Fourier-Transform Infrared Spectroscopic Analysis

Before SR-FTIR scanning, all FJ samples were prepared for fixation, decalcification, and cryosectioning as previously described. The 14 μm cryosections were placed onto the BaF2 substrate (Spectral Systems, Hopewell Junction, USA) and then examined by a Nicolet Continuum XL microscope (Thermo Fisher Scientific, Wilmington, MA, USA) coupled with a Nicolet 6700 spectrometer (Thermo Fisher). After a total of 32 scans, the spectra were acquired with a spectral resolution of 8 cm − 1 and an aperture of 20 × 20 μm. The maps were collected with OMNIC 9 software (Thermo Fisher Scientific). Raw spectra were baseline corrected and then smoothed by Savitzky‒Golay, and five points of each sample were selected randomly for semiquantitative analysis (Guo et al. 2020 (link)).