Site-specific C content by quantitative isotopic C Nuclear Magnetic Resonance spectrometry: A pilot inter-laboratory study

Isotopic ¹³C NMR spectrometry, which is able to measure intra-molecular ¹³C composition, is of emerging demand because of the new information provided by the ¹³C site-specific content of a given molecule. A systematic evaluation of instrumental behaviour is of importance to envisage isotopic ¹³C NMR as a routine tool. This paper describes the first collaborative study of intra-molecular ¹³C composition by NMR. The main goals of the ring test were to establish intra- and inter-variability of the spectrometer response. Eight instruments with different configuration were retained for the exercise on the basis of a qualification test. Reproducibility at the natural abundance of isotopic ¹³C NMR was then assessed on vanillin from three different origins associated with specific δ¹³C_i profiles. The standard deviation was, on average, between 0.9 and 1.2‰ for intra-variability. The highest standard deviation for inter-variability was 2.1‰. This is significantly higher than the internal precision but could be considered good in respect of a first ring test on a new analytical method. The standard deviation of δ¹³C_i in vanillin was not homogeneous over the eight carbons, with no trend either for the carbon position or for the configuration of the spectrometer. However, since the repeatability for each instrument was satisfactory, correction factors for each carbon in vanillin could be calculated to harmonize the results.     

Functionalized Graphene as a Gatekeeper for Chiral Molecules: An Alternative Concept for Chiral Separation

We propose a new method of chiral separation using functionalized nanoporous graphene as an example. Computational simulations based on density functional theory show that the attachment of a suitable chiral “bouncer” molecule to the pore rim prevents the passage of the undesired enantiomer while letting its mirror image through.     

Desorption electrospray ionization mass spectrometry: direct toxicological screening and analysis of illicit Ecstasy tablets

Desorption electrospray ionization mass spectrometry (DESI-MS) was used as a simple and rapid way to analyze drug tablets and powders without sample preparation. Experiments were performed with a home-made DESI source coupled to a triple-quadrupole linear-ion trap (QqQ(LIT)) mass spectrometer. Twenty-one commercial drugs as well as some illicit Ecstasy tablets and powders were analyzed. MS spectra almost exclusively showed the protonated or deprotonated ion of the drug after directing the pneumatically assisted electrospray onto the tablet's surface. With some tablets, inhomogeneity of the surface resulted in different spectra depending on the spot analyzed, thus showing that DESI could be used for imaging. Directly triggered MS/MS spectra were used for confirmatory analysis, with analysis times often below 10 s per tablet. For illicit Ecstasy tablets, DESI-MS, GC/MS and LC/MS analyses provided similar qualitative results for the main analytes. With MS/MS spectra library comparison or exact mass measurements, this technique could become very powerful for the rapid analysis of unknown tablets and shows the great potential of desorption techniques as an alternative to solution-based analysis.     

Adsorption of the first row of transition metals on the perfect and defective MgO(1 0 0) surface

We present DFT calculations for adsorption of the first row of transition metal atoms on a MgO(1 0 0) surface and on a surface exhibiting defects. Some atoms exhibit a high adsorption energy on the defect (e.g. Co, Ni and Cu), but others (Ca, Sc) rather adsorb on a clean surface and another set is indifferent to the presence of defect. The adsorption becomes energetically unfavorable when the σ anti-bonding orbitals become populated; this is worse on a defective surface than on a terrace. The π back-donation to the surface contributes to favor the adsorption on the center.     

Development and validation of an LC–MS method for the simultaneous determination of sulfadiazine,trimethoprim, and N4-acetyl-sulfadiazine in muscle plus skin of cultured fish

An analytical method for the determination of both sulfadiazine (SDZ) and trimethoprim (TMP), and also N⁴-acetyl-sulfadiazine (AcSDZ), the main metabolite of SDZ, in fish muscle plus skin has been developed and validated. Dapsone was used as internal standard. The method involves extraction of the analytes from fish tissue by pressurized liquid extraction using water as extractant. Sample cleanup was carried out by solid phase extraction using Abselut Nexus cartridges. Target analytes were quantitatively determined by liquid–chromatography mass spectrometry using single ion monitoring. The developed method was validated according to the European Union requirements (decision 2002/657/EC). The limit of detection for SDZ and AcSDZ was 3.0 and 2.5 µg kg^?1 for TMP. The limit of quantification (LOQ) was 10 µg kg^?1 for SDZ and AcSDZ and 7.5 µg kg^?1 for TMP. The recovery experiments carried out included the concentration levels of 0.5, 1 and 1.5 times the MRLs for SDZ and TMP. Concentration levels for AcSDZ were the same as SDZ. The values obtained were higher than 92.0% with coefficient of variation (CV, %) below 8.6%. The precision of the method, calculated as CV (%), ranged from 0.2 to 6.8% and from 0.8 to 8.9% for intra–day and inter–day analysis, respectively. Decision limit (CC_α) was calculated as 104.3, 53.7 and 105.3 µg kg^?1 for SDZ, TMP and AcSDZ, respectively. Detection capability (CC_β) was calculated as 110.0, 58.8 and 109.7 µg kg^?1 for SDZ, TMP and AcSDZ, respectively. “Matrix effect” and “relative matrix effect” were also evaluated. The method was used for the analysis of fish samples purchased from local markets.     

On Thermodynamical Relation Between Rotating Charged BTZ Black Holes and Effective String Theory

In this paper we study the first law of thermodynamics for the （2＋1）-dimensional rotating charged BTZ black hole considering a pair of thermodynamical systems constructed with the two horizons of this solution. We show that these two systems are similar to the right and left movers of string theory and that the temperature associated with the black hole is the harmonic mean of the temperatures associated with these two systems.     

Quantum electronic transport: Linear and nonlinear conductance from the Keldysh approach

This is a review of the derivation of the Landauer conductance using the Keldysh non-equilibrium Green's function (NEGF) formalism and the equations-of-motion (EOM) method. We consider the elastic quantum electronic transport through a multi-lead device and treat the conductor in the mean-field approximation. This is suitable for open quantum dots as well as for several molecular systems where charging effects are negligible. The focus of the presentation is to unveil the technical issues involved in the formalism. We show how the Landauer conductance emerges as a linear term in the current-voltage I-V characteristics and indicate how to go beyond this regime. We address the connection of the NEGF approach to recent developments in molecular transport and discuss the problems that arise when one tries to include interaction effects beyond the mean field.