Diazinon is a nonsystemic organophosphate insecticide developed in the early 1950’s. It was heavily used as an all-purpose indoor/outdoor home pesticide, but was banned for home use in 2004. The thiophosphoric ester in diazinon can form toxic phosphine gases and phophorus oxides. The EURL included diazinon as a target pesticide for the EUPT-CF9. Obelisc R was used to retain diazinon and separate it from impurities. Obelisc R contains embedded ionic and hydrophobic groups which can assist in fine tuning separations. Method is LC/MS compatible and can be used as a general approach for analyzing diazinon and other organophosphate insecticides.