Electrochemical preparation of standard solutions of Pb(II) ions in ionic liquid for analysis of hydrophobic samples: The olive oil case

In this paper we present an electrochemical approach to prepare standard solutions of metal ions in a room temperature ionic liquid (IL), which can find useful application for analysis in hydrophobic matrices. The method, developed here for the case of lead ions, is based on the galvanostatic dissolution of a lead anode dipped directly in a suitable IL, namely tri-hexyl(tetradecyl)phosphonium bis (trifluoromethylsulfonyl) imide ([P14,6,6,6]+[NTf2]-). After each oxidation step, the metal dissolution process in the IL solutions was monitored by cyclic voltammetric measurements at a glassy carbon disk electrode. The results indicated that the peak current relevant to the reduction of the electro-generated Pb(II) increased linearly while increasing the oxidation time. By varying the oxidation time from 200 to 6000 s, a set of Pb(II)/[P14,6,6,6]+[NTf2]- solutions at concentrations ranging between 10 and 300 ug g-1 was prepared. To validate the efficiency of the electrochemical procedure to produce metal ion standard solutions, the Pb content was quantified by developing a microwave digestion procedure specifically suitable for the IL medium, followed by ICP-QMS analysis in the digested standards. The results indicated a satisfactory agreement between concentrations found by ICP-QMS and calculated from electrochemical data, with a coulometric efficiency of Pb(II) generation in ionic liquid ≥ 95.6%. Finally, the applicability of the Pb(II)/IL solutions as standards for analyses in hydrophobic media was tested by determining, by ICP-QMS, the Pb content in an extra-virgin olive oil spiked with known amounts of a Pb(II)/IL standard. Satisfactory Pb recoveries, ≥ 96%, were measured.