Standardless energy-dispersive X-ray fluorescence analysis using primary radiation monochromatized with LiF(200) crystal

Energy-dispersive X-ray fluorescence spectrometer (EDXRF) with primary radiation monochromatized by LiF(200) crystal was developed. In the constructed spectrometer, the radiation from the Ag target X-ray tube operated at 50 kV and 40 mA excites the secondary target (Cu, Se, Zr or Mo). The characteristic radiation (Cu K_α, Se K_α, Zr K_α or Mo K_α) of the target is monochromatized with LiF(200) crystal and excites elements in the analyzed sample. The X-ray spectra are collected by thermoelectrically cooled Si-PIN detector with resolution of 145 eV at 5.9 keV. The pinhole collimator placed in front of the X-ray detector allows reducing size of the analyzed area. Quantitative analysis is performed using standardless fundamental parameters (FP) method. Because sample is excited using highly monochromatized radiation, the calculations are much simpler and analysis error resulting from uncertainty of X-ray tube spectral distribution is completely eliminated. Moreover, EDXRF system allows obtaining very low background and appropriate secondary target can be selected for the best excitation of the determined elements and to avoid overlapping of the analyte peaks with characteristic radiation originating from the secondary target. The FP calculations were verified using several certified reference materials of stainless steel. The spectrometer was used for nondestructive analysis of mono- and polycrystals of selenide spinels of general formula M_xN_yCr_zSe₄ (where M, N are Cu²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Ge²⁺, Ni²⁺, V³⁺, Sb³⁺, Ga³⁺). The results from EDXRF were compared with those obtained by means of the wavelength-dispersive X-ray fluorescence spectrometry (WDXRF).     

Influence of Leu5 configuration on the equilibrium constants of complexes of [Leu]-enkephalin with β-cyclodextrin studied by fluorescence spectroscopy, microcalorimetry and 1H NMR spectroscopy

Natural enkephalins and their analogues are very important as potential therapeutic agents (analgetics). In this paper we describe the influence of Leu chirality of cyclic [Leu]enkephalins on the binding constant with β-cyclodextrin and spatial and mutual orientation of guest and host molecules. The formation of complexes is enthalpy driven for both cyclic [Leu]enkephalins. Moreover, d-configuration of Leu residue causes an increase of the binding constant of cyclic enkephalin compared to l-analogue. An analysis of 2D NMR spectra reveals that, apart from inclusion complex formed by penetration of cyclodextrin cavity from wider and narrow rims by Trp or Leu residue, a side and/or bottom association complexes are formed.     

Luminescent Blooming of Dendronic Carbon Nanotubes through Ion‐Pairing Interactions with an EuIII Complex

A multiwalled carbon nanotube (MWCNT) scaffold was covalently functionalized with a second‐generation polyamidoamine (PAMAM) dendron, presenting four terminal amino groups per grafted aryl moiety. These reactive functions were alkylated to obtain a positively charged polycationic dendron/carbon nanotube system ( d‐MWCNTs?Cl ), which eventually underwent anion exchange reaction with a negatively charged and highly luminescent Eu^III complex ( [EuL₄]?NEt₄ , in which L =(2‐naphtoyltrifluoroacetonate)). This process afforded the target material d‐MWCNTs?[EuL₄] , in which MWCNTs are combined with red‐emitting Eu^III centers through electrostatic interactions with the dendronic branches. Characterization of the novel MWCNT materials was accomplished by means of TGA and TEM, whereas d‐MWCNTs?Cl and d‐MWCNTs? [EuL₄] further underwent XPS, SEM and Raman analyses. These studies demonstrate the integrity of the luminescent [EuL₄]^? center in the luminescent hybrid, the massive load of the cationic binding sites, and the virtually complete anion‐exchange into the final hybrid material. The occurrence of the ion‐pairing interaction with MWCNTs was unambiguously demonstrated through DOSY NMR diffusion studies. Photophysical investigations show that MWCNTs?[EuL₄] is a highly soluble and brightly luminescent red hybrid material in which MWCNTs act as photochemically inert scaffolds with negligible UV/Vis absorption, compared with the grafted Eu complex, and with no quenching activity. The high dispersibility of MWCNTs?[EuL₄] in a polymer matrix makes it a promising luminophore for applications in material science.     

Thermoelectric power of CuCrxVySe4 p-type spinel semiconductors

Structural, electrical and magnetic measurements of polycrystalline CuCr_xV_ySe₄ spinels with x=1.79, 1.64 and 1.49 and y=0.08, 0.22 and 0.45, respectively, are presented. The compounds under study crystallize in regular system of a normal spinel type MgAl₂O₄ structure with the space group symmetry Fd3m. The chromium spins are coupled ferromagnetically and show both strong long- and short-range magnetic interactions evidenced by the large values of the Curie (T_C) and Curie–Weiss (θ_CW) temperatures, decreasing from T_C=407 K and θ_CW=415 K for y=0.08, via T_C=349 K and θ_CW=367 K for y=0.22 to T_C=283 K and θ_CW=293 K for y=0.45, respectively. In all the studied spinels a change of the electrical conductivity character from the semiconductive into the metallic one above 230 K was observed. A detailed thermoelectric power analysis showed a domination of diffusion thermopower component, maximum of phonon drag component at 230 K, a decrease of impurity component with increasing V content, as well as the weak magnon excitations at 40 K.     

连续波锁模P_r~（3＋）：YLF激光器

本文报导了采用氩离子激光器来泵浦Ｒｒ￣（３＋）：ＹＬＦ晶体，应用声光调制器实现了主动锁模；同时应用振动─高反射平面镜也实现了被动锁模，两种锁模均得到了ｐｓ光脉冲。据作者了解这是这种晶体材料的第一次锁模运转。     

Thermal behaviour of riboflavin

From the thermal decomposition curves of riboflavin at various heating rates in static air atmosphere, the stages of thermal decomposition of this compound were established. The following kinetic parameters were calculated for the first decomposition stage of this process: activation energy (E _a ), reaction order (n), preexponential factor (A) and reaction rate constant (k). A mechanism of the changes is proposed.

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Zusammenfassung Die Stadien der thermischen Zersetzung von Riboflavin wurden aus den Kurven der thermischen Zersetzung in Luftatmosphäre bei verschiedenen Aufheizgeschwindigkeiten ermittelt. Für den ersten Zersetzungsschritt dieses Prozesses werden die Werte für die Aktivierungsenergie (E _a ), die Reaktionsordnung (n), den präexponentiellen Faktor (A) und die Reaktionsgeschwindigkeitskonstante (k) berechnet. Für die Umwandlungen wird ein Mechanismus vorgeschlagen.

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, , . , (E _a ), (n), (A) (k). .