Analytical strategies for characterizing organic paint media using gas chromatography/mass spectrometry

Throughout history, artists have experimented with a variety of organic-based natural materials, using them as paint binders, varnishes, and ingredients for mordants in gildings. Artists often use many layers of paint to produce particular effects. How we see a painting is thus the final result of how this complex, highly heterogeneous, multimaterial, and multilayered structure interacts with light. The chemical characterization of the organic substances in paint materials is of great importance for artwork conservation because the organic components of the paint layers are particularly subject to degradation. In addition, understanding the organic content and makeup of paint materials allows us to differentiate between the painting techniques that have been used over history. Applying gas chromatography/mass spectrometry (GC/MS) analysis to microsamples of paint layers is widely recognized as the best approach for identifying organic materials, such as proteins, drying oils, waxes, terpenic resins, and polysaccharide gums. The method provides essential information for reconstructing artistic techniques, assessing the best conditions for long-term preservation, and planning restoration. In this Account, we summarize the more common approaches adopted in the study of the organic components of paint materials. Our progress in developing GC/MS analytical procedures in the field of cultural heritage is presented, focusing on problems that arise from (i) the presence of mixtures of many chemically complex and degraded materials, (ii) the interference of inorganic species, (iii) the small size of the samples, and (iv) the risk of contamination. We outline some critical aspects of the analytical strategy, such as the need to optimize specific wet-chemical sample pretreatments in order to separate the various components, hydrolyze macromolecular analytes, clean-up inorganic ions, and derivatize polar molecules for subsequent GC/MS separation and identification. We also discuss how to interpret the chromatographic data so as to be able to identify the materials. This identification is based on the presence of specific biomarkers (chemotaxonomy), on the evaluation of the overall chromatographic profile, or on the quantitative analysis of significant compounds. GC/MS-based analytical procedures have for 20 years provided important contributions to conservation science, but challenges and opportunities still coexist in the field of organic-based paint materials. We give selected examples and provide case studies showing how a better understanding of the chemical composition of organic paint materials and of their degradation pathways contribute to a better knowledge our cultural heritage, and to its preservation for future generations.