*2.1. X-ray Fluorescence Spectrometer (XRF)*

The portable XRF instrument Tracer III-SD (Bruker AXS) consists of an X-ray tube equipped with a Rh target, and a Peltier cooled Si drift XFlash detector having a resolution of 130 eV FWHM at 5.9 keV. The source was operated at 40 kV and 0.030 mA, with a data acquisition time of 30 s. This instrumental setup allows for the analysis of elements with an atomic number (Z) greater than 10. The X-rays emitted by the tube are collimated on the analyzed surface with a spot diameter of 4 mm. The spectra, corrected for the efficiency of the detector, were expressed as counts per second (cps).
