Solid-phase extraction of polar pesticides from environmental water samples on graphitized carbon and Empore-activated carbon disks and on-line coupling to octadecyl-bonded silica analytical columns

The suitability of Empore-activated carbon disks (EACD), Envi-Carb graphitized carbon black (GCB) and CPP-50 graphitized carbon for the trace enrichment of polar pesticides from water samples was studied by means of off-line and on-line solid-phase extraction (SPE). In the off-line procedure, 0.5-2 1 samples spiked with a test mixture of oxamyl, methomyl and aldicarb sulfoxide were enriched on EnviCarb SPE cartridges or 47 mm diameter EACD and eluted with dichloromethane-methanol. After evaporation, a sample was injected onto a C₁₈-bonded silica column and analysed by liquid chromatography with ultraviolet (LC-UV) detection. EACD performed better than EnviCarb cartridges in terms of breakthrough volumes (>2 1 for all test analytes), reproducibility (R.S.D. of recoveries, 4–8%, n=3) and smapling speed (100 ml/min); detection limits in drinking water were 0.05–0.16 μg/l. In the on-line experiments, 4.6 mm diameter pieces cut from original EACD and stacked onto each other in a 9 mm long precolumn, and EnviCarb and CPP-50 packed in 10×2.0 mm I.D. precolumn, were tested, and 50–200 ml spiked water samples were preconcentrated. Because of the peak broadening caused by the strong sorption of the analytes on carbon, the carbon-packed precolumns were eluted by a separate stream of 0.1 ml/min acetonitrile which was mixed with the gradient LC eluent in front of the C₁₈ analytical column. The final on-line procedure was also applied for the less polar propoxur, carbaryl and methiocarb. EnviCarb could not be used due to its poor pressure resistance. CPP-50 provided less peak broadening than EACD: peak widths were 0.1–0.3 min and R.S.D. of peak heights 4–14% (n = 3). In terms of analyte trapping efficiency on-line SPE-LC-UV with a CPP-50 precolumn also showed better performance than when Bondesil C₁₈/OH or polymeric PLRP-S was used, but chromatographic resolution was similar. With the CPP-50-based system, detection limits of the test compounds were 0.05–1 μg/l in surface water.