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This work presents a modified method to analyze selected pharmaceutical residues (clofibric acid, ibuprofen, carbamazepine, naproxen, ketoprofen and diclofenac) in water samples. Various solid-phase extraction cartridges were investigated. The newly developed Oasis HLB (polystyrene-divinylbenzene-N-vinyl pyrrolidone terpolymer) solid-phase extraction (SPE) cartridge provides the optimal sample extraction results. The analytes were then identified and quantitatively determined by gas chromatography-mass spectrometry (GC-MS) via on-line derivatization in the injection-port using a large-volume (10 mul) sample injection with tetrabutylammonium (TBA) salts. This injection-port derivatization technique provides sensitivity, fast and reproducible results for pharmaceutical residues analysis. Mass spectra of butylated derivatives and tentative fragmentation profiles are proposed. Molecular ions and some characteristic ions were used as the quantitation ions to obtain maximum detection sensitivity and specificity. The quantitation limits of these compounds ranged from 1.0 to 8.0 ng/l in 500 ml tap water samples. Recovery of these residues in spiked various water samples ranged from 50 to 108% while RSD ranged from 1 to 10%. The selected analytes were detected in concentrations of 30 to 420 ng/l in wastewater treatment plant effluent and river water samples. (C) 2005 Elsevier B.V. All rights reserved.