Preliminary comparison on three purification methods for the determination of organophosphorus pesticide residues from cereals

The Italian legislation has implemented EU directives and regulations to establish limits of pesticide residues in cereals and other products of their grinding. More stringent regulations are foreseen for products of organic agricolture. The recoveries of twenty organophosphorus pesticides (OPP) extracted from different kind of cereals, by different clean-up methods, were determined in this paper. Analytical problems associated with the analysis of pesticides in these food matrices are mainly due to the presence of high-molecular weight compounds that can interfere with the analytes and reduce the chromatographic separation efficiency. Liquid-liquid extraction (LLE) was used to isolate the analytes and three different clean-up procedures were compared. The pesticides were initially extracted from the sample using a mixture of petroleum ether:dichloromethane:acetone (50:25:25v/v). The resulting extract was centrifuged, filtered and evaporated to dryness. In the first procedure the dry residue was dissolved in n-hexane and was purified through a system of three different cartridges. The extract containing the OPP residues was loaded on a Extrelut NT-3 cartridge connected, on-line, to a silica and to a C18 cartridge. In the second method, purification of organic extracts was performed by dissolution of the dry residue in acetonitrile followed by a freezing step to allow the lipid precipitation and separation. In the third method, the extract was dissolved in petroleum ether and cleaned-up on a Alumina column. A mixture of n-hexane/dichloromethane was used as eluting solvent. The main purpose of this research is to compare and to asses the effectiveness of different purification methods for this class of pesticide compounds, by means of the evaluation of their recoveries. It has been noted that first method it’s not suitable for the clean up of the studied matrices. As regard the third method, while it showed a better performance in terms of purification to the second method, gave lower recoveries. The recovery tests were carried out spiking 25 g of each cereal sample with standard pesticide solutions to give a 0.01 mg/Kg concentration of pesticides in samples. The average recoveries were >70%. The OPP residues were firstly determined by GC/NPD and the results were confirmed by GC/MS.