Investigating the impact of internal standard heterogeneity on gold quantification with total reflection X-ray fluorescence: A simulation study

Total reflection X-ray Fluorescence (TXRF) is a powerful analytical tool with high detection sensitivity that has been applied to a variety of biological samples. While its ability to quantify gold nanoparticles (AuNPs) in cancer cells has been demonstrated, the extension to tissue slices would be of interest. To that end, the preservation of the underlying tissue microstructure requires samples to be measured as microtome slices. In this form, internal standard spiking is warranted. Thus, it is important to examine the impact of sample heterogeneity on the TXRF's quantification accuracy. To address these questions, a TXRF spectrometer along with 5 μm thin heterogeneous and homogeneous samples were modeled using TOPAS. The simulation model generated TXRF spectra which were then analyzed to obtain recovery rates of Au in both sample types. The results showed near 100% recovery regardless of the elemental spatial distribution in the samples. This provides insights into the quantification potential for AuNPs inside tumors that are histologically processed into thin tissue slices. In addition, this simulation toolkit provides the first practical means of modeling TXRF spectroscopy which will hopefully be of use to the TXRF community.     

A total reflection X‐ray fluorescence method for the determination of chlorine at trace levels in nuclear materials without sample dissolution

A total reflection X‐ray fluorescence (TXRF) method for the determination of chlorine at trace levels in nuclear fuel samples is described. Chlorine present in trace concentrations in nuclear fuel materials such as U₃O₈, (U,Pu)C, PuO₂ and Pu‐alloys was first separated from the solid matrix by pyrohydrolysis as HCl and was collected in 5 mM NaOH solution. This solution was analyzed for chlorine by TXRF spectrometry using Cl Kα analytical line excited by W Lα. Cobalt was used as internal standard. The precision for such chlorine determination was found to be within 27% (n = 4) when the analysis was carried out in air atmosphere. This could be improved to 8% by making TXRF measurement in flowing helium atmosphere. The results obtained from TXRF determinations were also compared with those obtained from ion chromatography (IC) and were in good agreement. The collection of distillate during pyrohydrolysis in NaOH helped in counterchecking loss of chlorine during TXRF sample preparation. The average deviation of TXRF‐determined values in helium‐purged TXRF measurements with IC determined values (as chloride) was 15% at a chlorine concentration level in the range of 1–70 µg/mL. Copyright © 2012 John Wiley & Sons, Ltd.     

Assessment of essential micronutrient levels in common beans (Phaseolus vulgaris) in Kenya by total reflection X-ray fluorescence

In Kenya, where malnutrition and hidden hunger still pose a significant challenge to growth and development, beans are an essential source of food for most of the population. However, data on micronutrient in beans are limited, and although they are required in very small amounts, they are essential for growth and development of plants as well as human beings. The aim of this study was to determine the concentrations of Mn, Fe, Cu, and Zn, in common beans in Kenya. Samples of both fresh bean leaves and dry grains of the most common bean varieties were collected from small-scale farmers in Muguga and Kyevaluki in Kiambu and Machakos Counties, respectively. They were analyzed using total reflection x-ray fluorescence (TXRF), a technique of increasing interest for food analysis since it is fast, easy, and reliable. Standard methods of sample preparation were used, and average elemental concentrations were compared with established sufficiency ranges. Bean leaves and dry bean grains from both sampling areas had sufficient concentrations of the four analyzed elements. Except for the Fe concentrations, the concentrations of the others elements were in the lower end of the sufficiency range for all bean species. The results obtained from this study are essential information for both farmers and policy makers and can be incorporated in programs to guide policy aimed at improving the nutritional quality of beans and thus food security in Kenya.     

