AbstractA systematic ESCA study of the solid residues of supercritical critical extraction ofPopulus tremuloïdesprepared in various conditions has been performed. Reference materials also examined by ESCA included the initial wood, Whatman no. 1 paper, and two lignins prepared by different procedures. Interest has been focused on the Clsand Olsspectra and on the determination of surface O/C atomic ratio. A difference between bulk O/C values measured by chemical analysis and surface O/C ratios obtained from ESCA data is considered diagnostic of a difference in chemical composition of the bulk and the surface. In the Clspeaks of wood and its constitutive polymers, the usual components peaks C1, C2, and C3were observed. For the solid residues, however, a fourth peak designated as C0appears, the importance of which increases steadily when the temperature of extraction is raised from 250 to 350°C. The component C0was ascribed to polyaromatic constituants. Its proportion is correlated with the fraction of the carbon in the residue comprised in the recondensed material and determined independently. The Olspeak shows also a complex structure with three components in the wood‐derived reference materials and four in some of the solid residues. These structures have not been discussed previously in the literature and a tentative assignment for the component peaks is propo
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