2.3. Attenuated Total Reflection Fourier-Transform Infrared Spectroscopy (ATR-FTIR)

The exposed surfaces of the historical and polymer reference samples were characterized by IR spectroscopy in attenuated total reflection (ATR-FTIR) with the Handheld 4300 FTIR spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA), equipped with a ZnSe beam splitter, a Michelson interferometer, and a thermoelectrically cooled DTGS detector. Spectra were acquired with a diamond ATR module, 128 scans, and 4 cm⁻¹ resolution, between 4000 and 650 cm⁻¹. This ATR module allows the analysis of samples with a minimum size of 200 µm. Background spectra were collected between every acquisition. The OriginPro 8 (OriginLab Corportation, Northampton, MA, USA) software was used to analyze the spectra, which are shown as acquired, without baseline corrections or normalizations. The same spot of the plastic surface was analyzed at each of the artificial aging intervals to guarantee monitoring accuracy.