# Analytical Pyrolysis and Mass Spectrometry to Characterise Lignin in Archaeological Wood
## 4 Conclusions  
This review discusses how analytical pyrolysis-based mass spectrometric techniques, such as direct exposure mass spectrometry (DE-MS), evolved gas analysis mass spectrometry (EGA-MS), and single and double-shot pyrolysis gas chromatography mass spectrometry (Py-GC/MS), can be exploited in the study of archaeological lignin and have the ability to provide information on archaeological lignin degradation. The following main conclusions can be drawn:  
* DE-MS facilitates the analysis of a high number of samples in a short time. Differences and similarities between and within wooden samples at a screening level are highlighted by applying principal component analysis (PCA) to the mass spectral data. This method highlights differences in wood degradation pathways, especially in terms of loss of carbohydrates and alteration/depolymerisation/oxidation undergone by lignin. * EGA-MS facilitates quali-quantitative investigations of wood components in terms of thermo-chemistry, chemical composition, and distribution of pyrolysis products. In addition to the information obtained by the thermographic profiles, the comparison of the relative amounts of lignin pyrolysis products with a carbonyl group at the benzylic position is a powerful approach to highlight the differences in oxidation of both guaiacyl and syringyl lignin. The changes in the relative amounts of lignin dimers are an indication of lignin depolymerisation. * Single-shot Py(HMDS)-GC/MS provides information on the chemical changes undergone by the residual lignin in archaeological wood, and is fundamental especially when the calculation of the H/L and S/G ratios is not sufficient to highlight differences in the preservation state. The categorisation of lignin pyrolysis products into classes highlights alterations in the side chains of the phenylpropanol units and oxidation. Figure 9: TIT of sample Ly-A1 from the Lyon2 ship obtained by EGA-MS (**a**) and pyrolytic profiles obtained by double-shot Py(HMDS)-GC/MS of the same sample: first shot at 320 \({}^{\circ}\)C (**b**) and second shot at 600 \({}^{\circ}\)C (**c**), showing the separation between wood and PEG pyrolysis products. Peak labelling refers to Table 3.  
Specific degradation products, such as \(p\)-hydroxybenzoic acid (Oseberg samples) or methyl esters (La Marmotta samples), provide key evidence of the possible causes of lignin degradation in archaeological wood. * Double-shot Py(HMDS)-GC/MS is a promising approach, which can be applied in combination with EGA-MS to some complex samples containing both wood and consolidating/restoration agents. It reduces the difficulty of interpreting pyrograms containing extremely large numbers of pyrolysis products by producing two simpler chromatograms containing the pyrolysis products of only a fraction of the sample. Future analytical developments aimed at a better understanding of the chemical composition and degradation of lignin in archaeological objects will entail the integration of Py-GC/MS with other analytical techniques. In particular, the analysis of soluble fractions and oligosaccharides by HPLC-MS, and the development of mass spectrometry methods with high resolution imaging capabilities, are interesting avenues for future research. Finally, integrating mass spectrometric approaches into more holistic multi-analytical investigations that target mechanical and physical properties of archaeological wood is fundamental for the study of composite archaeological objects containing inorganic materials and consolidating agents. The study which constituted the basis for this review was performed as a part of the University of Pisa project \"PRA_2018_26 Advanced analytical pyrolysis to study polymers in renewable energy, environment, cultural heritage (2018-2020)\", and was also the starting point for the selection of the analytical procedures to be used in the JPI-Cultural Heritage project \"STAR--Development of Storage and Assessment methods suited for organic Archaeological artefacts\" (2020-2023).