In order to generate a robust and reliable library, all the spectra included in the database were measured using the same instrumental configuration and acquisition conditions.

The FTIR spectra were registered in attenuated total reflection (ATR) mode, using a SpectrumTwo FTIR spectrometer (PerkinElmer, Waltham, MA, USA) equipped with a GladiATR accessory (monolithic diamond ATR crystal from Pike Technologies). Spectra were collected in the 4000-380 cm−1 mid-infrared spectral region at 4 cm−1 resolution. A total of 128 scans were used for an optimal signal-to-noise ratio. All FTIR spectra have been baseline corrected and are plotted in percent transmittance (%T).

XRF measurements were performed using a TRACER III-SD portable energy-dispersive instrument from Bruker, provided with a Rh anode X-ray tube and a 10 mm2 X-Flash Silicon Drift Detector (SDD), with an energy resolution of approximately 147 eV for the Mn Kα line at 200,000 cps. For powder samples, approximately 5 g was used for each sample, in order to simulate an infinite layer thickness, placed directly onto Mylar film and exposed to X-rays for 300 s. The detection mode was universal, optimized for the mid-energy range of the 0-40 keV domain, experimental parameters being set at 40 kV tube voltage, ~11 μA current intensity, with no filtering of the incident beam, in air atmosphere. The instrument’s detection limit varies for each element as a function of the physics of the sample, down to ppm level.