Ion-pair reversed-phase liquid chromatography–quadrupole-time-of-flight and triple-quadrupole–mass spectrometry determination of ethyl sulfate in wastewater for alcohol consumption tracing

Ethyl sulfate (EtS) is excreted in urine as a minor metabolite (0.010–0.016% on molar basis) after intake of alcoholic beverages, being a convenient biomarker for ethanol tracing after its determination in sewage. In this work, a new method for the direct determination of EtS in wastewater by liquid chromatography–(tandem) mass spectrometry (LC–MS(/MS)) has been developed. Different LC columns, mobile phases, and detection systems have been tested. Convenient retention by ion-pair reversed-phase LC was achieved by addition of 50 mM tetrabutylamonium bromide to the sample. Also, a triple-quadrupole (QqQ) instrument and a quadrupole time-of-flight (QTOF) system were compared. The repeatability of both systems and linearity was comparable, with RSD ≤ 10% in sewage samples. The QqQ instrument provided a better limit of detection (LOD = 0.1 μg L−1) than the QTOF system LOD (0.2 μg L−1). However, the LOD of this last instrument was still good enough for wastewater concentrations, while avoiding problems with interferences on the QqQ not permitting positive identification with this last system. The stability of EtS was tested and it has proven to be stable in wastewater for at least one week at room temperature and more than one month at −20 °C. The application of the method to samples collected during a week in a Galician (NW Spain) city showed EtS concentrations between 4 and 12 μg L−1. This translated into a per capita consumption of pure ethanol in the range from 9 to 24 mL day−1 inh−1, observing an increase during the weekend compared to weekdays.