A time-resolved total internal reflection aqueous fluorescence (TIRAF) microscope for the investigation of cell adhesion dynamics

To investigate dynamic processes of cell adhesion on smooth and structured surfaces we developed a time-resolved TIRAF microscope with high spatial and temporal resolution. A lateral image resolution, close to the resolution limit of 223 nm, is reached using a variable microchannel with maximum height of 210 μm for cell cultivation in combination with a ×60 oil immersion objective of numerical aperture 1.4. Using Rhodamine Green as aqueous volume marker, no significant cell membrane damage occurs within 100 min under continuous irradiation in our experimental conditions. In time-discrete experiments with L929 cells over a period of 11 h we observed interactions of adhesion areas with the surface to which the cell adhered. Short-term measurements of a defined area of a single cell containing only a few adhesion contacts showed interactions between single adhesion clusters and also putative formation of new clusters. Stationary TIRAF experiments with cells growing on structured surfaces or between microelectrodes are possible without producing additional reflections. We conclude from our first results that time-resolved TIRAF microscopy is a powerful technique for studying the mechanism of cell motion, especially under a variety of artificial conditions used to influence cell migration. Received: 21 December 1998 / Accepted: 6 January 1999 / Published online: 7 April 1999     

Stacking order and disorder in layered K3Me(CN)6 compounds studied by diffuse X-ray scattering: A realization of the ANNNI model?

Layered crystalline materials like K₃Me(CN)₆ with Me=Cr, Mn, Fe, Co may often exist in various polytypic forms, due to a variety of choices of layer stacking modes. For cases where the interlayer constellations can be limited to only two energetically almost equivalent ways, the buildup of the crystal may be described by a spin-1/2 Ising-like model. For the system presently being studied one can rationalize the layer stacking to a four-valued choice (i.e., a 1D 4-state Potts case), or use an Ising-like two-sublattice model. Previous diffraction studies of K₃Me(CN)₆ indicated that two long-range ordered structures prevailed, an orthohombic one named MDO₁, with one double layer per repetition unit, and a monoclinic one, MDO₂, with two double-layer units. Our studies reveal a more complex situation: The Fe material is for the most part of the MDO₂ type. But in addition, in some crystal samples, a hitherto unobserved phase also appears, with six double-layer repetition units, in fact a hybrid of MDO₁ and MDO₂. The Co material is for the most part of the MDO₂ type, but contains in addition a considerable contribution of stacking disorder, as evidenced by the presence of diffuse X-ray scattering lines. The lines do, however, contain distinct maxima, indicating the presence of several layer stacking modes with preference of two, three, four, five, and seven double-layer correlations. The findings can be qualitatively discussed in terms of the ANNNI model.     

A study of barium ultra-thin films on the SrTiO3(1 0 0) surface by soft X-ray photoelectron spectroscopy

The growth and the electronic properties of Ba ultra-thin films (coverage ≤ 2 ML) on the SrTiO₃(1 0 0) surface was studied using synchrotron radiation facilities. The investigation was carried out mainly by soft X-ray photoelectron spectroscopy measurements at low core levels and the valence band region. The results show that the Ba overlayer develops in a layer-by-layer mode. The first two layers do not create any states in the band gap, thus leaving intact the insulating character of the substrate. This is due to the oxidation of Ba through transfer of O²⁻ anions from the substrate into the Ba overlayer. It is observed that the valency of the Ti atoms at the interface does not change. This is attributed to an outwards diffusion of O²⁻ anions from deeper layers promoted by substrate annealing. Within the coverage examined, no evidence for Ba metallization is found, contrary to what happens on metallic substrates. Comparing the results of this work with previous ones on similar metal–SrTiO₃(1 0 0) adsorption systems, it is concluded that a single factor such as charge transfer or thermodynamic stability is not always sufficient to determine the oxidation process of the adsorbate layer. Kinetics on surfaces as well as substrate doping and defects can also play a decisive role.     

Structural study by X-ray diffraction and Mössbauer spectroscopy of a new synthetic iron phosphate: K4MgFe3(PO4)5

A new iron phosphate K₄MgFe₃(PO₄)₅ has been synthesized by the flux method and characterized by single-crystal X-ray diffraction and Mössbauer spectroscopy. It crystallizes in the tetragonal system with the space group and the unit cell parameters a=9.714(3) Å and c=9.494(5) Å. The crystal structure is of a new type. It exhibits a three-dimensional framework built up from corner-sharing MO₅ (M=0.75Fe+0.25Mg) trigonal bipyramids and PO₄ tetrahedra. The K⁺ ions are occupying large eight-sided tunnels running along c. A room temperature Mössbauer study confirmed the +3 valence state of iron and its five-coordination.     

