Application of isotope ratio monitoring gas chromatography–mass spectrometry to the analysis of organic residues of archaeological origin

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Abstract
Isotope ratio monitoring gas chromatography–mass spectrometry (irm GC–MS) was used to determine the δ13C values of individual compounds present in organic residues preserved in archaeological potsherds. The lipid fractions of the preserved organic residues were extracted from potsherds recovered from excavations performed as part of the Raunds Area Project, Northamptonshire, UK. The lipid extracts were analysed by high temperature GC and GC–MS. The leaf wax of a contemporary wild-type Brassica was also extracted and analysed for comparative purposes. Prior to irm GC–MS analysis, lipid extracts were fractionated; alkane and ketone fractions were obtained from the wild-type Brassica leaf wax, the total lipid extracts of two of the potsherds, and an archaeological soil sample using small-scale flash chromatography. The δ13C values obtained for the samples were consistent with the individual lipids investigated being of C3 plant origin. These data support the hypothesis that the lipids preserved in the potsherds were derived from a Brassica species such as cabbage. These results confirm the potential value of irm GC–MS to derive stable isotope ratios for individual lipid species of archaeological interest, and, more specifically, the potential for application of the technique to palaeodietary investigations.