Elemental and chemical-state imaging using synchrotron radiation

Robust framework and software implementation for fast speciation mapping

One of the greatest benefits of synchrotron radiation is the ability to perform chemical speciation analysis through X-ray absorption spectroscopies (XAS). XAS imaging of large sample areas can be performed with either full-field or raster-scanning modalities. A common practice to reduce acquisition time while decreasing dose and/or increasing spatial resolution is to compare X-ray fluorescence images collected at a few diagnostic energies. Several authors have used different multivariate data processing strategies to establish speciation maps. In this manuscript, the theoretical aspects and assumptions that are often made in the analysis of these datasets are focused on. A robust framework is developed to perform speciation mapping in large bulk samples at high spatial resolution by comparison with known references. Two fully operational software implementations are provided: a user-friendly implementation within the MicroAnalysis Toolkit software, and a dedicated script developed under the R environment. The procedure is exemplified through the study of a cross section of a typical fossil specimen. The algorithm provides accurate speciation and concentration mapping while decreasing the data collection time by typically two or three orders of magnitude compared with the collection of whole spectra at each pixel. Whereas acquisition of spectral datacubes on large areas leads to very high irradiation times and doses, which can considerably lengthen experiments and generate significant alteration of radiation-sensitive materials, this sparse excitation energy procedure brings the total irradiation dose greatly below radiation damage thresholds identified in previous studies. This approach is particularly adapted to the chemical study of heterogeneous radiation-sensitive samples encountered in environmental, material, and life sciences.

Development of XANES nanoscopy on BL7C at PLS-II

X-ray absorption near-edge structure (XANES) imaging is a powerful tool to visualize the chemical state distribution of transition-metal-based materials at synchrotron radiation facilities. In recent years, the electrochemical working rechargeable battery has been the most studied material in XANES imaging owing to the large increase of portable electronics and electric vehicles. This work acknowledges the importance of battery analysis and has developed the XANES imaging system on BL7C at Pohang Light Source-II (PLS-II). BL7C employs an undulator taper configuration to obtain an energy band >130 eV near the K-absorption edge of the target element with a minimum energy interval >0.2 eV. While measuring energy-dependent images, the zone plate translation maintains the best focus, and then various data processes such as background correction, image registration and clustering allow single XANES spectrum extraction and chemical distribution mapping. Here, the XANES imaging process is described, the XANES spectrum quality is identified and the chemical states of the partially charged cathode material used in lithium-ion batteries as an application example are examined.

Chemical State Analysis by X-Ray Fluorescence Using Absorption Edges Shifts

Recently synchrotron radiation (SR) sources have been extensively used for the study of materials science. The high intensity of tunable monochromatic X-rays from SR facilitates many types of spectroscopic/ diffraction studies which have otherwise not been possible. Regarding the X-ray fluorescence technique, significant improvement of the minimum detection limit has been performed and has enabled trace element analysis in the order of tens of ppb or 10-12 g.3-8 SR microanalyzers and near surface analysis using grazing incidence geometry are also attractive applications of synchrotron X-ray fluorescence technique. From a point of materials characterization, chemical state analysis is not less important than ordinary element analysis.

Development of Quantitative X-Ray Microtomography

We have developed several x-ray microtomography systems which function as quantitative three dimensional x-ray microscopes. In this paper we describe the evolutionary path followed from making the first high resolution experimental microscopes to later generations which can be routinely used for investigating materials. Developing the instrumentation for reliable quantitative x-ray microscopy using synchrotron and laboratory based x-ray sources has led to other imaging modalities for obtaining temporal and spatial two dimensional information.

Synchrotron Microtomography of Composites

X-ray computed tomography (CT) uses absorption profiles from many different viewing directions to reconstruct the two-dimensional distribution of x-ray absorptivity within a slice of the sample. The tunability, high brightness and parallelism of synchrotron radiation are critical to high resolution (0.001mm), high contrast (1%) CT or microtomography. In situ study of samples multiple times during the course of an experiment is exciting to consider.

Continuous fiber SiC/Al composites were deformed under three-point bending, and the resulting damage and fiber arrangement were revealed with synchrotron microtomography. Several hundred slices of 0.012 mm thickness were recorded simultaneously using 25 key radiation and a phosphor screen/charge coupled device (CCD) detector. Reconstruction was with the filtered back projection method. Low density regions were observed in the matrix in regions of highest stress where cracking is expected.

Micro-computed tomography-current status and developments

The recent rapid increase in interest in tomographic imaging of small animals and of human (and large animal) organ biopsies is driven largely by drug discovery, cancer detection/monitoring, phenotype identification and/or characterization, and development of disease detection methods and monitoring efficacies of drugs in disease treatment. In biomedical applications, micro-computed tomography (CT) scanners can function as scaled-down (i.e., mini) clinical CT scanners that provide a three-dimensional (3-D) image of most, if not the entire, torso of a mouse at image resolution (50-100 microm) scaled proportional to that of a human CT image. Micro-CT scanners, on the other hand, image specimens the size of intact rodent organs at spatial resolutions from cellular (20 microm) down to subcellular dimensions (e.g., 1 microm) and fill the resolution-hiatus between microscope imaging, which resolves individual cells in thin sections of tissue, and mini-CT imaging of intact volumes.

Quantitative, chemically specific imaging of selenium transformation in plants

Quantitative, chemically specific images of biological systems would be invaluable in unraveling the bioinorganic chemistry of biological tissues. Here we report the spatial distribution and chemical forms of selenium in Astragalus bisulcatus (two-grooved poison or milk vetch), a plant capable of accumulating up to 0.65% of its shoot dry biomass as Se in its natural habitat. By selectively tuning incident x-ray energies close to the Se K-absorption edge, we have collected quantitative, 100-microm-resolution images of the spatial distribution, concentration, and chemical form of Se in intact root and shoot tissues. To our knowledge, this is the first report of quantitative concentration-imaging of specific chemical forms. Plants exposed to 5 microM selenate for 28 days contained predominantly selenate in the mature leaf tissue at a concentration of 0.3-0.6 mM, whereas the young leaves and the roots contained organoselenium almost exclusively, indicating that the ability to biotransform selenate is either inducible or developmentally specific. While the concentration of organoselenium in the majority of the root tissue was much lower than that of the youngest leaves (0.2-0.3 compared with 3-4 mM), isolated areas on the extremities of the roots contained concentrations of organoselenium an order of magnitude greater than the rest of the root. These imaging results were corroborated by spatially resolved x-ray absorption near-edge spectra collected from selected 100 x 100 microm(2) regions of the same tissues.