Simultaneous liquid-liquid extraction and dispersive solid-phase extraction as a sample preparation method to determine acidic contaminants in river water by gas chromatography/mass spectrometry

A sample preparation procedure that combines a liquid-liquid extraction (LLE) with a dispersive solid-phase extraction (DSPE) has been devised to determine residues of four phenoxyacid herbicides, two aminopolycarboxylic acids and five acidic anti-inflammatory drugs in small volumes of river water samples. Two aliquots of acetone (3 and 0.5 mL) were used to extract the analytes from a 10 mL water sample at pH 2 containing 5 mg of octadecylsilane (ODS) sorbent and NaCl at a 5.5 M concentration. Acetone was isolated by the salting-out effect, collected, evaporated and the extract was treated with BF3/methanol to obtain the methyl esters of the analytes and determine them by GC with mass spectrometric detection. Recoveries were comprised between 82% and 114% with relative standard deviations about 5-15% (n=5) within a concentration range about 0.03-44 µg L−1. The amount of ODS added to sample resulted to be a significant factor in the recovery for most of the analytes as deduced from an experimental design; the sample pH was not a so critical factor. A robustness study of the proposed sample preparation was carried out as defined by Youden and Steiner and an estimation of the uncertainties of the measured concentrations was made following the EURACHEM/CITAC guidelines, too.