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 micrometers 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 (APS) 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 centimeters across, and a new prototype station with an axially-symmetric, single-bounce, capillary optic with slightly less flux, but slightly higher fluence (which affects achievable resolution at low metal concentration) and higher spatial resolution. 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× 10 ⁷ photons /(µm ² ·s), while the prototype capillary station provides δ _res = 6.3 µm at t _dwell = 50 ms and a fluence per time of 6.1× 10 ⁷ photons (µm ² ·s). 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.