Characterisation of archaeological waterlogged wood by pyrolytic and mass spectrometric techniques

Conservation of Waterlogged Wood—Past, Present and Future Perspectives

This paper reviews the degradation, preservation and conservation of waterlogged archaeological wood. Degradation due to bacteria in anoxic and soft-rot fungi and bacteria in oxic waterlogged conditions is discussed with consideration of the effect on the chemical composition of wood, as well as the deposition of sulphur and iron within the structure. The effects on physical properties are also considered. The paper then discusses the role of consolidants in preserving waterlogged archaeological wood after it is excavated as well as issues to be considered when reburial is used as a means of preservation. The use of alum and polyethylene glycol (PEG) as consolidants is presented along with various case studies with particular emphasis on marine artefacts. The properties of consolidated wood are examined, especially with respect to the degradation of the wood post-conservation. Different consolidants are reviewed along with their use and properties. The merits and risks of reburial and in situ preservation are considered as an alternative to conservation.

Microbial demethylation of lignin: Evidence of enzymes participating in the removal of methyl/methoxyl groups

Lignin is an abundant natural plant aromatic biopolymer containing various functional groups that can be exploited for activating lignin for potential commercial applications. Applications are hindered due to the presence of a high content of methyl/methoxyl groups that affects reactiveness. Various chemical and enzymatic approaches have been investigated to increase the functionality in transforming lignin. Among these is demethylation/demethoxylation, which increases the potential numbers of vicinal hydroxyl groups for applications as phenol-formaldehyde resins. Although the chemical route to lignin demethylation is well-studied, the biological route is still poorly explored. Bacteria and fungi have the ability to demethylate lignin and lignin-related compounds. Considering that appropriate microorganisms possess the biochemical machinery to demethylate lignin by cleaving O-methyl groups liberating methanol, and modify lignin by increasing the vicinal diol content that allows lignin to substitute for phenol in organic polymer syntheses. Certain bacteria through the actions of specific O-demethylases can modify various lignin-related compounds generating vicinal diols and liberating methanol or formaldehyde as end-products. The enzymes include: cytochrome P₄₅₀-aryl-O-demethylase, monooxygenase, veratrate 3-O-demethylase, DDVA O-demethylase (LigX; lignin-related biphenyl 5,5'-dehydrodivanillate (DDVA)), vanillate O-demethylase, syringate O-demethylase, and tetrahydrofolate-dependent-O-demethylase. Although, the fungal counterparts have not been investigated in depth as in bacteria, O-demethylases, nevertheless, have been reported in demethylating various lignin substrates providing evidence of a fungal enzyme system. Few fungi appear to have the ability to secrete O-demethylases. The fungi can mediate lignin demethylation enzymatically (laccase, lignin peroxidase, manganese peroxidase, O-demethylase), or non-enzymatically in brown-rot fungi through the Fenton reaction. This review discusses details on the aspects of microbial (bacterial and fungal) demethylation of lignins and lignin-model compounds and provides evidence of enzymes identified as specific O-demethylases involved in demethylation.

Analytical Pyrolysis and Mass Spectrometry to Characterise Lignin in Archaeological Wood

This review describes the capability of analytical pyrolysis-based techniques to provide data on lignin composition and on the chemical alteration undergone by lignin in archaeological wooden objects. Applications of 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) in archaeological lignin characterisation are described. With comparison to cellulose and hemicelluloses, lignin is generally less prone to most degradation processes affecting archaeological artefacts in burial environments, especially waterlogged ones, which are the most favourable for wood preservation. Nevertheless, lignin also undergoes significant chemical changes. As wood from waterlogged environments is mainly composed of lignin, knowledge of its chemical structure and degradation pathways is fundamental for choosing preventive conservation conditions and for optimising consolidation methods and materials, which directly interact with the residual lignin. Analytical pyrolysis coupled with mass spectrometry, used in several complementary operational modes, can gather information regarding the chemical modifications and the state of preservation of lignin, especially concerning oxidation and depolymerisation phenomena. Several applications to the analysis of wood from archaeological artefacts affected by different conservation problems are presented to showcase the potential of analytical pyrolysis in various scenarios that can be encountered when investigating archaeological waterlogged wood.

Sustainable Nanotechnologies for Curative and Preventive Wood Deacidification Treatments: An Eco-Friendly and Innovative Approach

Waterlogged wooden artifacts represent an important historical legacy of our past. They are very fragile, especially due to the severe phenomenon of acidification that may occur in the presence of acid precursors. To date, a satisfactory solution for the deacidification of ancient wood on a large scale has still not been found. In this paper, we propose, for the first time, eco-friendly curative and preventive treatments using nanoparticles (NPs) of earth alkaline hydroxides dispersed in water and produced on a large scale. We present the characterization of the NPs (by X-ray diffraction, atomic-force and electron microscopy, and small-angle neutron scattering), together with the study of the deacidification efficiency of our treatments. We demonstrate that all our treatments are very effective for both curative and preventive aims, able to assure an almost neutral or slightly alkaline pH of the treated woods. Furthermore, the use of water as a solvent paves the way for large-scale and eco-friendly applications which avoid substances that are harmful for the environment and for human health.

