Characterisation of degraded paper in the field of cultural heritage using SEC/MALS and CE/ESI-MS
Résumé
Better understanding the parameters influencing the durability of historic papers and lignocellulosic materials in general is a fundamental area of research in the field of preservation of historic paper-based artefacts. The state of degradation of cellulose determines the mechanical properties of paper, which ultimately is the criterion to be related to the useful lifespan of paper. The main cause of decay of paper is its acid content in combination with time and conditions of exposure, as upon ageing cellulose undergoes acid catalysed hydrolysis and oxidation reactions. The macromolecular characterisation of cellulose is thus a pivotal aspect in the research and sensitive analytical methods have to be developed to possibly enable diagnostics of the alteration levels with application to historic objects. In that respect, size-exclusion chromatography (SEC) is a technique of choice because it is very sensitive to evidence early degradation. Coupled on-line with multi-angle light scattering (MALS) detection, the information on the molar mass and the size distributions, as well as the polydispersity of a polymer is extremely precise as absolute values of the molar mass averages are determined. The unique advantage of MALS experiments in chromatography mode is that no calibration is required. SEC/MALS thus provides a fine and accurate characterisation of cellulose upon dissolution of paper in an appropriate solvent (lithium chloride/N,N-dimethylacetamide).
Another aspect to be considered for the comprehensive study of the deterioration of cellulosic cultural heritage relates to the by-products and fragments from paper degradation. A large part of the molecules formed have low-molar mass, and depending on which polymer they originate from in the paper (cellulose, hemicelluloses, lignin), they are aliphatic and aromatic compounds with carbonyl, and carboxyl functionalities, and carbohydrates. Due to sampling constraints, a primary aim in the field of cultural heritage preservation science is to establish useful micro-destructive, and whenever possible non-invasive, methodologies. As material sampling is often unavoidable, these techniques should evolve towards sample miniaturisation. Capillary electrophoresis (CE) is well adapted to minute amounts of sample. The performance of this technique in terms of selectivity, efficiency and resolving power is even enhanced when coupled to high sensitivity detectors such as mass spectrometers (MS), which provide molar mass (m/z) and some structural information. CE/ESI-MS (electrospray ion source) in particular is especially adapted for the analysis of complex samples.
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