ARCHAEOLOGY: USES OF CHROMATOGRAPHY IN

Abstract

Although recent advances in analytical methods have accelerated the study of these materials, the analysis and identiRcation of ancient organic residues has a long history. An early example, in the 1920s, was the use of wet chemical techniques by the chemist Alfred Lucas to study organic material from pottery and mummiRed human remains from the tomb of Tutankhamun. Over the last 20 years or so, the analysis of organic residues has grown into a recognized Reld in its own right. Examples of organic residues include the debris associated with the remains of food and other natural products as a result of their manipulation in pottery containers (e.g. cooking of food), the balms in the wrappings of mummiRed bodies and traces of colouring dyes impregnated in ancient textiles. Given the amorphous character of organic residues, the most effective approach to their identiRcation lies in their chemical composition. Characterization of organic residues generally relies upon the principles of chemotaxonomy, where the presence of a speciRc compound or distribution of compounds in an unknown sample is matched with its presence in a contemporary natural substance. The use of such molecular markers is not without its problems, since many compounds are widely distributed in a range of natural materials, and the composition of an ancient residue may have changed signiRcantly during burial. In general, molecular markers belong to the compound class deRned as the lipids, a heterogeneous group of molecules which includes fats and oils and molecules with common solubilities, such as the constituents of resins and waxes. Early work in this Reld relied heavily on either thin-layer chromatography (TLC) or gas chromatography (GC) alone to characterize residues. Today, combined GC}mass spectrometry (GC-MS) and, to a lesser extent, high-performance liquid chromatography}MS (LC-MS) are demonstrating considerable value in identifying ancient organic matter. The wider availability of these techniques and, in particular, the introduction of GC}isotope ratio mass spectrometry (GC}IRMS), is contributing to more speciRc identiRcations than was possible before. GC}IRMS allows the ratios of abundances of stable isotopes of elements such as carbon and nitrogen to be determined for individual compounds introduced via a gas chromatograph. Stable isotope ratios are of particular importance to studies of foodwebs due to the characteristic isotope signatures of plants utilizing different photosynthetic pathways. These distinctive ratios are passed along the food chain to herbivores and carnivores. The method requires very small samples and is being applied to trace organic residues in pottery vessels to establish their origin with a high degree of precision.