# Analytical Pyrolysis and Mass Spectrometry to Characterise Lignin in Archaeological Wood
## 3 Results and Discussion
### Single-Shot Py(HMDS)-GC/ms
Generally, with regards to archaeological wood, this index provides information on the preferential loss of one wood component (mainly holocellulose) by comparing the values obtained for degraded and sound wood of the same species. Table 4 shows the H/L ratios obtained by Py-GC/MS analysis of selected samples from Biskupin [7], La Marmotta [62], Oseberg [8], and reference sound wood samples, such as oak wood for the samples from Biskupin and La Marmotta, and birch wood for the Oseberg samples. The data obtained analysing wood from La Marmotta and the Oseberg collection could apparently suggest that all the samples have similar states of preservation, since they had all undergone a substantial loss of polysaccharides. The results obtained for the Biskupin samples, on the other hand, showed a decreasing trend in preservation from the innermost sample (4C) to the outermost sample (4A). The Oseberg samples 185-1 and 185-6 showed very low H/L ratio, indicative of an almost complete absence of polysaccharide components, especially for sample 185-6. Nevertheless, it is evident that the H/L ratio does not provide information on the chemical changes occurring in the single wood components, nor on the state of preservation of the residual wood materials. A useful parameter in the evaluation of the preservation level of hardwood lignin by Py-GC/MS is the syringil/guiaacyl (S/G) ratio, which is related to alterations of methoxy groups in lignin [93]. The S/G ratios of the analysed samples are reported in Table 4. The reference samples showed, as expected, very different values: 1.3 for oak wood and 2.4 for birch wood, indicating that the two species have a different distribution of guiaacyl and syringyl components in the lignin network. The interpretation of the results obtained for archaeological samples is not straightforward. The most common scenario is to obtain a lower S/G ratio for degraded samples, as the syringyl component of lignin is less resistant to chemical changes than the guiaacyl component, due to the presence in the syringyl lignin of additional methoxy groups on the aromatic ring that can be partially cleaved [90; 91]. This was observed in the Oseberg samples, but not in all the samples from the other archaeological sites. However, S/G value may also be affected by several factors. For example, if both syringyl and guiaacyl units undergo demethylation/demethoxylation, the phenomenon is not highlighted by the S/G ratio. Consequently, although a change in the S/G ratio may be a first indication that the lignin has also undergone chemical changes, a more detailed data analysis is often needed. In order to highlight the chemical modification undergone by the residual lignin, a strategy based on the categorisation of lignin pyrolysis products strategy can be adopted. The pyrolysis products generated from the lignin are classified according to their molecular structure and pyrolytic formation [7; 8; 62]. The sum of the peak areas of the pyrolysis products assigned to each category (L. cat., Table 3) is expressed as a percentage of the total relative abundance of all lignin pyrolysis products. In sound wood, the lignin pyrolysis products are usually distributed in a similar way, irrespective of the wood species [7]. Monomeric structures (coniferyl and sinapyl alcohols) account for the 40-50% of the total. The rest are composed of guiaacyl and syringyl units \(p\)-substituted by \"short\" (up to 2 carbon atoms) and \"long\" (3 carbon atoms) alkyl chains (about 20%), carbonyl