Determination of 15 organophosphorus pesticides residues in milk by dual column capillary gas chromatography

A previous method was improved in order to determine 15 organophosphorus pesticides (OPPs) in raw milk by dual column capillary gas chromatography using Nitrogen-Phosphorus Detection (NPD). With a single injection it was possible to measure the following OPPs: acephate, azinphos ethyl, chlorpyriphos ethyl, chlorpyriphos methyl, demeton-S-methyl sulfoxide, diazinon, disulfoton, methacrifos, methamidophos, methidation, parathion ethyl, phorate, pirimiphos methyl, pyrazophos, triazophos. All of these substances have a maximum residue levels (MRL) fixed by the European Commission. The isolation was performed by liquid partition followed by a single cleanup with a C18 monofunctional solid phase cartridge The GLC analysis was carried out with two capillary columns of different polarity, connected in parallel by a glass dual column adapter. The chromatographic temperature program was from 80°C to 100 °C at 2°C/min., followed by a second rate of 5°C/min. up to 300°C. The quantification was obtained using parathion methyl as an internal standard. The analyte identification was confirmed by comparing the retention times in the two columns. The recovery and precision for each compound were determined using 18 aliquots of a blank milk. The aliquots were divided in 3 groups of six and fortified using 0.5, 1 and 1.5 times the permitted limit each, according to the European Commission Decision 2002/657/EC. Although there was an increase in the number of analytes, the GLC conditions were further simplified without any changes in the analytic performances. The method was developed in a UNI EN ISO 9001:2000 certified laboratory.