Caf2 windows

Rejuvenating oils, i.e., VCO, WCO and VEO, were chemically analysed by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The analyses were performed in a single-reflection Golden Gate^TM accessory (Specac, Orpington, UK) equipped with a diamond crystal. The IR spectra of oils were recorded over 32 scans in the range of 4000 cm⁻¹ to 600 cm⁻¹, with a resolution of 4 cm⁻¹ using a Nicolet iS20 Spectrometer equipped with a DTGS detector and CaF₂ windows (Thermo Fisher Scientific, Waltham, MA, USA). The comparison and analysis of spectra were performed after signal processing, i.e., baseline, ATR correction for Diamond crystal, peak identification, and normalisation ( $a_{norm}$ ) according to the procedure presented by Hofko et al. [31 (link)]. Second-derivative spectra (Savitzky–Golay algorithm) were calculated to resolve the overlapping peaks and remove of the effects of baseline drifts.

A Thermo Nicolet 6700 FTIR spectrometer (Thermo Electron Corporation, Waltham, MA, USA), controlled with OMNIC 7.3 SP1 (Thermo Scientific) software, was used to record FTIR spectra. Liquid samples were held in a Thermo HT-32 demountable cell with CaF₂ windows (Thermo Scientific, Waltham, MA, USA) and a copper or lead spacer between the windows (usually a 0.025 mm copper spacer). A total of 40 µL of liquid sample was injected into the cell. A background air spectrum, using an empty sample compartment (no cell), was recorded just before a sample was analysed. The spectra consist of 32 scans recorded at a resolution of 4 cm⁻¹. Spectra were collected at room temperature, with the cell temperature measured with a Thermo Scientific CAL 9500 thermostat (Waltham, MA, USA).