Submicron hard X-ray fluorescence imaging of synthetic elements

Synchrotron-based X-ray fluorescence microscopy (XFM) using hard X-rays focused into sub-micron spots is a powerful technique for elemental quantification and mapping, as well as microspectroscopic measurements such as μ-XANES (X-ray absorption near edge structure). We have used XFM to image and simultaneously quantify the transuranic element plutonium at the L3 or L2-edge as well as Th and lighter biologically essential elements in individual rat pheochromocytoma (PC12) cells after exposure to the long-lived plutonium isotope 242Pu. Elemental maps demonstrate that plutonium localizes principally in the cytoplasm of the cells and avoids the cell nucleus, which is marked by the highest concentrations of phosphorus and zinc, under the conditions of our experiments. The minimum detection limit under typical acquisition conditions with an incident X-ray energy of 18 keV for an average 202 μm2 cell is 1.4 fg Pu or 2.9 × 10−20 moles Pu μm−2, which is similar to the detection limit of K-edge XFM of transition metals at 10 keV. Copper electron microscopy grids were used to avoid interference from gold X-ray emissions, but traces of strontium present in naturally occurring calcium can still interfere with plutonium detection using its Lα X-ray emission.

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► Actinide elements are mapped with L-edge X-rays and better than 400 nm resolution.
► A typical detection limit was 2.9 × 10−20 moles Pu μm−2.
► XANES measurements provide chemical information in 0.1 μm2 spots.
► Selection of materials for encapsulation is important for avoiding interferences.