Thermo nicolet is50 ftir spectrometer

Fourier-transform infrared (FTIR) spectra of the lignin samples were recorded using Thermo Nicolet iS50 FTIR spectrometer with iS50 ATR-crystal (Thermo Fisher Scientific, Vantaa, Finland). Analysis of the spectral area (3800–600 cm⁻¹) was carried out as duplicate measurements with 32 scans from each sample and averaged prior normalization, which was based on peak area using Excel (Microsoft, Espoo, Finland).

The soil samples were removed from the fresh bags, fully air dried, ground and passed through a 0.149 mm soil sieve. The sieved naturally air-dried soil samples were mixed with dried potassium bromide (spectrally pure) at a mass ratio of 1 : 90, ground manually with an agate mortar and pestle, compacted by a tablet press at 15 kPa, and placed in a Thermo Nicolet iS50 FTIR spectrometer (ThermoFisher Scientific, USA) to be measured. With KBr as the background value, the spectra were collected in the range of 400–4000 cm⁻¹, with a resolution of 4 cm⁻¹, and the scanning frequency was 64 times. Each sample was repeated five times, and the average value was taken after similarity comparison; the sample IR spectra were obtained after deducting the background value. The raw spectral data were corrected using the calibration function that comes with OMNIC to remove the influence of baseline, and then standard normalization was performed to remove the difference in weighing between different samples.