*2.4. Reflection Infrared Spectroscopy*

The portable infrared spectrophotometer ALPHA-R (Bruker Optik GmbH) is equipped with a Globar IR source, a patented interferometer (RockSolid™, insensitive to external vibrations and able to work in any spatial orientation), and a DLaTGS room temperature detector. The working optical layout for reflection measurements is 22◦/22◦ (specular optics), with about 15 mm of working distance. The infrared spectra have been acquired in the spectral range 7500–375 cm−<sup>1</sup> with a spectral resolution of 4 cm−<sup>1</sup> and 200 interferograms. The sampling area was 28 mm<sup>2</sup> . A background correction using a reference spectrum from a gold flat mirror was applied for representing the reflectance profile (R), expressed in the graphs as pseudo-absorbance, log (1/R).
