Tom Schoonjans

Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft

Can you spot a speck of space dust? NASAs Stardust spacecraft has been collecting cosmic dust: Aerogel tiles and aluminum foil sat for nearly 200 days in the interstellar dust stream before returning to Earth. Citizen scientists identified most of the 71 tracks where particles were caught in the aerogel, and scanning electron microscopy revealed 25 craterlike features where particles punched through the foil. By performing trajectory and composition analysis, Westphal et al. report that seven of the particles may have an interstellar origin. These dust particles have surprisingly diverse mineral content and structure as compared with models of interstellar dust based on previous astronomical observations. Science, this issue p. 786 Analysis of seven particles captured by aerogel and foil reveals diverse characteristics not conforming to a single model. Seven particles captured by the Stardust Interstellar Dust Collector and returned to Earth for laboratory analysis have features consistent with an origin in the contemporary interstellar dust stream. More than 50 spacecraft debris particles were also identified. The interstellar dust candidates are readily distinguished from debris impacts on the basis of elemental composition and/or impact trajectory. The seven candidate interstellar particles are diverse in elemental composition, crystal structure, and size. The presence of crystalline grains and multiple iron-bearing phases, including sulfide, in some particles indicates that individual interstellar particles diverge from any one representative model of interstellar dust inferred from astronomical observations and theory.

Waterborne versus dietary zinc accumulation and toxicity in Daphnia magna: A synchrotron radiation based X-ray fluorescence imaging approach

Recent studies have suggested that exposure of the freshwater invertebrate Daphnia magna to dietary Zn may selectively affect reproduction without an associated increase of whole body bioaccumulation of Zn. The aim of the current research was therefore to investigate the hypothesis that dietary Zn toxicity is the result of selective accumulation in tissues that are directly involved in reproduction. Since under field conditions simultaneous exposure to both waterborne and dietary Zn is likely to occur, it was also tested if accumulation and toxicity under combined waterborne and dietary Zn exposure is the result of interactive effects. To this purpose, D. magna was exposed during a 16-day reproduction assay to Zn following a 5 × 2 factorial design, comprising five waterborne concentrations (12, 65, 137, 207, and 281 μg Zn/L) and two dietary Zn levels (49.6 and 495.9 μg Zn/g dry wt.). Tissue-specific Zn distribution was quantified by synchrotron radiation based confocal X-ray fluorescence (XRF). It was observed that the occurrence of reproductive inhibition due to increasing waterborne Zn exposure (from 65 μg/L to 281 μg/L) was accompanied by a relative increase of the Zn burdens which was similar in all tissues considered (i.e., the carapax, eggs, thoracic appendages with gills and the cluster comprising gut epithelium, storage cells and ovaries). In contrast, the impairment of reproduction during dietary Zn exposure was accompanied by a clearly discernible Zn accumulation in the eggs only (at 65 μg/L of waterborne Zn). During simultaneous exposure, bioaccumulation and toxicity were the result of interaction, which implies that the tissue-specific bioaccumulation and toxicity following dietary Zn exposure are dependent on the Zn concentration in the water. Our findings emphasize that (i) effects of dietary Zn exposure should preferably not be investigated in isolation from waterborne Zn exposure, and that (ii) XRF enabled us to provide possible links between tissue-specific bioaccumulation and reproductive effects of Zn.

Spatially resolved 3D micro-XANES by a confocal detection scheme

A confocal setup based on polycapillary half-lenses was used to demonstrate three-dimensional (3D) spatially resolved mu-XANES in fluorescence detection mode at the DUBBLE XAS station of the ESRF (BM26A). The incoming beam was focused using a polycapillary half-lens and a second glass polycapillary was placed in front of the energy dispersive detector to establish the confocal detection. The full-width-half-maxima along the main axes of the resulting ellipsoidal detection volume were 18.5 x 12.0 x 10.0 microm(3) at the Cu K-edge. The confocal mu-XANES mode is applied in the 3D resolved study of mineral inclusions in rare natural diamonds at the Fe K edge.

Spatially resolved (semi)quantitative determination of iron (Fe) in plants by means of synchrotron micro X-ray fluorescence

AbstractIron (Fe) is an essential element for plant growth and development; hence determining Fe distribution and concentration inside plant organs at the microscopic level is of great relevance to better understand its metabolism and bioavailability through the food chain. Among the available microanalytical techniques, synchrotron μ-XRF methods can provide a powerful and versatile array of analytical tools to study Fe distribution within plant samples. In the last years, the implementation of new algorithms and detection technologies has opened the way to more accurate (semi)quantitative analyses of complex matrices like plant materials. In this paper, for the first time the distribution of Fe within tomato roots has been imaged and quantified by means of confocal μ-XRF and exploiting a recently developed fundamental parameter-based algorithm. With this approach, Fe concentrations ranging from few hundreds of ppb to several hundreds of ppm can be determined at the microscopic level without cutting sections. Furthermore, Fe (semi)quantitative distribution maps were obtained for the first time by using two opposing detectors to collect simultaneously the XRF radiation emerging from both sides of an intact cucumber leaf. FigureElemental distribution maps within intact tomato roots as determined by confocal micro X‐ray fluorescence

Dual detection X-ray fluorescence cryotomography and mapping on the model organism Daphnia magna