Atom geometry of nanostructured Fe films grown on c(2 × 2)-N/Cu(1 0 0) surface: An investigation by X-ray absorption spectroscopy with multishell analysis

We investigated the growth of Fe nanostructured films on c(2 × 2)-N/Cu(1 0 0) surface with Fe K-edge X-ray absorption fine structure (XAFS) in the near edge and in the extended energy region. The high photon flux of the incident X-rays allowed us to perform multishell analysis of the XAFS oscillations for Fe coverage Θ_Fe < 1 ML. This data analysis yields a detailed investigation of the atom geometry and some insights in the film morphology. At Θ_N < 0.5 ML (N saturation coverage) there is absence of contribution to XAFS from N atoms. First shell analysis of linearly polarized XAFS gives Fe-Fe (or Fe-Cu) bond length values varying between R₁ = 2.526 ± 0.006 Å at the highest Fe coverage (3 ML ) and R₁ = 2.58 ± 0.01 Å at Θ_Fe = 0.5 ML, Θ_N = 0.3 ML, with incidence angle Θ = 35°. These values are different from the case of bcc Fe (R = 2.48 Å), and compatible with fcc Fe (R₁ = 2.52 Å) and fcc Cu (R₁ = 2.55 Å). At the Fe lowest coverage (Θ_Fe = 0.5 ML) the dependence of R₁ on the incidence angle indicates expansion of the outmost layer. Near edge spectra and multishell analysis can be well reproduced by fcc geometry with high degree of static disorder. At N saturation pre-coverage (Θ_N = 0.5 ML) the XAFS analysis has to keep into account the Fe-N bonding. The results suggest two different adsorption sites: one with Fe in a fcc hollow site, surrounded by other metal atoms as nearest neighbours, and one resulting from an exchange with a Cu atom underneath the N layer.     

Fe/ZnO (0001)体系界面相互作用中薄膜厚度效应的光电子能谱研究

利用同步辐射光电子能谱(SRPES)和X射线光电子能谱(XPS)技术,系统研究了室温下Fe/ZnO界面形成过程中Fe薄膜与氧结尾的ZnO(000 1 )衬底之间的相互作用,结果显示初始沉积的Fe明显被表面氧氧化为Fe²⁺离子,在Fe覆盖度为0—3 nm的范围内,分别观察到与界面电荷传输、化学反应以及薄膜磁性相关的三个有意义的临界厚度,这一结果将有助于基于Fe/ZnO界面的相关器件的设计和研发.     

A potential green-emitting phosphor Ca8Mg(SiO4)4Cl2:Eu for white light emitting diodes prepared by sol-gel method

A potential green emitting phosphor Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺ was prepared by modified sol-gel method. The factors those affect the photoluminescence intensity including heating temperature, the usage of the chlorine source CaCl₂ and the concentration of dopant Eu²⁺ were also investigated in detail. As comparison, the phosphor prepared by solid-state reaction was also prepared. The phosphors show intense absorption in the range of 375-450 nm, which makes it a potential candidate of green emitting phosphor used for near-UV or blue light excited white LEDs.     

Thermal decomposition of Mo(CO)6 on thin Al2O3 film: A combinatorial investigation by XPS and UPS

A thin and homogeneous alumina film was prepared by deposition and oxidation of aluminum on a refractory Re(0 0 0 1) substrate under ultrahigh vacuum conditions. X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron-energy-loss spectroscopy (HREELS) demonstrate that the oxide film is long-range ordered, essentially stoichiometric and free from surface hydroxyl groups. The chemisorption and thermal decomposition of Mo(CO)₆ on the Al₂O₃ film were investigated by means of XPS and UPS. Mo(CO)₆ adsorbs molecularly on the oxide film at 100 K; however, thermal decomposition of the adsorbate occurs upon annealing at high temperatures. Consequently the metallic molybdenum clusters are deposited on the thin alumina film via complete decarbonylation of Mo(CO)₆.     

