Spikes were excluded from Raman spectra using the algorithm developed by Sabin et al. . The data cube was unfolded to a 2D matrix NM × λ, where M is the number of pixels at x axis, N the number of pixels at y axis and λ the number of spectral variables. The asymmetric least squares (AsLS) algorithm was used to correct the baseline and the spectra were normalized using unit vector. The spectral range chosen to build the models was 1804-600 cm -1 .