My research has primarily focused on the design of x-ray fluorescence (XRF) systems to measure elemental body composition in vivo. In general, such measurement systems are developed for assessing exposure of industrial populations in the workplace, as the existing monitoring techniques do not provide direct information regarding levels in biologically relevant tissues/organs, tending instead to rely on the inference of worker health from measuring levels in blood and urine.
Through the development of a Monte Carlo photon transport code for this work, I have investigated the optimal design of several in vivo XRF systems. Over the past several years, in collaboration with Physics department colleagues Iain Campbell and Ralf Gellert, I have modified this code to investigate the effect of water content in minerals on the Compton-to-coherent scattering ratio detected when probed with an x-ray source. The original Monte Carlo simulation has been expanded to include a host of additional elements, and variance reduction techniques have been introduced in order to increase the computing speed of the code. In 2006, we published an article in X-ray Spectrometry, describing our proposed method of comparing simulated and experimental Compton-to-coherent ratios in order to determine the presence of “invisible” OH and H2O. We found that the simulated ratios were consistent with the experimental values for an extensive set of geo-chemical reference standards. Furthermore, we demonstrated that this approach would be sufficiently sensitive to observe the effects from the presence of 6% water or more in individual samples. We are continuing the development of this Monte Carlo code: surface layers are being added in order to better model the new water-bearing compounds being measured experimentally. In addition, full modeling of interactions in the collimators and the detector are being added.
Our analysis approach has now been applied to determine the water content of soils and rocks encountered by the Spirit rover on Mars. Typical basaltic surface soils were found to be essentially dry (<1% water by weight) and basaltic rocks were similarly low in water content (<3.5 % water by weight). However, four bright subsurface soils in Gusev Crater were found to have water content in the range of 6 to 18% by weight. These soils also contain extremely high sulfur levels relative to other soils in the region. Together with constraints from mineralogy, our findings indicate that highly hydrated ferric sulfates are important carriers of bound water in these four locations. We reported this work in the Journal of Geophysical Research: Planets in 2008. Our research received substantial coverage in the national media in October, 2007, including appearances in the Globe and Mail, Toronto Star and on CTV news.
In addition, I continue to collaborate with colleagues at McMaster University in the field of elemental body composition. We have recently acquired a new system for measuring strontium in bone, which we will be exploring for use in monitoring patients being treated for osteoporosis with a compound that contains strontium.