Dichtebestimmungen

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X-ray magneto-optics of lanthanide materials: principles and applications

Lanthanide metals are a particular class of magnetic materials in which the magnetic moments are carried mainly by the localized electrons of the 4f shell. They are frequently found in technically relevant systems, to achieve, e.g., high magnetic anisotropy. Magneto-optical methods in the X-ray range are well suited to study complex magnetic materials in an element-specific way. In this work, we report on recent progress on the quantitative determination of magneto-optical constants of several lanthanides in the soft X-ray region and we show some examples of applications of magneto-optics to hard-magnetic interfaces and exchange-coupled layered structures containing lanthanide elements. PACS 78.20.Ls; 78.70.Dm; 78.70.Ck; 78.20.Ci; 75.70.-i     

Near-infrared reflection spectroscopy and partial least squares regression for determining the total carbon coverage of silica packings for liquid chromatography

Near-infrared reflection spectroscopy (NIRS) was used in combination with principal component analysis (PCA) and partial least squares (PLS) regression to determine the silica packing properties, whether endcapped (EC) or non-endcapped, and the amount of surface total carbon coverage (%). A preparation technique for the reproducible analysis of bulk materials is covered as well as spectral data pretreatments to enhance prediction accuracy of the PLS models. Especially derivatives and scatter correction methods turned out to be well suited. A standard error of prediction (SEP) of 0.57% C for the determination of the total carbon load of octadecylated (C₁₈) silica was found. In case of the C₁₈-EC silica a SEP of 0.51% carbon gives rise to a robust and accurate model. The presented method allows the simultaneous determination of several parameters, e.g. particle and pore size or surface modifications, from a single spectrum and is amenable to implementation for in-line or on-line analysis in the silica producing industry.     

Zur Verteilung der Wurzeln algebraischer Gleichungen

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Über die Abschätzung der Wurzeln algebraischer Gleichungen

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Fast, noninvasive and simultaneous near-infrared spectroscopic characterisation of physicochemical stationary phases' properties: from silica particles towards monoliths

The design of novel stationary phases is a permanent demanding challenge in chromatographic separation science to enable analysis with enhanced selectivity, specificity and speed. Therefore, the characterisation of chemical and physical properties is next to calculation of chromatographic parameters essential. Conventionally, chemical parameters including surface coverage are determined by burning combustion or frontal analysis, physical parameters including particle size, pore size, pore volume and surface area are determined by SEM, mercury intrusion porosimetry (MIP) and Brunauer-Emmett-Teller (BET). All these methods are time consuming, invasive and require besides special equipment some special trained laboratory staff. Therefore, we introduced near-infrared spectroscopy (NIRS) as a noninvasive, easy-to-handle technology with wavenumber ranging from 4000 to 10,000 cm(-1) enabling analysis within only a few seconds at higher precision than the conventional methods. Investigated materials comprise porous and nonporous silica gel, carbon-based nanomaterials (fullerenes), polymer beads and monoliths. Different carriers themselves and their kind of derivatisations (RP, normal-phase, ion-exchanger, IMAC (immobilised metal affinity chromatography), affinity) can be determined by applying principal component analysis (PCA) of recorded spectra. Partial least square regression (PLSR) enables the determination of particle size, pore size, pore volume, porosity, total porosity and surface area with one single measurement. For the optimised design of well-defined polymer beads and monoliths, real-time in situ monitoring to control, e. g. particle and pore sizes as well as monomer content during the polymerisation process, can be extremely helpful. In this article, the advantages of this fast, noninvasive high-throughput NIRS methods are summarised, discussed in detail and different applications of the individual characterised materials are shown.     

Synthesis and synchrotron light-induced luminescence of ZnO nanostructures: nanowires, nanoneedles, nanoflowers, and tubular whiskers

ZnO nanostructures, including single-crystal nanowires, nanoneedles, nanoflowers, and tubular whiskers, have been fabricated at a modestly low temperature of 550 degrees C via the oxidation of metallic Zn powder without a metal catalyst. Specific ZnO nanostructures can be obtained at a specific temperature zone in the furnace depending on the temperature and the pressure of oxygen. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) studies show that ZnO nanostructures thus prepared are single crystals with a wurtzite structure. X-ray excited optical luminescence (XEOL) from the ZnO nanostructures show noticeable morphology-dependent luminescence. Specifically, ZnO nanowires of around 15 nm in diameter emit the strongest green light. The morphology of these nanostructures, their XEOL, and the implication of the results will be discussed.     

The effect of the surface of SnO2 nanoribbons on their luminescence using x-ray absorption and luminescence spectroscopy

X-ray excited optical luminescence (XEOL) and x-ray absorption near-edge structure in total electron, x-ray fluorescence, and photoluminescence yields at Sn M5,4-, O K-, and Sn K-edges have been used to study the luminescence from SnO2 nanoribbons. The effect of the surface on the luminescence from SnO2 nanoribbons was studied by preferential excitation of the ions in the near-surface region and at the normal lattice positions, respectively. No noticeable change of luminescence from SnO2 nanoribbons was observed if the Sn ions in the near-surface region were excited selectively, while the luminescence intensity changes markedly when Sn or O ions at the normal lattice positions were excited across the corresponding edges. Based on the experimental results, we show that the luminescence from SnO2 nanoribbons is dominated by energy transfer from the excitation of the whole SnO2 lattice to the surface states. Surface site specificity is not observable due to its low concentration and weak absorption coefficient although the surface plays an important role in the emission as a luminescence center. The energy transfer and site specificity of the XEOL or the lack of the site specificity from a single-phase sample is discussed.