Synchrotron dendrochemistry: Exploring the comparison of multi-dimensional elemental analysis methodologies on tree cores using synchrotron techniques

The research conducted in this thesis assesses the feasibility of using X-ray fluorescence (XRF) spectroscopy to determine concentrations (i.e., ppm and ppb) of heavy metals in tree core samples. The samples used to determine this feasibility came from the arsenic (As)-laden region of Yellowknife, NT, which has had past gold mining activity for several decades. With an area of known As contamination, it was hoped that the element could be used as a tracer to determine if XRF counts could in fact be compared directly to concentrations obtained from inductively coupled plasma-mass spectrometry (ICP-MS). There were eight sample locations within the study site, and they all bordered different cardinal directions of the perimeter of the Giant Mine. At each site multiple tree cores and soil samples were taken, as well as heights and diameters of the sampled trees. The first manuscript is focussed on determining the best parameters for XRF spectroscopy tree core scans. Determining a methodology of conducting synchrotron dendrochemistry is important because there is little-to-no research on the best parameters to use to obtain meaningful, repeatable results. Using the IDEAS beamline at the CLS synchrotron allowed us to test XRF scanning parameters on tree cores using different preparation methods, dwell times, and core orientations with respect to incoming incident light. We were able to determine the most accurate and time-efficient way to scan tree cores using XRF without sampling and preparation methods leaving contamination on the core that could be picked up by the hypersensitive detector on the beamline. We determined that a perpendicular orientation, with a dwell time of 2 s and a jog distance of 0.25 mm resulted in the best line scans that used cores that were sanded flat by iterations of subsequent sandpaper grits. The second manuscript is focussed on creating a relationship between the results obtained from two heavy metal analysis methodologies used in dendrochemistry; XRF and ICP-MS. Using a synchrotron, we obtained results of what elements were present in the wood by measuring photons emitted by the individual elements when excited by incident X-rays. Using ICP-MS we sampled the same tree from the same location in time to obtain precise concentrations of heavy metals from different portions of the same tree core. Although hypothesized that a relationship between the two methodologies could be found, no correlation was found; instead, two-dimensional maps of the tree cores suggested a reason why the hypothesis was not confirmed. Two-dimensional maps indicated that elements within trees likely vary in all three dimensions in a much more random fashion than was originally expected. This variation has implications for not only specific questions in dendrochemistry like radial translocation but necessarily alters the way all past and future synchrotron dendrochronological studies should be viewed.

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