Identification of the vanadyl terminated V2O3(0 0 0 1) surface by NEXAFS spectroscopy: A combined theoretical and experimental study

In this work we present results from density functional theory (DFT) cluster studies to determine polarization-dependent near edge X-ray absorption fine structure (NEXAFS) spectra of the vanadyl termination of the V₂O₃(0 0 0 1) surface. The oxygen K edge spectra are calculated for the relaxed surface geometry where geometric parameters are taken from recent periodic DFT work. A detailed analysis of energetic peak positions, relative intensities, and final state orbitals allows a deep understanding of the complex angular dependence of the calculated spectra on the basis of the local binding environment of differently coordinated oxygen species. Further, our theoretical analysis can assign and explain various spectral details in the experimental NEXAFS data, in particular, those related to vanadyl oxygen. This allows us to support the experimentally suggested vanadyl surface termination.     

An example of chemistry-morphology interaction: making up for the geometric and energetic heterogeneities of the (1 0 0) surface of single crystalline silicon by high-temperature treatments in H2

A lot of work has been carried out to prepare chemically homogeneous (1 0 0) silicon surfaces. The hydrogen-terminated (1 0 0) silicon surfaces are the most promising ones, especially in view of their remarkable environmental stability. The simplest way to produce hydrogen-terminated surfaces (attack in water solution of HF of a sacrificial, thermally grown, oxide) results in strongly heterogeneous rough surfaces (although with prevailing dihydride terminations). These surfaces can, however, be flattened and homogenized by treating them in H₂ at high temperature (>850 C). The morphological and chemical changes undergone by the surface during the treatment are studied X-ray photoelectron spectroscopy, atomic force microscopy, scanning tunnelling microscopy, infrared absorption spectroscopy in the attenuated total reflection mode, reflection high energy electron diffraction and thermal programmed desorption, and the mechanisms responsible for them are discussed.     

The 2 × 1 reconstruction of the rutile TiO2(0 1 1) surface: A combined density functional theory, X-ray diffraction, and scanning tunneling microscopy study

An extensive search for possible structural models of the (2 × 1)-reconstructed rutile TiO₂(0 1 1) surface was carried out by means of density functional theory (DFT) calculations. A number of models were identified that have much lower surface energies than the previously-proposed ‘titanyl’ and ‘microfaceting’ models. These new structures were tested with surface X-ray diffraction (SXRD) and voltage-dependent STM measurements. The model that is (by far) energetically most stable shows also the best agreement with SXRD data. Calculated STM images agree with the experimental ones for appropriate tunneling conditions. In contrast to previously-proposed models, this structure is not of missing-row type; because of its similarity to the fully optimized brookite TiO₂(0 0 1) surface, we call it the ‘brookite (0 0 1)-like’ model. The new surface structure exhibits two different types of undercoordinated oxygen and titanium atoms, and is, in its stoichiometric form, predicted to be rather inert towards the adsorption of probe molecules.     

Tetrahedral coordination for Zn in hexacyanometallates: Structures of Zn3A2[M(CN)6]2·xH2O with A=K, Rb, Cs and M=Ru, Os

Zinc hexacyanoruthenate (II) and hexacyanoosmate (II) were prepared and studied from X-ray diffraction (XRD), infrared (IR), and thermogravimetric (TG) data. These compounds were found to be isomorphous with the iron analogues, crystallizing with a rhombohedral unit cell (R−3c space group), where the zinc atom has tetrahedral coordination to N ends of CN groups. For Cs, compounds with formula unit ZnCs₂[M(CN)₆] and a cubic unit cell (Fm−3m) were also obtained. The crystal structures for the eight compositions were refined from the corresponding X-ray powder diffraction patterns using the Rietveld method. Related to the tetrahedral coordination for the Zn atom, the rhombohedral phase has a porous framework with ellipsoidal cavities of about 12.5×9×8 Å, communicated by elliptical windows of ∼5 Å. Within these cavities the exchangeable alkali metal ions are found. The filling of the cavity volume is completed with water molecules. IR spectrum senses certain charge delocalization from the inner metal, through the π-back donation mechanism. For Os compounds this effect is particularly pronounced, related to a more diffuse d orbitals for this metal.