High-pressure, high-temperature x-ray absorption fine structure transmission cell for the study of aqueous ions with low absorption-edge energies

We describe a method to acquire x-ray absorption fine structure (XAFS) spectra of low-Z atoms dissolved in high-pressure liquids and supercritical fluids. The method is applicable to energies at and below the Ca K edge (4038.5 eV). The cell design incorporates 25-μm-thick × 700 μm diameter diamond x-ray windows in a Poulter-type seal geometry. X-ray focusing mirrors were used to reduce both the horizontal and vertical beam size to about 200 μm so that the incident beam would pass cleanly through the 300 μm aperture of the cell. Pathlengths in the range from 50 to 1000 μm can be selected with appropriate spacers. The maximum operating conditions of the flow-through cell design are 500°C and 1 kbar. We show that only a relatively small number of XAFS scans are required to generate high signal-to-noise ratio spectra for a supercritical water solution (400°C) containing 1 m CaCl ₂ using a bending-magnet beamline (20-BM, PNC-CAT, Advanced Photon Source, Argonne National Laboratory).