Session: Parallel session 5 - Forensic and cultural heritage

Oil Paints identification by advanced mass spectrometry

Bayan ALMASRI^1,2, Giulia GEMELLI¹, Christian ROLANDO^1,3

¹Miniaturization for Synthesis, Analysis & Proteomics (MSAP), UAR 3290, CNRS, University of Lille, Faculty of Sciences & Technologies, Villeneuve d'Ascq, France
²Laboratory of Applied Chemistry (LAC), Lebanese University, Faculty of Sciences, Tripoli, Lebanon
³Shrieking Sixties, 1-3 Allée Lavoisier, Villeneuve d'Ascq, France

Oil paintings are invaluable cultural heritage objects which can be preserved from centuries. Oil paints are formed of inorganic pigments mainly and a vegetable siccative oil as binder, the most common ones being linseed, poppy and walnut oils. Linseed oil triacylglycerols (TAGs) are mostly linolenic acid, C18 fatty acid with 3 double bonds (C18:3), whereas poppy and walnut oils contain mostly linoleic acid, C18 fatty acid with 2 double bonds (C18:2). In the presence of light and atmospheric oxygen, these oils polymerize by siccativation forming a 3D insoluble network which is directly linked to their content of polyunsaturated fatty acids. We developed an original methodology for oil paints analysis based on mild transamidation cleavage of glycerol esters, leading to the identification of modified and crosslinked fatty acids and oil degradation markers. The N,N-dimethylpropane-1,3-diamine (DMAPA) was chosen due to the presence of the tertiary amine that improves MS ionization either in MALDI or ESI ionization. We also developed a labelling of the polyols by derivatizing them with p-dimethylaminobenzoyl chloride. The oil depolymerization protocol is improved by swelling the oil paint flake in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) before transamidation. The depolymerized oil was analyzed by MALDI FTICR MS and ESI LC-MS using an Orbitrap. After having optimized our protocols, we applied them to mock-up samples made with different pigments after different ageing periods. The spectra obtained show traces of the remaining polyunsaturated fatty acids and the formation of dimers linked by C-C or C-O-C bonds. The distribution of products in the 3D reticulated network depends both on the siccative oil and also on the pigment. We scaled our methodology down to the analysis of tenths of micrograms size samples, the tiny quantities commonly available from artworks in museums. Example of analysis of art paintings from the XIXth to the XXth centuries will be presented.