# Analytical Pyrolysis and Mass Spectrometry to Characterise Lignin in Archaeological Wood
## 3 Results and Discussion
### Direct Exposure Mass Spectrometry (DE-MS)  
The instrumental assets where pyrolysis products are directly identified by MS without chromatographic separation step are referred to as Pyrolysis Mass Spectrometry (Py-MS), Direct Exposure Mass Spectrometry (DE-MS) or Direct Temperature resolved Mass Spectrometry (DT-MS). Apart from some differences in the instrumental apparatus, the three analytical techniques are based on the same principle: The sample is desorbed or pyrolysed by controlled heating directly into the ion source [46, 50, 51, 52, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80]. Pyrolysis coupled directly to mass spectrometry allows for reduction of analysis time and extends the detectability of pyrolysis fragments to structures too large for gas chromatography analysis. At the same time, the results are generally difficult to interpret, as an overall mass spectrum of all pyrolysis products is obtained. For this reason, multivariate analysis of mass spectral data, such as principal component analysis (PCA), has sometimes been used to compare, differentiate, and classify the samples [38, 48, 49, 50, 51, 52, 54]. DE-MS and PCA were applied to study the state of degradation of archaeological waterlogged wood and, in particular, investigate the chemical modification of lignin fraction in the objects from two Italian archaeological sites (the Ancient Harbour of Pisa San Rossoe [54] and La Marmotta [33]) and from the two Polish archaeological sites (Biskupin and Zolte [39, 53]). The results were compared to those obtained for sound non-degraded wood of the same species, such as pine (_Pinus pinaster_), oak (_Quercus robur_), elm (_Ulmus minor_), alder (_Almus glutinosa_), and beech (_Fagus sylvatica_). Analysis of DE-MS mass spectral data corresponding to the mass range 50-350 \(m/z\) was performed by principal component analysis (PCA, Nipals algorithm) [81] on the covariance matrix of centred data, after row normalisation of the full 50-500 \(m/z\) spectra. The region 50-350 \(m/z\) was selected because it contains all the pyrolysis fragments corresponding to lignin monomers and dimers. The software used was XLSTAT 2009.1.02. (Addinsoft, Paris, France). Figure 1 shows the mass spectra obtained by the analysis of a reference oak wood sample, one sample from the outermost part of a pile from La Marmotta, and one sample from fragment 6 (oak wood) collected from the Biskupin site. The mass spectra showed high complexity. The results were interpreted on the basis of the attribution of \(m/z\) values [33]. Table 2 lists the fragments identified and the most probable assignments, based on the mass spectra of guaiacyl and syringl compounds produced in the pyrolysis of lignin. \begin{table}
\begin{tabular}{c c c} \hline \(m/z\) & **Molecular Origin** & **Wood Component** & **References** \\ \hline
65 & present in many pyrolysis products of both lignin and carbohydrates & L/C & [82, 83, 84] \\ \hline
77 & present in many pyrolysis products of both lignin and carbohydrates & L/C & [82, 83, 84] \\ \hline
91 & alkylbenzes & L & [83] \\ \hline
94 & phenol & L & [52, 82, 83, 84] \\ \hline \end{tabular}
\end{table} 
Table 2: Molecular assignment of the most abundant \(m/z\) values observed in the DE-MS mass spectra of lignin. L—lginin; C-carbohydrates; G—guaiacyl-lignin; S—syringyl-lignin. The archaeological samples, as expected, showed low signals of carbohydrates compared to the reference sample [54]. The sample from Biskupin still contained some carbohydrates, as highlighted by the intensity of the peaks at \(m/z\) 65 and 77 [52], whereas the sample from La Marmotta was mainly composed of lignin (Figure 1). In addition to the possibility of evaluating the state of degradation of wood from the polysaccharide point of view, it is also possible to observe some degradation processes occurring in the lignin fraction following some characteristic \(m/z\) values.