Thermal Desorption Gas Chromatography–Mass Spectrometry Methods for Minimally Invasive Organic Residue Analysis of Archeological Potsherds

Organic residue analysis (ORA) is widely used to investigate lipids preserved in archeological pottery. However, the amount of sample typically involved (2 g) precludes its application to particularly precious, small, or brittle sherds. This paper presents a novel, minimally invasive, protocol to overcome this limitation. By using a thermal separation probe (TSP) coupled to gas chromatography-mass spectrometry (GC-MS), this approach utilizes thermally assisted hydrolysis and methylation (THM), reducing sample sizes to as little as 1 mg. Testing with standard solutions and modern lipid-doped sherds confirmed that the method enables recovery of fatty acids (FAs) and determination of their carbon isotope compositions. The new approach was then validated using a range of archeological sherds previously analyzed using established liquid extraction-ORA approaches. In addition to fatty acids, other lipid biomarkers such as miliacin and ω-(o-alkylphenyl)alkanoic acids (APAAs) were recovered. With the presented THM method, results comparable to established ORA methods are obtained but, crucially, sample sizes are reduced by nearly 3 orders of magnitude. This widens the application of ORA, for example, by enabling the study of valuable objects, permitting lipid profiling on different areas of one sherd, or serving as a rapid screening method ahead of compound-specific radiocarbon dating.