A novel analytical strategy for the assessment of trace heavy metal contamination in olive oils

Free fatty acids (FFAs) content in olive oil is a key parameter routinely determined to classify and/or assess the quality, the freshness, and the economic value of the final product available on the market. The official method, suggested by ECC Regulation, for the quantification of the olive oil acidity - expressed as mass percentage of free oleic acid -, is a classical acid/base volumetric titration, performed in non-aqueous solvents [1]. The procedure is simple, but very slow, laborious and organic solvent consuming. Considering the large number of routine measurements necessary for marketing purposes, the development of fast, simple, low-cost, solvent-free and reliable instrumental analytical methodologies for quantifying the olive oil acidity is a task of great interest. To this purpose, in the present study an electroanalytical strategy for a direct evaluation of the acidity in olive oils is presented. The experimental conditions for exploiting the reduction process of the acidic species present in this food matrix were defined and optimised, under both deoxygenated and oxygenated conditions, taking as acid probe the oleic acid (OA), which is the prevailing long-chain FFA component of olive oil. Oil samples were prepared by adding, as supporting electrolyte, the room temperature ionic liquid (RTIL) trihexyl( tetradecyl)phosphonium bis(trifluoromethylsulfonyl) imide ([P14,6,6,6]+[NTf2]-), that is soluble in vegetable oil and enhance the matrix conductivity [2,3]. The measurements were performed in a 2 mL glass vial as electrochemical cell, using a 12.5 mm radius Pt microdisk as working electrode. By applying a properly optimised chronoamperometric setup, reliable acidity data in some extravirgin, virgin, and lamp olive oil samples were found. The results obtained indicates that the electrochemical procedure developped can be advantageously exploited as a fast, low-cost and solvent-saving analytical tool for the determination of olive oil acidity.