Method development for the determination of residual pesticides and heavy metals in complex samples using modern preconcentration techniques

Abstract

In this work, modern pre-concentration techniques, namely dispersive liquid-liquid micro-extraction (DLLME) and QuEChERS, were used to analyse pesticides and heavy metals in complex matrices. The work is divided into six papers. In Papers 1, the recent developments and applications of DLLME during analysis of pesticides in food matrices were reviewed. The DLLME technique has captured the interests of many researchers in recent years. The major advantage, among others, of DLLME is miniaturisation in which the acceptor-to-donor ratio is reduced tremendously leading to high enrichment compared to other sample preparation techniques. In the present work, the different complex matrices where the DLLME technique has been employed for the analysis of pesticides are reviewed as well as the challenges associated with this technique. Papers II reviewed the recent applications and developments of the QuEChERS technique during the analysis of pesticides in food matrices. QuEChERS is a versatile pre-concentration method whose application spans the whole breath of organic compounds. There are three common standard methods used during QuEChERS and these are the original QuEChERS, AOAC and the EN methods. In this paper, recent developments and applications of QuEChERS techniques in the analysis of pesticides in food samples were reviewed. In Paper III, green pre-concentration techniques employed during analysis of pesticides were reviewed. Recently, the parameter of “greenness” during sample pre-concentration of pesticides in food matrices is as important as selectivity in order to avoid using large amounts of harmful organic solvents during sample preparation. Developing new green pre-concentration techniques is one of the key subjects in green chemistry in order to minimize the release of large volume of toxic organic solvents into the environment. Thus, to reduce the impact on the environment during trace analysis of pesticides in food matrices, new developments in pesticide pre-concentration have gone in three separate directions (which are reviewed in this paper): one is the search for more environmentally friendly solvents, the second one is miniaturization and the third one is the development of solvent-free pre-concentration techniques. Eco-friendly solvents such as supercritical fluids, ionic liquids and natural deep eutectic solvents have been developed for use as extraction solvents during pre-concentration of pesticides in food matrices. Also miniaturized pre-concentration techniques such as QuEChERS, dispersive liquid-liquid micro-extraction and hollow-fibre liquid phase micro-extraction have been used during trace analysis of pesticides in food samples as well as solvent-free techniques such as solid phase micro-extraction and stir bar sorptive extraction. All these developments are geared to ensure that pesticide pre-concentration in food matrices is green and were reviewed in this paper. The effect of vehicular emissions on the concentrations of selected heavy metals was investigated in Paper IV. The samples were pre-concentrated using DLLME prior to analysis with flame atomic absorption spectroscopy. Dithizone, chloroform and methanol were used as chelating agent, extraction solvent and dispersion solvent respectively during the DLLME technique. The pH of the sample was adjusted to around 8 using sulphuric acid or sodium hydroxide solution. The influential DLLME parameters, such as pH volume and type of extraction solvent, and voume of disperser solvent, were optimized prior to the application of the developed method to real samples (roadside dust, fruits and vegetables). In Paper V, chromium speciation in fruits and vegetables was studied. The chromium in fruit and vegetable sample juices was pre-concentrated using DLLME prior to analysis with flame atomic absorption spectroscopy. Diphenylcarbazide (DPC) was used as a chelating agent in this study, and salting out of the complex from the aqueous medium into the organic phase was effected using sodium acetate. Chloroform and methanol were used as extraction and dispersion solvents respectively in the DLLME method for the determination of chromium (VI). For total chromium, the trivalent chromium was oxidised using acidified KMnO4 to hexavalent chromium before performing the DLLME technique. The concentration of chromium (III) was determined by finding the difference between total chromium and concentration of chromium (VI). The important parameters that influence the efficiency of the DLLME technique were also optimized using the univariate approach. After optimization, the developed method was applied to real samples. In Paper VI, the concentration of malathion pesticide in fruits was determined using QuECHERS for pre-concentration and UV-Vis spectrophotometry for instrumental analysis. Acetonitrile was used as the extraction solvent and Z-sep+/PSA sorbent combination was used for sample clean-up. The acetonitrile extract from QuEChERS was then hydrolysed using KOH followed by reaction with acidified potassium bromate for colour development. The coloured product formed was then analysed using UV-Vis spectrophotometry. Among the fruits analysed, Oranges had no malathion residue in them. However, trace amounts of malathion, below WHO maximum allowable limits, were found in pears and apples.