Organic Residue Analysis
We do lipid analysis and the extraction and interpretation of archaeological information recovered at the molecular level.
What are lipids?
Lipids are a heterogeneous group of molecules which includes fats, oils and waxes. Lipids are soluble in common organic solvents and are more resistant to water leaching and degradation than proteins, DNA etc.
What we do?
We can identify materials such as:
- resins to the level of Pinacea = pines and firs (also frankincense, myrrh, pistacia - but unlikely in N Europe!).
- waxes (such as beeswax).
- fats and oils (sometimes separating animal from plant) also processed materials such as pine pitches and birch bark tar.
- and of course mixtures of the above.
We identify samples by comparing the abundance and distribution of individual components with our experience of authentic modern samples.
One problem however is degradation, and although lipids are more resistant than other materials, the yields can be low and some samples can be so degraded that it is difficult to distinguish the exact source - e.g. plant oils are rich in oleic acid, but this degrades to give a fatty acid distribution similar to animal fats.
Characterisation of organic residues generally relies upon the principles of chemotaxonomy, where the presence of a specific 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 (biomarkers) is not without its problems since many compounds are widely distributed in a range of natural substances, and the composition of an ancient residue may have changed significantly during burial.
Techniques of analysis
Techniques of analysis
Gas chromatography (GC)
A sensitive separation technique in which the components of a volatile sample are partitioned between two phases; a mobile gaseous phase, and a stationary liquid phase bonded onto the inside of a column. The ‘chromatogram’ shows a trace of the variation in component concentration against time and may be used to obtain qualitative and quantitative information.
Combined gas chromatography - mass spectrometry (GC-MS)
Coupling a mass spectrometer to the effluent of a GC combines the powerful analytical capability of mass spectrometry with the high degree of separation possible with GC.
At the simplest level, the mass spectrometer ionises molecules, then identifies the ions according to their mass-to-charge (m/z) ratio resulting in the generation of a mass spectrum (ion abundance against the m/z value).
In many cases, mass spectra allow the individual compounds present to be detected with some certainty. GC/MS has been used widely in the identification of ancient lipid residues, resins, waxes and so on.
GC-combustion-isotope ratio mass spectrometry (GC-C-IRMS)
The recent introduction of gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-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 utilising different photosynthetic pathways.
These distinctive ratios are passed along the food chain to herbivores and carnivores. There is evidence that there may be differences in δ13C values between ruminants and non-ruminants.
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.
Dr Ben Stern
Tel: + 44 (0)1274 23 6054