# Analytical Pyrolysis and Mass Spectrometry to Characterise Lignin in Archaeological Wood
## 3 Results and Discussion
### Single-Shot Py(HMDS)-GC/ms
\begin{table}
\begin{tabular}{c|c|c c c c c c c|c c} \hline \multirow{2}{*}{} & \multirow{2}{*}{**Oak Ref**} & \multicolumn{3}{c}{**Biskupin**} & \multicolumn{3}{c|}{**La Marmotta**} & \multirow{2}{*}{**Birch Ref**} & \multicolumn{2}{c}{**Oseberg**} \\ \cline{2-3} \cline{5-10}  & & **4A** & **4B** & **4C** & **2212A** & **2212D** & **2212G** & & **185-1** & **185-6** \\ \hline Sum H (\%) & 77.1 & 38.0 & 63.8 & 74.4 & 14.4 & 14.4 & 21.1 & 75.6 & 20.3 & 3.2 \\ \hline Sum L (\%) & 22.9 & 62.0 & 36.2 & 25.6 & 85.6 & 85.6 & 78.9 & 24.4 & 79.7 & 96.8 \\ \hline H/L & 3.4 & 0.6 & 1.8 & 2.9 & 0.2 & 0.2 & 0.3 & 3.1 & 0.3 & 0.0 \\ \hline S/G & 1.6 & 1.1 & 1.7 & 2.3 & 1.3 & 1.9 & 2.3 & 2.4 & 1.4 & 1.7 \\ \hline \end{tabular}
\end{table} 
Table 4: Holocellulose (H) and lignin (L) content expressed as percentage values based on chromatographic areas; H/L ratios and syringyl/guiaacyl (S/G) ratios of reference and archaeological wood samples. compounds (10-15%), and demethylated/demethoxylated compounds (15-20%). A very small abundance of pyrolysis products bearing an acidic functionality is usually present in the pyrolysis profiles of sound wood (3-5%). The characterisation of archaeological wood samples led to totally different data (Figure 8). The relative abundance of \"monomers\" was usually lower than that obtained for sound wood. On the other hand, an increase in \"short chain compounds\" was detected. The formation of the side chain compounds is connected to depolymerisation of lignin during the burial period [94] or to the higher yield of secondary pyrolytic reactions for aged wood compared to sound wood [7; 87; 88]. In both cases, the increase in the relative abundance of short side-chain phenols and the decrease in the relative abundance of monomeric structures can be taken as an indication that alterations in the side chain of the phenylpropane units of lignin have occurred during degradation. Long side-chain lignin pyrolysis products, which are defunctionalised monomers, also appeared with a higher relative abundance in samples from the Biskupin site and samples taken from the inner part of the pile from La Marmotta than in sound wood. The oxidation of lignin was highlighted by an increase in carbonyl and carboxyl functionalities [23; 27; 30], but the extent of oxidation observed in the Oseberg samples is exceptional even for very degraded archaeological wood [8]. In terms of the detailed information that can be provided by Py(HMDS)-GC/MS, specific degradation products can be identified and related to specific degradation pathways. _P_-hydroxybenzoic acid (#69) was identified in all pyrograms obtained from the Oseberg samples. This pyrolysis product has not been previously reported as a lignin break-down product in naturally-aged archaeological samples [30; 31; 32; 37; 95]. The presence of this acid can be related to the oxidising conditions used in industrial processes to separate lignin from holocellulose, such as acidolysis and hydrolysis [96; 97; 98; 99; 100]. Thus, the results obtained for the Oseberg collection samples suggest that the aim treatment induced extreme oxidising conditions for lignin in the long term. The generation of _p_-hydroxybenzoic acid can be due to several reactions, including \(\beta\)-O-4' cleavage, demethoxylation, and side chain cleavage accompanied by extensive oxidation of lignin units. This was a robust proof of the role of the alum treatment in the dramatic degradation of these archaeological objects. In addition, the percentage abundance of acid lignin pyrolysis products was found to correlate with the concentrations of inorganic elements (Al, Fe, Ca) present in these samples [8]. Figure 8: Abundances of the categories of lignin pyrolysis products expressed as percentages in the archaeological and the reference wood samples. The relative standard deviation obtained by Py-GC/MS of lignocellulosic materials is about 7%, as reported in the literature [7; 30].