A measuring system for the fast simultaneous isotope ratio and elemental analysis of carbon, hydrogen, nitrogen and sulfur in food commodities and other biological material

The isotope ratio of each of the light elements preserves individual information on the origin and history of organic natural compounds. Therefore, a multi-element isotope ratio analysis is the most efficient means for the origin and authenticity assignment of food, and also for the solution of various problems in ecology, archaeology and criminology. Due to the extraordinary relative abundances of the elements hydrogen, carbon, nitrogen and sulfur in some biological material and to the need for individual sample preparations for H and S, their isotope ratio determination currently requires at least three independent procedures and approximately 1 h of work. We present here a system for the integrated elemental and isotope ratio analysis of all four elements in one sample within 20 min. The system consists of an elemental analyser coupled to an isotope ratio mass spectrometer with an inlet system for four reference gases (N(2), CO(2), H(2) and SO(2)). The combustion gases are separated by reversible adsorption and determined by a thermoconductivity detector; H(2)O is reduced to H(2). The analyser is able to combust samples with up to 100 mg of organic material, sufficient to analyse samples with even unusual elemental ratios, in one run. A comparison of the isotope ratios of samples of water, fruit juices, cheese and ethanol from wine, analysed by the four-element analyser and by classical methods and systems, respectively, yielded excellent agreements. The sensitivity of the device for the isotope ratio measurement of C and N corresponds to that of other systems. It is less by a factor of four for H and by a factor of two for S, and the error ranges are identical to those of other systems.     

Mechanical Testing of Solid–Solid Interfaces at the Microscale

In order to determine the influence of internal interfaces on the material’s global mechanical behavior, the strength of single interfaces is of great interest. The experimental framework presented here enables quantitative measurements of the initiation and propagation of interfacial cracks on the microscale. Cantilever beams are fabricated by focused ion beam milling out of a bulk sample, with an interface of interest placed close to the fixed end of the cantilever. Additionally, a U-notch is fabricated at the location of the interface to serve as a stress concentrator for the initiation of the crack. The cantilevers are then mechanically deflected using a nanoindentation system for high resolution load-displacement measurements. In order to determine the onset and propagation of damage, the stiffness of the cantilevers is recorded by partial unloads during the test as well as by making use of a continuous stiffness technique. A finite element model is used to normalize the load and stiffness in order to establish the framework for comparisons between different interfaces.     

Intensity distribution in the vibronic Γ7(2 T 2g )→Γ8(4 A 2g ) transition of ReBr 6 2− in Hexahalogenostannate host crystals

The phosphorescence spectra of ReBr ₆ ^2– doped A₂SnX₆ (A = K, Rb, Cs; X = Cl, Br) have been measured at 10 K. The spectra consist of a weighted sum of progressions associated with the local modes of the ReBr ₆ ^2– center. By a fit to a generalized Lorentzian line shape function the totally symmetric distortion of the ₇(² T _2g ) excited state relative to the ₈(⁴ A _2g ) ground state has been determined.Dedicated to Professor Dr. H.-H. Schmidtke on the occasion of his 50th birthday.     

3He NMR: from free gas to its encapsulation in fullerene

The ³He nuclear magnetic shieldings were calculated for single helium atom, its dimer, simple models of fullerene cages (He@C_n), and single wall carbon nanotubes. The performances of several levels of theory (HF, MP2, DFT‐VSXC, CCSD, CCSD(T), and CCSDT) were tested. Two sets of polarization‐consistent basis sets were used (pcS‐n and aug‐pcS‐n), and an estimate of ³He nuclear magnetic shieldings in the complete basis set limit using a two‐parameter fit was established. Theoretical ³He results reproduced accurately previously reported theoretical values for helium gas, dimer, and helium probe inside several fullerene cages. Excellent agreement with experimental values was achieved. ³He nuclear magnetic shieldings of single helium atom approaching various points of benzene ring were tested, and an impact of ³He confinement within fullerene cages of different size on the ³He chemical shift was determined. Copyright © 2013 John Wiley & Sons, Ltd.     

Essential methodological improvements in the oxygen isotope ratio analysis of N‐containing organic compounds

The quantitative conversion of organically bound oxygen into CO, a prerequisite for the ¹⁸O/¹⁶O analysis of organic compounds, is generally performed by high‐temperature conversion in the presence of carbon at ～1450°C. Since this high‐temperature procedure demands complicated and expensive equipment, a lower temperature method that could be utilized on standard elemental analyzers was evaluated. By substituting glassy carbon with carbon black, the conversion temperature could be reduced to 1170°C. However, regardless of the temperature, N‐containing compounds yielded incorrect results, despite quantitative conversion of the bound oxygen into CO. We believe that the problems were partially caused by interfering gases produced by a secondary decomposition of N‐ and C‐containing polymers formed during the decomposition of the analyte. In order to overcome the interference, we replaced the gas chromatographic (GC) separation of CO and N₂ by reversible CO adsorption, yielding the possibility of collecting and purifying the CO more efficiently. After CO collection, the interfering gases were vented by means of a specific stream diverter, thus preventing them from entering the trap and the mass spectrometer. Simultaneously, a make‐up He flow was used to purge the gas‐specific trap before the desorption of the CO and its subsequent mass spectrometric analysis. Furthermore, the formation of interfering gases was reduced by the use of polyethylene as an additive for analytes with a N:O ratio greater than 1. These methodological modifications to the thermal conversion of N‐containing analytes, depending on their structure or O:N ratio, led to satisfactory results and showed that it was possible to optimize the conditions for their individual oxygen isotope ratio analysis, even at 1170°C. With these methodological modifications, correct and precise δ¹⁸O results were obtained on N‐containing analytes even at 1170°C. Differences from the expected standard values were below ±1‰ with standard deviations of the analysis <0.2‰. Copyright © 2010 John Wiley & Sons, Ltd.