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. Experimental settings were set-up as to obtain high quality spectra, with minimal noise. However, spectra acquired via fiber optic spectrometers (such as Raman analysis) are typically lower in quality compared to those from a benchtop equipment.
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. Samples were applied directly on the ATR crystal, with no sample preparation prior to spectral measurements. Before each measurement, the ATR crystal was cleaned using isopropanol and optical wipes. 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.
Raman spectra were recorded using a Wasatch Photonics system (WP 785 ER Raman) equipped with a standard fiber optic probe, providing a spatial resolution of 7 cm-1. A near infrared diode laser (785 nm) was employed as excitation source. The laser spot size is ̴ 170 μm, while the collection area is 1 mm diameter at 11 mm working distance (lens to sample). Acquisition time was on the order of 5 s and the laser power on the order of a few mW on the samples. The Raman spectra were acquired on solid samples without any sample pretreatment. Some of the investigated materials could not be characterized because of the strong luminescence they exhibit with the 785 nm excitation wavelength. All the spectra have been acquired in the absence of room lights to avoid any interference.
