Determination of dimethoate and fenitrothion in estuarine samples by C-18 solid-phase extraction and high-resolution gas-chromatography with nitrogen-phosphorus detection

Dimethoate [S-((2-(methylamino)ethyl)-2-oxoethyl)phosphorodithioate] and fenitrothion [O,O-dimethyl O-(4-nitro-m-tolyl)phosphorothioate], both organophosphorus pesticides (OPs), have been successfully enriched on monofunctional C-18 solid-phase extraction (SPE) cartridges and quantified by high-resolution capillary gas-chromatography with nitrogen-phosphorus detection (GC-NPD). The method was optimised for estuarine conditions. The recoveries varied significantly with the pH values of spiked seawater samples in the range pH 6.5–9.0 for both compounds. Salinity did not affect the recoveries of fenitrothion whereas the recoveries of dimethoate increased with salinity. The highest recoveries from water samples spiked at trace concentrations of 100 ng l−1 were 68% and 91% for dimethoate and fenitrothion, respectively, and were achieved at pH 6.5 and a salinity of 36‰. Recoveries from ultrasonically extracted sediments, spiked at 100 ng g−1 dry weight were 91% for dimethoate and 86% for fenitrothion. The detection limits (sn = 3) for dimethoate and fenitrothion in water and sediment samples were 1 ng l−1 and 2 ng g−1 dry weight, respectively. The reproducibility (RSD) of all determinations was < 10%. Comparison of the GC-EI-MS spectra showed that ions mz 87,93 and 229 for dimethoate and mz 109, 260 277 for fenitrothion are suitable for selective ion monitoring. The method was not suitable for extraction and quantification of dimethoxon, an oxygen analogue of dimethoate, as the recoveries were always below 20%. GC column lifetime was significantly improved after calibrations were performed with standard solutions prepared in OP-free extracts of natural samples.