Direct exposure electron ionization mass spectrometry and gas chromatography/mass spectrometry techniques to study organic coatings in archaeological amphorae

Two different analytical approaches, direct exposure electron ionisation mass spectrometry (DE-MS) and gas chromatography/mass spectrometry (GC/MS), were compared in a study on archaeological resinous materials. DE-MS was found to be an efficient fingerprinting tool for the fast screening of organic archaeological samples and for providing information on the major components. GC/MS appeared to be more efficient in unravelling the sample composition at a molecular level, despite the length of analysis time and the need for a wet chemical pre-treatment. Both analytical procedures were applied to characterise the organic material present as coatings in Roman and Egyptian amphorae. DE-MS successfully identified abietanic compounds, thus a diterpenic resinous material could be identified and its degree of oxidation assessed. GC/MS enabled us to identify dehydroabietic acid, 7-oxo-dehydroabietic acid, 15-hydroxy-7-oxo-dehydroabietic acid, 15-hydroxy-dehydroabietic acid, retene, tetrahydroretene, norabietatriene, norabietatetraene, and methyl-dehydroabietate. These oxidised and aromatised abietanes provided evidence that the amphorae examined were waterproofed with a pitch produced from resinous wood of plants from the Pinaceae family. The chemometric evaluation of the GC/MS data highlighted significant chemical differences between the pitches found in the two archaeological sites, basically related to differences in the production techniques of the materials and in their degradation pathways.