Multifunctional bending magnet beamline with a capillary optic for X-ray fluorescence studies of metals in tissue sections

Scanning fluorescence X-ray microscopy lets one non-destructively and quantitatively map the distribution of most biologically important metals in cells and tissues. For studies on large-scale tissues and organs, a spatial resolution of several micrometres is often sufficient; in this case, bending magnets at synchrotron light sources provide abundant X-ray flux. We describe here the use of bending magnet beamline 8-BM-B at the Advanced Photon Source with two distinct microscopy stations: a pre-existing one with Kirkpatrick-Baez (KB) mirror optics for slightly higher throughput and the ability to accommodate samples tens of centimetres across, and a new prototype station with an axially symmetric, single-bounce, capillary optic with slightly less flux, but finer resolution at similar fluence per time. The KB station provides δ_res = 10.5 µm spatial resolution at a per-pixel exposure time of t_dwell = 100 ms and a fluence per time of 5.8 × 10⁷ photons µm^-2 s^-1, while the prototype capillary station provides δ_res = 6.5 µm at t_dwell = 50 ms and a fluence per time of 5.6 × 10⁷ photons µm^-2 s^-1. We used image power spectral density to estimate the achieved spatial resolution δ_res from individually acquired images, with δ_res depending on the optic, the fluorescence signal strength of the sample being imaged, and the method used to process raw fluorescence spectral data.