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Fig. 3


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Standardized spectra reveal three signatures (upper part) visible in the Van Krevelen diagram (lower part) with a singular limit for (H/C)at. ratio value of 0.50. Upper this limit (with an unknown (O/C)at. limit value, in blue signals), the Std spectra systematically increase while decrease when lower than 0.50 (red signals). A third signature (orange signals) with increasing and decreasing slope is also evidenced between the region 670–700 nm and corresponds to samples with very low (H/C)at. – (O/C)at. ratio values.

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