Probing Chemical Changes in Holocellulose and Lignin of Timbers in Ancient Buildings

Wooden structures in China's ancient buildings hold highly historical and cultural values. There is an urgent need to repair and replace the damaged wooden structures after hundreds and thousands of years of exposure to weather. Unfortunately, to date there is still a lack of insightful understanding on how the chemical structure, composition, and micro-morphology evolve over the long-term natural aging before artificial ancient timbers can be developed. This work aims to systematically examine the outer surface, middle layer, and inner surface of the same piece of Chinese fir (Cunninghamia lanceolate) collected from an ancient Chinese building. Based on qualitative and quantitative analysis, both cellulose and hemicellulose in aged woods are found to experience significant degrees of degradation. The crystalline regions of cellulose are also determined to undergo moderate degradation as compared to the control fresh wood. In comparison, the lignin basically remains unchanged and its content in the inner layer slightly increases, as evidenced by more free phenol groups determined. Relative to the outer and inner layer, the middle layer of the ancient wood shows the lowest degree of degradation close to that of the fresh wood. This work offers guidelines for fabricating artificial ancient woods to repair the destroyed ones in China's ancient architectures.

How to Detect Life on Icy Moons

The icy moons of the outer Solar System present the possibility of subsurface water, habitable conditions, and potential abodes for life. Access to evidence that reveals the presence of life on the icy moons can be facilitated by plumes that eject material from the subsurface out into space. One instrument capable of performing life-search investigations at the icy moons is the MAss SPectrometer for Planetary EXploration/Europa (MASPEX), which constitutes a high-resolution, high-sensitivity multibounce time-of-flight mass spectrometer capable of measuring trace amounts (ppb) of organic compounds. MASPEX has been selected for the NASA Europa Clipper mission and will sample any plumes and the surface-sputtered atmosphere to assess any evidence for habitability and life. MASPEX is capable of similar investigations targeted at other icy moons. Data may be forthcoming from direct sampling but also impact dissociation because of the high speed of some analytes. Impact dissociation is analogous to the dissociation provided by modern analytical pyrolysis methods. Radiolytic dissociation on the europan surface before or during the sputtering process can also induce fragmentation similar to pyrolysis. In this study, we have compiled pyrolysis mass spectrometry data from a variety of biological and nonbiological materials to demonstrate the ability of MASPEX to recognize habitability and detect life in any plumes and atmospheres of icy moons. Key Words: Europa-Icy moons-Life detection-Mass spectrometry-Organic matter. Astrobiology 18, 843-855.

Agarotetrol: a source compound for low molecular weight aromatic compounds from agarwood heating

Agarwood is known to generate a distinct fragrance upon heating and is used as both a medicine and a fragrant wood. Low molecular weight aromatic compounds (LACs) such as benzylacetone are emitted from agarwood on heating and have a sedative effect on mice. These are detected exclusively in the headspace vapor of heated agarwood and are absent in the wood itself; hence, some compounds in agarwood are thought to be converted to LACs by the process of heating. In this study, different fractions obtained from agarwood were analyzed to reveal the source compounds of LACs. Some LACs detected in the resinous agarwood were absent from the non-resinous parts and confirmed as characteristic of the resinous parts. The essential oil and hydrosol of agarwood obtained by distillation were analyzed by gas chromatography-mass spectrometry (GC-MS). Sesquiterpenes were detected in the essential oil, and sesquiterpenes and a variety of LACs were detected in the hydrosol. A hot water extract of agarwood remaining in the distillation flask after distillation was analyzed by high-performance liquid chromatography (HPLC), and agarotetrol was found to be the main compound. Purified agarotetrol was heated in a glass vial and its headspace vapor was analyzed by solid-phase microextraction GC-MS. Benzylacetone and other LACs were detected. These results indicate that agarotetrol, a chromone derivative, contributes to the fragrance of agarwood through the generation of LACs upon heating.

Multi-analysis of chemical transformations of lignin macromolecules from waterlogged archaeological wood

A large number of archaeological wooden building poles have been excavated from the Hai Menkou site (Yunnan province, China). Lignin can be transformed and altered accompanied with significant loss of carbohydrates during this process. Elucidation of chemical and structural transformations of lignin is of primary importance for understanding both the nature of degradation processes and the structure of waterlogged archaeological wood, and crucial for developing proper consolidation technology and restoring artifacts of historical and cultural value. In this study, state-of-the-art analytical techniques, including SEM, FT-IR, XRD, CP-MAS ¹³C NMR, 2D-HSQC NMR, ³¹P-NMR, CRM, GPC and TG analysis, were all employed to elucidate the structural characteristics of lignin in waterlogged and reference Pinus wood. The results interpreted by NMR analysis demonstrated the depolymerization of lignin via cleavage of β-O-4, β-5, -OCH₃ and some LCC linkages, leading to a higher amount of free phenol OH groups in the lignin from the ancient waterlogged wood as compared to that of the reference wood. Microscopically, it was found that extensive degradation of carbohydrates in cell walls was mainly occurred in secondary cell walls, while the lignin concentrations were relatively increased in CCML and S regions in the plant cell wall of the ancient wood.

