Upper part: Distribution of the three sets of samples and the experimental samples from Trompovski et al. (2005) in a Van Krevelen diagram. Red circles correspond to samples characterised by a decreasing Std signals, green triangles corresponds to samples from Tompowsky et al. (2005), green squares to the three sets of samples discussed in text. These later samples were subjected to a progressive pyrolysis as highlighted by the lowering of (H/C)_at. and (O/C)_at. ratio values. A) 1-7 samples correspond to the Eucalyptus grandis species and B) 9-14 to Eucalyptus saligna. Both species show a transition in Std signatures in the Std600/750 (upper part) that is more distinguishable in CR650/700 (Lower part) with a shift that occurs around 400 °C where the spectra increasing trend disappears.