Potential of Solid Phase Extraction Disks to Aid Determination of Dislodgeable Foliar Residues of Chlorpyriphos, Malathion, Diazinon, and Acephate

Abstract

The utility of solid phase extraction (SPE) for concentrating four organophosphate insecticides from solutions of water and sodium dioctyl sulfosuccinate, a surfactant, was evaluated. Reverse phase (C18, octadecyl bonded silica) sorbent in the form of a disk was the SPE medium evaluated. Chlorpyriphos, malathion, and diazinon, but not acephate, were retained on and eluted from the SPE disks. For pesticides that were retained on SPE disks, recoveries from the disks were equal to or higher than recoveries achieved by solvent partitioning. Dislodgeable foliar residues of acephate were successfully concentrated for analysis by lyophilization of water–surfactant solutions. Recoveries of pesticides from SPE disks stored at −15°C for one week were equal to or higher than those of pesticides stored in water–surfactant for one week at −15°C. Malathion- and diazinon-fortified samples in water–surfactant and on SPE disks were prepared in one state and shipped for analysis in another state. Pesticides in the water–surfactant samples were concentrated by solvent partitioning and were underestimated by 41% (diazinon) and 16% (malathion). Conversely, diazinon samples on the SPE disks were on average underestimated by 3% and malathion was overestimated by an average of 55%. The overestimation of malathion was attributed to a matrix effect during analysis associated with the presence of surfactant, which was retained on and subsequently eluted from the SPE disks. The retention of surfactant by the SPE disks and its subsequent elution may considerably limit their usefulness in determination of dislodgeable foliar residues.