February 2011 Archives

A German group recently developed an X-ray fluorescence imaging system with a pnCCD-based camera. They performed a test using a laboratory 30 μm microfocus X-ray tube and synchrotron radiation at the BAM beamline, BESSY II. It was found that the system simultaneously records ca. 70,000 spectra with an energy resolution of 152 eV (at Mn Kα) with a spatial resolution of 50 μm over a viewing area of 12.7 mm squared. For more information on pnCCD detectors, for example, the following Web page could be useful, http://www.pnsensor.de/Welcome/Detector/pn-CCD/index.html For more information on the whole system for X-ray fluorescence imaging, see the paper, "Compact pnCCD-Based X-ray Camera with High Spatial and Energy Resolution: A Color X-ray Camera", O. Scharf et al., Anal. Chem., 83, 2532 (2011).

Recently, a European international research group led by Professor K. Janssens (Antwerp University, Belgium) has succeeded in solving the scientific mechanism of color darkening in the paintings of Vincent van Gogh. Some readers may remember a previous news article, "Synchrotron XRF revealed Van Gogh's hidden painting", No.5, Vol. 37 (2008), which explained how synchrotron X-ray spectroscopy and imaging are powerful tools in the analysis of such paintings. In the present work, the research group discusses the change in color from yellow to dark brown in two Van Gogh paintings, Bank of the Seine (1887) and View of Arles with Irises (1888). They also systematically studied the aging process of artificial samples using pigments. The chrome yellow pigment is chemically lead chromate (PbCrO₄), which may include some amount of PbSO₄ and/or PbO. During their research based on X-ray micro-spectroscopy, it was found that part of the material is transformed into hydrated chromium oxide (Cr₂O₃・2H₂O), which is known as viridian, i.e., a blue-green pigment under sunlight or UV light irradiation. They also noted the formation of other Cr(III) compounds. Their conclusion was that the color change is due to the reduction from Cr(VI) to Cr(III) on the surface of the paintings, and the formation of a thin layer containing Cr(III). This would be the reason for the brownish color. Most of the experiments were done at beamline ID21 at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). For more information, see the papers, "Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy and Related Methods. 1. Artificially Aged Model Samples" and "2. Original Paint Layer Samples", L. Monico et al., Anal. Chem., 83 1214-1231 (2011).

A Japanese research group led by Professors J. Kawai (Kyoto University) and T. Yamamoto (Tokushima University) has recently published a series of high-resolution X-ray fluorescence spectra for supported vanadium oxide catalysts. The measurement was done with a double crystal spectrometer (Si 220 reflections with (+, +) arrangement), and the typical energy resolution was around 2.5 eV. The authors were successful in discussing quantitatively the difference in chemical states among the catalysts supported on the different oxides, amorphous SiO₂, γ-Al₂O₃, and TiO₂ (anatase/rutile = 7/3). For more information, see the paper, "Quantitative Chemical State Analysis of Supported Vanadium Oxide Catalysts by High Resolution Vanadium Kα Spectroscopy", T. Yamamoto et al., Anal. Chem., Article ASAP (DOI: 10.1021/ac102681z Publication Date (Web): February 8, 2011).

Dr. J. M. Fernandez-Varea (Universitat de Barcelona, Spain) and his colleagues have recently studied the emission of Lα, Lβ, and Lγ characteristic X-rays by the impact of electrons on Hf, Ta, W, Re, Os, Au, Pb, and Bi atoms. They calculated the ionization cross sections of the LI, LII, and LIII subshells of these atoms within the distorted-wave Born approximation, and compared them with the published experimental data. For more information, see the paper, "Lα, Lβ, and Lγ x-ray production cross sections of Hf, Ta, W, Re, Os, Au, Pb, and Bi by electron impact: Comparison of distorted-wave calculations with experiment", J. M. Fernandez-Varea et al., Phys. Rev. A83, 022702 (2011).

Two very exiting experimental reports have been published on the application of an X-ray free electron laser (XFEL) at Linac Coherent Light Source (LCLS, Stanford, USA). An international research team led by Dr. H. Chapman (DESY, Hamburg, Germany) and Professor J. Hajdu (Uppsala University, Sweden) has demonstrated a new advanced stage of protein crystallography, which uses only tiny proteins instead of preparing large-size crystals. This could open up new possibilities for the analysis of proteins that have been difficult or even impossible to prepare so far. The technique has been basically known as coherent X-ray diffraction imaging. The present research is the first experimental application of extremely brilliant femtosecond XFEL pulses. In addition to the demonstration of snapshots of nano-crystalline proteins, they have reported the first single-shot images of intact viruses. For more information, see the papers, "Femtosecond X-ray protein nanocrystallography", H. N. Chapman et al., Nature, 470, 73 (2011) and "Single mimivirus particles intercepted and imaged with an X-ray laser", M. M. Seibert et al., Nature, 470, 78 (2011).

Scientists in Japan have been using two synchrotrons, the SPring-8 and the Photon Factory, to analyze the dust particles collected by the HAYABUSA Asteroid probe, which returned from Asteroid Itokawa on June 13, 2010. HAYABUSA, which means "Falcon" in Japanese, was launched from the Uchinoura Space Center in Japan on May 9, 2003, and arrived at Itokawa in September 2005. The HAYABUSA particles were initially analyzed using electron microscopes, and then forwarded to the above synchrotron facilities in January 2011. Many interesting 3D images were collected at BL20XU, SPring-8, and the structure and chemical compositions were also analyzed at BL-3A, Photon Factory, KEK. For more information on the HAYABUSA project, visit the web page of the Japan Aerospace Exploration Agency (JAXA), http://www.isas.jaxa.jp/e/enterp/missions/hayabusa/index.shtml

One of hottest topics related to the application of an X-ray free electron laser (XFEL) is how to determine the structure of non-crystalline membrane proteins. There has been a clear conflict between the incident brightness required to achieve diffraction-limited atomic resolution and the electronic and structural damage induced by such illumination. Professors K. A. Nugent and H. M. Quiney (ARC Centre of Excellence for Coherent X-ray Science, University of Melbourne, Victoria, Australia) have recently published their theoretical research on this problem. They have improved the imaging model by using optical coherence theory and quantum electrodynamics, and concluded that the analysis is far more tolerant of electronic damage than believed so far. For more information, see the paper, "Biomolecular imaging and electronic damage using X-ray free-electron lasers", H. M. Quiney et al., Nature Physics, 7, 142 (2011).