A Combined Electrochemical and Spectroscopic Approach for Trace Lead Analysis in olive Oils using RTIL Electrolyte

Trace heavy metals dangerous to human health can be present not only in aqueous solutions, but also in non-aqueous matrices such as oil or fat. The analysis of metal ions in such highly viscous organic matrix by using conventional analytical procedures is rather challenging since it requires the application of series of delicate and time-consuming pretreatment steps which can be a source of contamination of the sample, possibly reflecting in scarce accuracy and precision. In this work we present a novel analytical approach that combines electrochemical preconcentration with spectroscopic analysis, focusing on the determination of lead in oil as a case study. In order to perform electrochemical experiments in such a complex and low-conductive food matrix, the room temperature ionic liquid (RTIL) [P14,6,6,6]+[NTf2]-, which is soluble in vegetable oils, was used as supporting electrolyte [1,2]. For the development and validation of the analytical procedure, at first, standard solutions of lead in non-aqueous medium were generated by galvanostatic anodic dissolution of high-purity Pb in RTIL. By controlling the dissolution time, a set of Pb2+/[P14,6,6,6]+[NTf2]- standards with C = 10 ÷ 200 mg Kg-1, was prepared. Such a procedure was validated by AAS or ICP-MS analysis of the Pb2+ standards so produced, after careful mineralization in a microwave unit by a digestion protocol properly set up. The final analytical strategy developed for the assessment of Pb content in oils include: mixing of the oil with the RTIL; potentiostatic electrochemical metal deposition from the tested oil sample; potentiostatic anodic stripping of the metal deposit after transfer to aqueous solution; spectroscopic (ICP-MS or GF-AAS) or voltammetric analysis of the solution collected. The feasibility and performances of the analytical protocol were tested and validated in standard solutions of Pb2+ in RTIL, and in oil samples mixed with RTIL and spiked with Pb. The proposed procedure was finally applied to the determination of Pb content in some Italian olive oils. The combination of a simple and fast electrochemical preconcentration/stripping protocol followed by spectroscopic analytical quantification represents a promising analytical strategy for the assessment of trace metal contamination in edible oils, avoiding mineralization or other time-consuming pretreatments of the complex food matrix.