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Synchrotron science @ The Wark

The Wark routinely accesses synchrotron facilities around the world. Staff and students have visited facilities at BESSY (Berlin, Germany), The Advanced Photon Source (Chicago, USA), Aladdin (University of Wisconsin, USA), Daresbury (UK), NSRRC (Hsinchu, Taiwan), The Photon Factory (Tsukuba, Japan) and the Canadian Light Source (Saskatoon, Canada), using a wide range of techniques.

Synchrotron light has a unique combination of properties that can be controlled to suit the type of analysis required. Some key properties include:

• Wide range of continuously accessible wavelengths from IR through UV and soft X-rays to hard X-rays
• High photon fluxes, many orders of magnitude more intense than laboratory sources
• Fine wavelength tuning for absorption edge studies and variable surface sensitivity in photoemission
• Can be finely focused for imaging techniques and small samples
• Polarization control

These properties enable a large range of analyses that are conventionally difficult due to detection limits, spatial/spectral resolution and source intensity. Some measurements may only be performed using a synchrotron source (e.g X-ray absorption edge spectroscopy).

The Wark’s many research areas have benefited greatly from access to synchrotron techniques. The following examples give a snapshot of Wark synchrotron science over the last 10 years.

Mineral Electronic Structure, Surface States and Reactivity
Synchrotron radiation XPS (SR-XPS) enables both bulk and surface state contributions to coreline and valence band spectra to be determined. The capability of varying the incident X-ray energy allows different depths to be sampled. The poor cleavage of pyrite (FeS₂) along the (100) direction ruptures both Fe-S and S-S bonds, resulting in the exposure of low co-ordination S monomers and S dimers at the surface. These two sites are all but invisible to conventional XPS, seen only as a small shoulder at the low binding energy side of the S 2p spectrum. SR-XPS dramatically reveals the two surface S sites due to increased surface sensitivity at lower incident photon energy (Figure 1). Ab initio calculations of the electronic distribution within the fractured surface support the spectroscopic interpretation of these S sites. Moreover the theoretical treatment also supports the charge transfer from the Fe site associated with the surface S monomer (i.e. Fe²⁺ è Fe³⁺) with commensurate low-to-high spin electronic configuration change. This example demonstrates how the combination of spectroscopic and theoretical tools can be used to shed light on the reactivity of sulfide surfaces exposed on fracture, particularly in the grinding of ores in flotation minerals processing. This methodology has been developed as part of the ongoing collaboration between A/Prof Bill Skinner and Dr Gunhild von Oertzen of The Wark and Prof Wayne Nesbitt of the University of Western Ontario.

Figure 1 Sulfur 2p core-line XPS spectrum for fractured pyrite using synchrotron radiation XPS (hv=250 eV). Ab initio, quantum mechanical calculation of electron spin distribution (inset) at the surface.

Forensic Analysis of Trace Evidence
The need for greater levels of certainty in trace evidence characterization has driven forensic science toward more advanced analysis methods. Non-destructive techniques are preferred so that many types of analysis may be performed on the same sample, retaining the evidence for later, newer measurements. The Wark’s involvement in forensic analysis has been developed through collaboration with SA Forensic Science over the past 8 years. Highly successful research in this time has established Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) as a powerful tool for the analysis of forensic trace evidence such as gunshot residue (GSR), glass, hair and inks. Through the Australian Synchrotron Research Program (ASRP) Research Fellowship of Dr Ivan Kempson, synchrotron science has been applied to similar materials, yielding even higher levels of discrimination and information previously unobtainable. Examples include the use of imaging X-ray fluorescence (XRF)/X-ray absorption spectroscopy (XAS) or synchrotron microprobe to determine trace element distribution and form within human and animal hair structures (Figure 2a) and 3-D distributions of Pb in fine glass particles from GSR by computer-aided micro-tomography (Figure 2b). The small-spot, high flux properties of synchrotron radiation enable extremely small samples and regions (~100 nm) to be analysed which, conventionally, would not be possible. These techniques have also found application in the anthropological/archaeological context and environmental biomonitoring through our collaboration with Profs Ron Martin and Andrew Nelson at the University of Western Ontario.

Figure 2a Synchrotron XRF microprobe elemental maps from a cross-section of human hair

Figure 2b Synchrotron micro-tomography of a 150 micron glass GSR particle. An overlay of orthogonal sections through the particle (right) and the 3D reconstruction of Pb distribution within the particle (far right) using Pb K-edge contrast.

Environmental Science
Mineral and biological aspects of environmental chemistry have been examined across the rhizosphere. The rhizosphere comprises the volume of soil and community of micro-organisms on or around plant roots. Leaching and weathering of soild minerals, oxidation/reduction of nutrient metals and uptake mechanisms by plant roots may be examined by synchrotron microprobe, with appropriate sample preparation. ToF-SIMS was also employed in parallel and examples were presented in the 2003 Annual Report. This work has been extended to the examination of aluminium toxicity effects on roots in smelter-impacted soils, through our ongoing collaboration with Profs Ron Martin and Shiela McFie at the University of Western Ontario and Prof Francois Courchesne et al at the University of Montreal, Canada. The second UniSA Collaborative project between CERAR and The Wark, looking at metal sequestration and co-adsorption on soil minerals (Dr Markus Gräfe and Dr David Beattie) involves a large component of X-ray absorption spectroscopy (XAS) and Extended X-ray Absorption Fine Structure (EXAFS) in particular.

Colloid Surface Science
Micro-tomography has been used to investigate the wetting behaviour of mineral particles in water (Mr Nate Stevens, Prof John Ralston, Dr Rossen Sedev). This study was carried out at the Advanced Photon Source (Chicago, USA). The objective is visualize where water is held within a packed bed of particles, a problem of wide relevance to minerals, petroleum, pharmaceutical and environmental research. In another application, the structure of water bound to hydrophobic and hydrophilic interfaces, using the evanescent X-Ray technique is being studied. This work employs the X-ray evanescent wave technique in collaboration with Prof Harald Reichert (Max Plank Institute for Solid State Physics, Stuttgart) which in 2005, commenced a joint PhD research program enrolled through the University of Stuttgart. 

Biointerfaces and Biomedical Applications
X-ray Photo-Emission Electron Microscopy (XPEEM) has been used for the analysis of plasma-patterned micro-array substrates with immobilized model oligonucleotides. The work, by Ms Phuong-Cac Nguyen and Prof Hans Griesser, was conducted at Aladdin (University of Wisconsin) in collaboration with Prof Adam Hitchcock (McMaster University) and Prof Stephen Urquhart (University of Saskatoon). The application of these micro-arrays is in diagnostic chips for the assay of complementary DNA or proteins.

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