Gas and liquid chromatography approaches coupled to mass spectrometry for the evaluation of thermal-oxidation lipid products in aged studies

Lipids are the main chemical class of molecules studied in archaeological pottery, thanks to their hydrophobicity that makes them particularly durable over time compared to other organic components such as carbohydrates and proteins [1]. However, most of lipids are featured by reactive functional groups that fatally lead to their degradation in archaeological site, furtherly complicating the interpretations on the origin of lipid matter [2]. Thermal decomposition, oxidation and hydrolysis reactions have been already investigated in laboratory in order to simulate the degradation of lipids. Such ageing studies can provide key elements to better address the origin of animal fats and plant oils resulting partially or totally altered over time. For this purpose, thermal-oxidative treatments have been carried out in laboratory in order to simulate the natural degradation of the lipid matter. In detail, triglyceride standards (tristearin, triolein, trilinolein and trilinolenin) and extra-virgin olive oil (EVOO) samples were kept in an oven at 120°C for three weeks. Degradation products were investigated by using gas chromatographic (GC) and liquid chromatographic (LC) techniques coupled to mass spectrometry (MS) detection. GC-amenable substances were investigated according to their volatility as follows: hyper- and medium volatility species, including short-chain fatty acids, aldehydes, ketones, lactones and esters, were analyzed by using head-space solid-phase microextraction (HS-SPME) techniques coupled to GC-MS analyses, while low-volatility and polar compounds, such as medium- and long-chain fatty acids or (α,ω)-dicarboxylic acids, were analyzed by GC-MS after conversion into trimethyl silyl ether (TMS) derivatives. Intact lipids were investigated by using Non-Aqueous Reversed Phase High Performance Liquid Chromatography coupled to Atmospheric Pressure Chemical Ionization Mass Spectrometry. Thermo-oxidation products were uniquely identified by the use of two different identification parameters: spectral similarity and linear retention index (LRI) correspondence for both analytical techniques. [1] Stern, B.; Heron, C.; Tellefsen, T.; Serpico, M., New investigations into the Uluburun resin cargo. J. Archaeol. Sci. 2008, 35, (8), 2188-2203. [2] Regert, M. Analytical strategies for discriminating archeological fatty substances from animal origin. Mass. Spectrom. Rev. 2011, 30, (2), 177-220.