Discrimination of Acacia gums by MALDI-TOF MS: applications to micro-samples from works of art

This paper reports the discrimination of Acacia gums from the two most widely used and commercially significant species, A. senegal and A. seyal, using an innovative strategy based on matrix assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS) after enzymatic digestion of the polysaccharide component. MALDI-TOF MS/MS experiments were carried out to determine the composition of the most intense ions of A. seyal gum and elucidate its mass fingerprint. The analytical method was applied to the analysis of gums from 12 other Acacia species and an attempt was made to find specific oligosaccharides that could be used to discriminate among them. Statistical analysis was performed to further explore the capability to differentiate these species. The implementation of a library of MALDI mass profiles of plant gums, with an initial focus on Acacia gums, represents the first MS-based attempt to facilitate reliable, species-level identification of these materials in unknown samples. The technique was successfully applied to the analysis of micro-samples from art objects of different types, with regard to geographical origin and date, including artefacts from ancient Egypt and a 20th century painting by Georges Braque, in the collections of The Metropolitan Museum of Art, New York, and the Art Institute of Chicago. Results demonstrated the possibility to detect Acacia gums from different species even in complex and aged micro-samples, providing important insights into the materials and condition of the objects. This research shows the potential to open new avenues of art historical and technical investigation into the specific plant sources, trade, and selection of materials by artists.