TAG profiling by HPLC/MS for the identification of lipids in historical samples

The identification of the source of lipid materials in archaeological or historical objects can yield important archaeometric information and contribute to the knowledge of past technologies and to the planning of conservation strategies. Lipids can be encountered as paint media or as residues of content in vessels. In particular oils and fats were used not only as food but also as illuminants or ingredients of cosmetics and medicines. Identification of the botanical or animal source of lipids represents an analytical challenge at the present state of the art. A recent approach (1), alternative to GC/MS analysis after saponification, is based on the determination of the overall triglycerides (TAGs) profile determined by HPLC/MS. The possibility to separate and determine intact TAGs without any modification of the samples (which can cause loss of information) is the main advantage of this technique (2). In this work we compare two detection systems for the LC analysis of the TAGs profiles: atmospheric pressure chemical ionization with single quadrupole MS (HPLC-APCI/MS) and electrospray ionisation with quadrupole/time of flight tandem MS (HPLC-ESI-Q/TOF). In both instrumental assets positive-ion ionization was used, and the recognition of TAGs was based on evaluations of the peaks in extracted ion chromatograms of the parent ion mass ([M+H]+ for APCI, [M+Na]+ for ESI) and of known fragments deriving from each TAG. The use of the two detection systems appeared complementary. On the one hand, the fragments are directly formed in APCI due to the excess of energy involved in the corona discharge ionization process and thus APCI-MS can be used for qualitative preliminary evaluation of the overall TAG profile in the sample. On the other hand, thanks to its higher mass resolving power and to the availability of tandem MS, ESI-Q-TOF can be used for the ultimate identification and quantification of selected TAG, chosen as biomarkers. We analyzed reference oils and fats, and organic residues found in XVI century historical ointments of Museo Aboca in San Sepolcro (Arezzo). We identified more than 20 species corresponding to DAG, TAG and to high molecular weight condensation products. Evaluation of the data included application of principal components analysis (PCA) (3).