*2.5. XRF Imaging Using Synchrotron Radiation*

XRF images of the plate were recorded at the microprobe station at CLS@APS at the ID beamline of sector 20 of the Advanced Photon Source at Argonne National Laboratory [16]. The ID line was equipped with a Si(111) double crystal monochromator and a KB mirror capable of focussing the X-ray beam down to 5 micrometres routinely. We used an excitation energy of 13 keV; this energy is just above the Hg L<sup>3</sup> (12,284 eV) and Au L<sup>3</sup> edge (11,919 eV), producing Hg Lα1,2 (9989 eV and 9898 eV) and Au Lα1,2 (9713 eV and 9628 eV) X-ray fluorescence lines, as well as other lines of interest, e.g., K<sup>α</sup> of first row transition elements [17]. The incident beam was tuned to a spot size of ~ 50 µm to optimize the data acquisition efficiency. The experiment was conducted when APS was running in a top-up mode, 24 bunches with a total current of 100 mA. This mode, together with the incident focussed beam (Io) being monitored with an ionization chamber, ensured the beam stability and proper normalization, which is essential as it normally takes several hours to scan the entire plate. In this run, the photon flux was approximately 10<sup>12</sup> photons per second over a spot size of ~50 µm with a step size of 50 µm. The illumination time was 50 ms per pixel, number of pixels of the map was 881 X 1061, and the total scan time was 13 h 24 min and 23 s. It should be noted that synchrotron XRF imaging has been widely used and the scope of its application can be found in a recent contribution [18].

The daguerreotype plate was mounted on a three-axis platform and the scanning was done by moving the plate across the beam pixel by pixel. The XRF image was obtained by setting the energy window of interest, e.g., Hg L<sup>α</sup> and Au Lα, in the X-ray fluorescence spectra collected by a four-element Vortex-Me4 solid state detector (~250 eV energy resolution) and stored in a multichannel analyser (MCA). The experimental set-up is shown in Figure S2, and a snapshot of the MCA display is shown in Figure S3, where the fitting of the overlapping fluorescence X-rays with which we could obtain more accurate elemental distribution and better contrast were obtained.
