Chemical investigations of bitumen from Neolithic archaeological excavations in Italy by GC/MS combined with principal component analysis

An analytical protocol involving microwave assisted solvent extraction and fractionation on silica gel columns followed by gas chromatography/mass spectrometry (GC/MS) and principal component analysis (PCA) of the chromatographic data was used for the characterization of bituminous residues sampled from Neolithic flint flakes and potsherds recovered from archaeological excavations in Abruzzo and Apulia (Italy). The analytical protocol was optimized and primarily tested in the study of geological bitumen (used as reference materials) from rocks and sediments of central-southern Italy (Abruzzo, Sicily and Lazio), and subsequently used to characterize the archaeological bitumen. Since bitumen is usually present in very low amounts in archaeological objects, we paid attention to improve the extraction efficiency of terpanes and steranes, the main soluble components of bitumen. The highest efficiency was obtained using microwave assisted extraction with a mixture of n-hexane/dichloromethane/methanol (80:15:5, v/v/v). Given that the composition of the bitumen varies depending on the area of origin, the results obtained from the archaeological materials allowed us not only to draw hypotheses on the possible function of tools/objects from which the bitumen is sampled, but also to obtain information on its geographical origin. In particular, PCA, used as a tool for an extensive analysis of chromatographic data, enabled us to correlate the quantitative chemical composition and the geographical origin of the samples, and finally to distinguish bitumen originating from the different Italian sites, based on their molecular profiles.