Electrochemical preconcentration coupled with spectroscopic techniques for trace lead analysis in olive oils

Highlights

• The electrochemical preconcentration of Pb is performed directly in olive oil.

• A Pt working electrode is used as deposition/reoxidation device of the metal ions.

• D-Optimal Design is applied to optimise the experimental electrochemical parameters.

• After medium exchange, the Pb content is quantified by GFAAS or ICP-QMS.

Abstract

In this paper we present a novel combined electrochemical-spectroscopic approach suitable to monitor trace levels of heavy metals directly in edible oils. The method is based on the electrochemical preconcentration/extraction of the analyte from the tested real matrix by cathodic deposition onto a Pt working electrode, then transfer and anodic re-oxidation of the metallic deposit to a “clean” aqueous solution, suitable for the subsequent spectroscopic analysis. The procedure has been here focused to the determination of lead in extra virgin olive oil (EVOO), performed by applying ICP-QMS or GFAAS techniques. To this aim, the EVOO samples were mixed with proper amounts of the room temperature ionic liquid (RTIL) [P_14,6,6,6]⁺[NTf₂]^-, in order to obtain a non-aqueous supporting electrolyte suitable for the electrodeposition process. The feasibility and performance of the analytical strategy were at first tested in standard solutions of Pb(II) in RTIL, produced by anodic dissolution of lead in the RTIL, as well as in olive oil samples mixed with 0.5 M RTIL and spiked with known amounts of Pb(II). The optimisation of the electrochemical parameters was achieved by applying a D-Optimal Design, properly set up to optimise the efficiency of the deposition and re-oxidation steps, quantitative recovery and measurement time. Finally, the analytical procedure was applied to the determination of Pb content in some Italian EVOOs, without any need of performing mineralization pretreatments. Data obtained with the proposed procedure satisfactorily agree with those achieved by ICP-QMS analysis after microwave digestion, being differences between the two approaches within 10%, with the advantage of reducing to half the pretreatment time, operating at room temperature and avoiding the use of aggressive solvents.

Introduction

Trace amounts of heavy metals can be present in olive oil because of contaminations originating from different sources, such as soil and fertilizers, production or storage procedures, or exposition of the olive plants to vehicular and industrial emissions [[1], [2], [3], [4], [5]]. The quality of the oil is strictly related to the concentration of metal species present in the final product, since trace elements like Cu, Fe, Ni and Zn may catalyse reactions that promote the oxidative degradation of the edible oil. Moreover, other metals, such as Pb, Cd or Hg, are potentially toxic for human consumption [4,5]. Thus, the determination of trace metals content in edible oil is crucial for assessing the quality, both from health and economic points of view. However, this analytical goal constitutes a challenging task, due to the very low concentration levels of these analytes, as well as to the high complexity of the organic matrix of vegetable oils. In particular, the analysis of metal ions in oil using conventional analytical instrumental techniques requires the application of complex and time-consuming pre-treatment steps, which are a potential source of contamination of the sample or loss of analyte, possibly reflected in scarce accuracy and precision. By far, atomic spectroscopic techniques, including graphite furnace atomic absorption spectrometry (GFAAS) [4,[6], [7], [8], [9], [10], [11], [12], [13], [14]], flame atomic absorption spectrometry (FAAS) [4,8,15,16], inductively coupled plasma atomic emission spectrometry (ICP-AES) [[6], [7], [8]] and inductively coupled plasma mass spectrometry (ICP-MS) [3,5,17,18] are the techniques most commonly used for the analysis of trace metal in edible oils. A few papers have been also reported on the use of electroanalytical methods, such as derivative potentiometric [1,2,19] and voltammetric stripping analysis [20] using Hg-based electrodes.

Whatever the analytical technique used, sample preparation is a critical step in the whole analytical process. Various pre-treatment procedures have been proposed and applied to the analysis of edible oils, most of them including dry or wet ashing [1,8,10], microwave-assisted acid digestion [[3], [4], [5], [6], [7], [8],17,18,20], dilution with organic solvents [8,9,21], liquid-liquid extraction [10,11,13,19,22,23], emulsion or microemulsion preparations [8,16]. All approaches show some advantages, but also several drawbacks, especially in terms of time-consuming procedures, aggressive conditions, use of hazardous solvents even in large amounts, low recovery rates, contamination risks [[10], [11], [12], [13],22]. Accordingly, improvements and optimisation of the sample pre-treatment step are highly demanded.

With the goal of developing a fast and reliable analytical approach, specifically suitable to monitor trace levels of heavy metals directly in edible oils, in the present study a novel strategy is presented, which combines electrochemical preconcentration with spectroscopic analysis, focusing on the determination of lead in extra-virgin olive oil as a case study. In the literature, the introduction of an electrochemical preconcentration (EC) step as an auxiliary tool for subsequent ICP-MS or ICP-AES determination of trace metals in complex aqueous matrices, has been proposed [[24], [25], [26], [27]]. For instance, such an approach has been used to detect As (III) and Se (IV) [25], Cr (VI) and V(V) [26], Cu and Cd [27] in environmental and biological samples, and Hg in process and lagoon waters [24].

The alternative approach developed here is based on the use of a platinum spiral electrode as preconcentration/extraction tool, where the metal species of interest, namely Pb, is at first electrochemically deposited. After suitable medium exchange, the deposited metal is anodically re-dissolved in a “clean” aqueous solution, suitable for ICP-MS or GFAAS analysis, or other kind of technique. In order to perform the electrochemical preconcentration of the metal from a complex and low-conductive matrix such as olive oil, the room temperature ionic liquid (RTIL) tri-hexyl (tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide [P_14,6,6,6]⁺[NTf₂]^-, which is soluble in vegetable oils, was used as the supporting electrolyte [[28], [29], [30], [31]]. For evaluating quantitatively the performances of the procedure, blank oil samples were spiked with known amounts of Pb in RTIL. For this purpose, standard solutions of lead in [P_14,6,6,6]⁺[NTf₂]^- were produced by galvanostatic anodic dissolution directly in the RTIL medium of a lead anode, using the electrochemical procedure recently developed in our laboratory [32].

Optimisation of the electrochemical parameters ruling deposition and re-oxidation of lead onto and from the Pt working electrode was performed by resorting to D-Optimal design [[33], [34], [35]]. A key feature of D-Optimal design – exploited in the present study – is the possibility of considering experiments previously performed, obtaining a good design by adding to them a relatively small number of new experiments [36].

Finally, the selected analytical protocol was applied to the combined electrochemical preconcentration and GF-AAS or ICP-QMS determination of the Pb content in real samples of Italian extra-virgin olive oil, without any mineralization of the matrix.