Development of a three–phase system based on liquid–liquid–liquid extraction and its application in sample preparation of different cereals flour samples for the extraction of nine selected pesticides and analysis by high–performance liquid chromatography–diode array detection

Pesticides are widely used to protect crops and plants from pests and diseases with the objective of increasing agricultural productivity. But the presence of residues of these compounds in food products may be a risk to consumers health, due to their potential toxicity [1]. Many control authorities have established maximum residue limits or tolerances to protect the environment and consumer health. Nowadays, the control of pesticide residues in food commodities is a requirement for compliance with the legislation, ensuring the safety of the population and international and national trade [2]. Thus, it is of great significance to develop an accurate and reliable analytical method, which is applicable to trace levels of pesticide residues in food and environmental samples. Sample preparation is a crucial step in most analytical procedures. Traditional sample preparation methods such as liquid–liquid extraction [3] and solid phase extraction [3, 4] have some drawbacks. This work deals with the development of a reliable sample preparation method based on liquid–liquid–liquid extraction combined with dispersive liquid–liquid microextraction and its application in the extraction and preconcentration of some pesticide residues in different cereals flour samples. This method consists of a three–phase system including aqueous phase (sample solution), acetonitrile and n–hexane for clean–up and the microextraction method for preconcentration of the analytes before their injection into the separation system (HPLC–DAD). The influence of several variables on the extraction efficiency were investigated and optimized. Under the optimal conditions, extraction recoveries were obtained in the range of 55–90% and the calibration curves were linear in wide ranges with a correlation coefficient ≥0.9987. Intra– (n = 6) and inter–day (n = 5) precisions of the method were satisfactory with relative standard deviations less than or equal to 7.0% at two concentrations of 10 and 50 ng g−1 of each analyte. Moreover, the detection limits and enrichment factors of the target analytes were obtained in the ranges of 0.16–0.60 ng g−1 and 221–359, respectively. It was found that the three–phase system was a good clean–up procedure and reduced the risk of potential interferences related to the complexity of the matrix. Finally, the proposed method was applied on different types of cereals including wheat, buck wheat, barley, rice, corn, chickpea, soya, semolina, whole wheat, wheat sprout flours, and wheat grain. Clodinafop–propargyl was determined at the concentrations of 21 ± 2 and 16 ± 1 ng g-1 (n =3), respectively, in whole wheat and wheat sprout flours.