Upper part: loading plots of the principal components PC1 and PC2 which
account for 33.4% and 17.1% respectively, of the total variance in the FT-Raman
spectra of the 37 reference materials studied. The letters A, B and C refer to three
spectral regions described in the text. Lower part: CH spectral region of 5 different
chemical families: proteins, triterpenoid resins, diterpenoid resins, triglycerides
and polysaccharides. This figure indicates which spectral features of the Raman
CH region have an important contribution to PC1 and PC2.