volume fraction of organic modifier (φ) in a binary aqueous‐organic mobile is adopted. Utility of the developed methodology was tested on the separation of a mixture of 21 amino acids carried out with 14 different gradient elution programs (from simple linear to multilinear and curved shaped) using ternary eluents in which a mixture of methanol and water (1:1, v/v) was the strong eluting member and acetonitrile was the weak solvent. Starting from at least two gradient runs, the prediction of solute retention obtained under all the rest gradients was excellent, even when curved gradient profiles were used. Development of such methodologies can be of great interest for a wide range of applications. 相似文献