Application of FTIR spectroscopy to the characterization of archeological wood

Two archeological wood samples were studied by attenuated total reflectance Fourier transform infrared (FTIR-ATR) spectroscopy. They originate from a shipwreck in Ribadeo Bay in the northwest of Spain and from a beam wood of an old nave of the Cathedral of Segovia in the central Spain. Principal component analysis was applied to the transposed data matrix (samples as columns and spectral bands as rows) of 43 recorded spectra (18 in the shipwreck and 25 in the beam wood). The results showed differences between the two samples, with a larger proportion of carbohydrates and smaller proportion of lignin in the beam than in the shipwreck wood. Within the beam wood, lignin content was significantly lower in the recent than the old tree rings (P=0.005). These variations can be attributed to species differences between the two woods (oak and pine respectively), with a mixture of guaiacyl and syringyl in hardwood lignin, whereas softwood lignin consists almost exclusively of guaiacyl moieties. The influence of environmental conditions on the FTIR fingerprint was probably reflected by enhanced oxidation of lignin in aerated conditions (beam wood) and hydrolysis of carbohydrates in submerged-anoxic conditions (shipwreck wood). Molecular characterization by analytical pyrolysis of selected samples from each wood type confirmed the interpretation of the mechanisms behind the variability in wood composition obtained by the FTIR-ATR.

Analytical Approaches Based on Gas Chromatography Mass Spectrometry (GC/MS) to Study Organic Materials in Artworks and Archaeological Objects

Gas chromatography/mass spectrometry (GC/MS), after appropriate wet chemical sample pre-treatments or pyrolysis, is one of the most commonly adopted analytical techniques in the study of organic materials from cultural heritage objects. Organic materials in archaeological contexts, in classical art objects, or in modern and contemporary works of art may be the same or belong to the same classes, but can also vary considerably, often presenting different ageing pathways and chemical environments. This paper provides an overview of the literature published in the last 10 years on the research based on the use of GC/MS for the analysis of organic materials in artworks and archaeological objects. The latest progresses in advancing analytical approaches, characterising materials and understanding their degradation, and developing methods for monitoring their stability are discussed. Case studies from the literature are presented to examine how the choice of the working conditions and the analytical approaches is driven by the analytical and technical question to be answered, as well as the nature of the object from which the samples are collected.

Snapshots of lignin oxidation and depolymerization in archaeological wood: an EGA-MS study

Evolved gas analysis-mass spectrometry (EGA-MS) was used for the first time to study archaeological wood, in order to investigate its chemical degradation. The archaeological wood was from an oak pile from a stilt house found in the Neolithic 'La Marmotta' village (Lake Bracciano, Rome, Italy). The sampling was performed from the external to the internal part of the pile, following the annual growth rings in groups of five. In addition, sound oak wood and isolated wood components (holocellulose and cellulose) were also analyzed, and the results were used to highlight differences because of degradation. Our study demonstrated that EGA-MS provides information on the thermo-chemistry of archaeological wood along with in-depth compositional data thanks to the use of MS. Our investigations not only highlighted wood degradation in terms of differences between carbohydrates and lignin content, but also showed that lignin oxidation and depolymerization took place in the archaeological wood. Mass spectral data revealed differences among the archaeological samples from the internal to the external part of the pile. An increase in the formation of wood pyrolysis products bearing a carbonyl group at the benzylic position and a decrease in the amount of lignin dimers were observed. These were related to oxidation and depolymerization reactions, respectively.

Gas chromatography/mass spectrometry and pyrolysis-gas chromatography/mass spectrometry for the chemical characterisation of modern and archaeological figs (Ficus carica)

Gas chromatography/mass spectrometry (GC/MS) after alkaline hydrolysis, solvent extraction and trimethylsilylation, and analytical pyrolysis using hexamethyldisilazane (HMDS) for in situ derivatisation followed by gas chromatographic/mass spectrometric analysis (Pyrolysis-silylation-GC/MS) were used to investigate the hydrolysable and soluble constituents, and the polymerised macromolecules of an archaeological fig (Ficus carica) recovered in Zaragoza (Spain), as well as of modern figs. The main aim was to study the compositional alterations undergone by the fig tissues in a particular archaeological environment: the fig was in a vessel and covered by a layer of a mixture of orpiment and gypsum. A comparison between the GC/MS results from modern and archaeological figs revealed that degradative reactions took place, leading to the disappearance/depletion of reactive (unsaturated fatty acids) and sensitive compounds (phytosterols and triterpenes). Py-silylation-GC/MS data provided evidence of a significant degradation of the saccharide and lipid components of the fig tissue, which left a residue enriched in polyphenols and polyesters.