Determination of oseltamivir phosphate and its metabolite with other pharmaceuticals in surface waters using solid phase extraction and isotope dilution liquid chromatography/tandem mass spectrometry

Solid phase extraction and isotope dilution (ID) liquid chromatography/tandem mass spectrometry (LC/MS/MS) were used to quantify concentrations of oseltamivir phosphate and its metabolite (oseltamivir carboxylate) in addition to four pharmaceuticals (caffeine, carbamazepine, triclosan, triclocarban) in surface waters. Results simultaneously and effectively elucidate the occurrence, fate, and effects of those compounds in surface waters. Higher response intensities of target compounds were obtained using an ultra-purified water/methanol gradient containing 0.01% formic acid with 0.5 mM ammonium acetate as the mobile phase. Methanol/ethyl acetate = 1:4 (v/v) efficiently eluted target compounds from a hydrophilic–lipophilic balanced polymer cartridge. Sample extraction at pH 2.0–3.5 did not affect recovery. This developed method showed satisfactory linearity (r² > 0.9957) and accurate quantification using ID method with each labelled internal compound. The recoveries from the spiked ultra-purified water and river water samples were 81–127% (1.2–16% relative standard deviation, RSD) and 89–106% (0.4–19%RSD), respectively. The method detection limit was found as 0.2–1.9 ng L⁻¹ from spiked levels of lower concentrations in ultra-purified water. This method, when applied to samples from a small river basin in Akita prefecture, Japan, detected most target compounds at low to high nanogram-per-litre concentrations.