New markers of natural and anthropogenic chemical alteration of archaeological lignin revealed by in situ pyrolysis/silylation-gas chromatography-mass spectrometry

Analytical pyrolysis coupled with gas chromatography and mass spectrometry with in situ silylation using hexamethyldisilazane (Py(HMDS)-GC/MS) was used to investigate the chemical alteration patterns of a set of archaeological waterlogged oak and silver fir woods. The samples were collected from five piles removed from stilt houses found in a Neolithic village (Bracciano lake, Rome, Italy) and from various parts of the roof of a Roman house (Herculaneum, Italy).We discuss on how the molecular information provided by Py(HMDS)-GC/MS revealed the causes and effects of natural and anthropogenic alteration and degradation of lignin, and how the adoption of silylation reactions lead to the detection of very informative pyrolysis products. Very particular pyrolytic patterns were obtained for the archaeological samples investigated, which were mainly characterised by the presence of oxidised compounds such as vanillin, acetovanillone, vanillic acid, syringaldehyde, acetosyringone, and syringic acid.We also report the first ever identification of the methyl esters of vanillic and syringic acids in their silylated form using this method. The results are consistent with heating processes undergone by archaeological wood due to natural or anthropogenic causes: the wooden roof from Herculaneum was naturally exposed to high temperatures during the eruption of Vesuvius, and the wood piles may have been artificially heated by people in the Neolithic Age to enhance the waterproof properties of wood.Due to the importance of identifying lignin pyrolysis products, the identification and mass spectra of sixty lignin pyrolysis products in their silylated form are also presented.