Supramolecular solid-phase extraction of ibuprofen and naproxen from sewage based on the formation of mixed supramolecular aggregates prior to their liquid chromatographic/photometric determination

Abstract

Sorbents made up of sodium dodecyl sulphate (SDS) hemimicelles, formed onto γ-alumina, were proposed for the quantitative and practically solvent-free solid-phase extraction (SPE) of ibuprofen and naproxen from sewage samples. The formation of drug–SDS mixed aggregates was proved by the pseudophase separation model and their composition as a function of the amount of drug was calculated. The overall hemimicellar SPE procedure consumed only 0.6 mL of methanol since non-organic solvent was required for cartridge conditioning and the drugs were completely eluted using 2 mL of a 0.3 M NaOH:methanol (70:30, v/v) solution. Breakthrough volumes of around 0.75 L and above 1 L were obtained for naproxen and ibuprofen, respectively. No clean-up steps were necessary for the determination of these drugs in sewage because the direct analysis of the eluates by liquid chromatography/UV was matrix effect-free. The identification of the analytes was based on retention times and UV spectra and it was confirmed by on-line fluorescence detection. The detection limits for naproxen and ibuprofen were 0.8 and 9 ng L −1 in wastewater influents and 0.5 and 7 ng L −1 in effluents, respectively. These limits were similar to or lower than those achieved by methods based on conventional sorbents (e.g. C 18–silica or polymeric resins), which invariably require the evaporation of the eluates. The accuracy and precision of the proposed method were assessed by analysing influent and effluent wastewater samples spiked with 2 and 0.4 μg L −1 of each analyte, respectively. The recoveries obtained and the corresponding standard deviations were in the ranges 93–101% and 2–9%. The method was applied to the determination of the target drugs in wastewater from three sewage treatment plants (STPs) in the south of Spain. The concentration of ibuprofen and naproxen ranged between 2.0 and 7.4 μg L −1 and 0.9 and 3.3 μg L −1 in influents and 0.4 and 1.4 μg L −1 and 0.2 and 0.8 μg L −1 in effluents, respectively.