High-energy interference-free K-lines synchrotron X-ray fluorescence microscopy of rare earth elements in hyperaccumulator plants

Synchrotron-based micro-X-ray fluorescence analysis (µXRF) is a nondestructive and highly sensitive technique. However, element mapping of rare earth elements (REEs) under standard conditions requires care, since energy-dispersive detectors are not able to differentiate accurately between REEs L-shell X-ray emission lines overlapping with K-shell X-ray emission lines of common transition elements of high concentrations. We aim to test REE element mapping with high-energy interference-free excitation of the REE K-lines on hyperaccumulator plant tissues and compare with measurements with REE L-shell excitation at the microprobe experiment of beamline P06 (PETRA III, DESY). A combination of compound refractive lens optics (CRLs) was used to obtain a micrometer-sized focused incident beam with an energy of 44 keV and an extra-thick silicon drift detector optimized for high-energy X-ray detection to detect the K-lines of yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd) without any interferences due to line overlaps. High-energy excitation from La to Nd in the hyperaccumulator organs was successful but compared to L-line excitation less efficient and therefore slow (∼10-fold slower than similar maps at lower incident energy) due to lower flux and detection efficiency. However, REE K-lines do not suffer significantly from self-absorption, which makes XRF tomography of millimeter-sized frozen-hydrated plant samples possible. The K-line excitation of REEs at the P06 CRL setup has scope for application in samples that are particularly prone to REE interfering elements, such as soil samples with high concomitant Ti, Cr, Fe, Mn, and Ni concentrations.