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XPS was used to characterize the chemical changes occurring after drying or applying a heat-treatment to beech wood samples. Our results indicate that the surface of this air-exposed material could be strongly affected either by the ambient atmosphere during storage or by the complex atmosphere in the oven during drying or heat-treatment. However, the O/C ratio measured after removal of a thin slice of a few millimetres of an untreated sample is in reasonable agreement with that calculated from the wellestablished chemical composition of beech. Through this methodology (equivalent to scraping for hard materials) it is expected to get a realistic characterization of the wood. The reliability and repeatability of the XPS measurements have been checked and the method applied to the study of the chemical changes of the beech samples subjected to heat-treatment. Heating at 240°C induces a significant decrease of the O/C ratio from 0.55 before to 0.44 after the treatment. Heat-treatment induces also a decrease of the C 2 carbon contribution (carbon atom bound to a single non-carbonyl oxygen) associated with an increase of the C 1 carbon contribution (carbon atoms bound only to carbon or hydrogen atoms), in agreement with chemical modifications reported previously in the literature. Thanks to the small analysed area of the equipment used in this study, different spots were analysed to demonstrate the presence or absence of a gradient of chemical composition due to thermal degradation or migration of extractives from within the wood structure to its surface. At the scale of our observations, the different wood samples investigated (dried or heat treated) appear to be homogeneous.

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