Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Recently, the role of microbial activity in mineral formation has become of great interest. In this case, the question arose for platinum–palladium nuggets recovered from an alluvial deposit (unconsolidated sediments deposited by streams) in the state of Minas Gerais in Brazil [1]. The morphological features of these aggregates of noble metals intuitively suggest that biological processes play a role in the formation of these particles. However, a geochemical fingerprint of microbial activity in precious-metal fixation is lacking. Iodine can provide such a fingerprint because it is a strongly biophile element. However, the detection of iodine at the µg/g level in a heavy matrix is quite challenging.
There are various techniques for the quantitative chemical analysis based on the detection of X-rays. Two of these techniques are PIXE (Particle Induced X-ray Emission) and XRF (X-Ray Fluorescence). In the past, only PIXE could be combined with a focussed ion beam to obtain elemental distribution maps with micrometer lateral resolution. PIXE was performed using a 3 MeV and 2 MeV H⁺ beam focussed to about 5x5 µm² coming from a 3 MV Tandetron accelerator at FZD, Dresden [2]. Another possibility is a mobile micro-XRF spectrometer, called MICROTAX or ArtTAX [2], in which X-rays from an X-ray tube are focussed by a polycapillary lens to a spot size of less than 100 µm. Both techniques, PIXE and µ-XRF, failed in detecting any iodine. The best detection limit obtained by PIXE was 70 µg/g for I in this matrix.
Nowadays, Synchrotron Radiation-induced XRF (SR-XRF) can also achieve spot sizes in the micrometer range. The SR-XRF analysis was carried out at the high energy beam line of the BAM, BAMline, at BESSY II. The excitation energy was 40 keV. An ~1 µm² spot size was achieved by focusing with an compound refractive lens CRL [3]. Only SR-XRF succeeded and indicated a significant positive correlation between iodine and palladium concentrations.
The measurements confirm that SR-XRF is very suitable for the detection of trace elements in heavy-metal matrices, which is mainly due to the tunability of the X-ray energy. The results of the SR-XRF measurements suggest that iodine may be used as a tracer for biologically mediated mineral formation.
[1] A.R. Cabral et al., Econ. Geol. 104, 1265-1276 (2009)
[2] F. Herrmann and D. Grambole, Nucl. Instr. Meth. B 104, 26-30 (1995)
[3] H. Bronk et al., Fresenius J Anal Chem 371, 307–316 (2001)
[4] E. Strub et al., Nucl. Instr. Meth. B 266, 2165-2168 (2008)