Micro X-ray fluorescence (-XRF) is a rapidly evolving analytical technique which allows visualising the trace level metal distributions within a specimen in an essentially non-destructive manner. At second generation synchrotron radiation sources, detection limits at the sub-ppm level can be obtained with micrometer resolution, while at third generation sources the spatial resolution can be better than 100 nm. Consequently, the analysis of metals within biological systems using micro and nano X-ray fluorescence imaging is a quickly developing field of research. Since X-ray fluorescence is a scanning technique, the elemental distribution within the sample should not change during analysis. Biological samples pose challenges in this context due to their high water content. A dehydration procedure is commonly used for sample preparation, enabling an analysis of the sample under ambient temperature conditions. Unfortunately, a potential change of elemental redistribution during the sample preparation is difficult to verify experimentally and therefore can not be excluded completely. Creating a cryogenic sample environment allowing an analysis of the sample under cryogenic condition is an attractive alternative, but not available on a routine basis. In this article, we make a comparison between the elemental distributions obtained by micro SR-XRF within a chemically fixed and a cryogenically frozen Daphnia magna, a model organism to study the environmental impact of metals. In what follows, we explore the potential of a dual detector set-up for investigating a full ecotoxicological experiment. Next to conventional 2D analysis, dual detector X-ray fluorescence cryotomography is illustrated and the potential of its coupling with laboratory absorption micro-CT for investigating the tissue specific elemental distributions within this model organism is highlighted.

The xraylib library for x-ray-matter interaction cross sections: new developments and applications

This work presents the recent developments of xraylib, an ANSI C library designed to provide convenient access to a large number of X-ray related databases, with a focus on quantitative XRF applications. The discussed enhancements include improved XRF production cross sections that take into account cascade effects and M-lines, as well as revised line energies, atomic level widths, Compton broadening profiles etc. A full overview is presented of the complete API.

F-54 Stardust Cometary Matter Analyzed By Synchrotron Nano-XRF: New Results and Developments

Stardust is an interplanetary spacecraft launched on February 7, 1999, by NASA, whose primary purpose was to investigate the make up of comet Wild2 and its coma. A second goal of the project was the acquisition of interstellar dust. In order to facilitate the capture of the particles, the spacecraft was equipped with aerogel (highly porous silica foam with an extremely low density) collectors. After the return of Stardust to Earth on January 15, 2006, the acquired samples were distributed among more than 150 researchers worldwide, including the X-Ray Micro-spectroscopy and Imaging group of Ghent University (XMI-UGent). Each presented sample contains an elongated cavity (called track) produced by the impact and deceleration of a single comet coma particle through the aerogel. Multiple experiments were performed on such tracks and their terminal particle at the ESRF ID13 beamline using several non-destructive X-ray based methods such as conventional 2D scanning nano X-ray fluorescence (XRF) and confocal nano-XRF imaging. The latter technique is applied by mounting a polycapillary half-lens in front of the energy dispersive detector, thereby effectively reducing the volume defined by the intersection of the incoming beam and the sample to a volume defined by the acceptance of the polycapillary. A linear Fresnel lens assembly or a KB-mirror system, were used to reduce the dimensions of the incoming X-ray beam to the 200-300 nm level. The recorded XRF data have been evaluated using XMI-quant, a software package made by the XMI-UGent group in order to perform quantitative evaluation of micro-XRF spectra with special focus on data from confocal XRF imaging experiments. This software package includes an implementation of the fundamental parameter method allowing the analysis of the complex Stardust particles. The presented quantitative results confirm the earlier findings concerning the presence of Ca-Al-rich inclusions in the cometary dust.

F-39 A TOP-DOWN APPROACH USING X-RAY IMAGING TECHNIQUES: INSTRUMENTAL DEVELOPMENTS AND APPLICATIONS IN LIFE SCIENCE

Synchrotron radiation X-ray fluorescence micro- and nanobeam techniques at second- and third generation SR sources offer the potential of non-destructive multi-element analysis down to trace concentration levels with unrivalled spatial resolution among X-ray based analytical techniques. At these sources, relative detection limits at the sub-ppm (fg/ng) level can be achieved. With respect to absolute detection limits (DL), sub-micron sized X-ray beams can offer DLs below 1 ag for the most efficiently excited transition elements, with a potential lateral resolution level better than 100 nm. These characteristics of micro/nanobeam SR-XRF allow spatially resolved multi-element determination of major, minor and trace constituents in microscopic sub areas and volumes within biological specimens in an essentially non-destructive/non-invasive manner. However, the complexity of performing such an experiment is often quite considerable, involving dedicated sample preparation, transportation towards and experimenting at the synchrotron facility, installing an appropriate experimental set-up and performing a thorough data analysis on large amounts of spectral data. The ecotoxicological research on Daphnia magna, a frequently used model organism for investigating the mechanisms of toxicity of metals, has often been difficult because many analytical techniques are not able to investigate trace metal distributions in a spatially resolved manner at a (sub)microscopic resolution. As illustrated by this presentation, SR-XRF microanalysis allows to fill this gap and moreover, due to the variety in sizes of X-ray beams available, this research can be performed from the organism level towards the tissue and cellular level, representing a top-down approach.

F-46 Invited—Three-Dimensional Nano-Xrf On Cometary Matter Returned by Nasa'S Stardust Mission

X-ray fluorescence tomography and polycapillary based confocal XRF imaging using synchrotron radiation are among the most sensitive, non-destructive microanalytical methods which can provide potentially three-dimensional (3D), quantitative information on the elemental distributions in the probed sample volume with trace-level detection limits. Lateral resolution is ultimately limited by the X-ray beam size, which is typically in the 1-10 μm range at a typical second or third generation synchrotron source while sub-micron resolution levels are becoming feasible (down to 30-100 nm) using 3 generation sources coupled with advanced X-ray focusing optics.