Quenchers in advanced oxidation technologies for analysis of micropollutants by liquid chromatography coupled to mass spectrometry: Sodium sulphite or catalase?

Abstract This work aimed to investigate the possible effect of 2 quenchers commonly used in H 2 O 2 -based advanced oxidation technologies (AOTs), i.e. catalase and sodium sulphite (Na 2 SO 3 ), on the analytical signal of 3 detectors coupled to liquid chromatography (LC): tandem mass spectrometry (LC-MS/MS), fluorescence detection (LC-FD) and LC-diode array detection (LC-DAD). The observation of analytical interferences for a group of compounds when studying the removal by continuous mode UV/H 2 O 2 of 26 micropollutants (MPs) from a spiked surface water (SW), for which the residual H 2 O 2 in the samples was quenched by Na 2 SO 3 , triggered the need of understanding this effect and thus catalase was used as comparative quencher. From the 26 micropollutants (MPs) having a wide range of polarity and p K a , those monitored after electrospray ionization (ESI) under positive ionization (PI) mode and presenting a p K a higher than 5.9 revealed a great signal suppression, but only when using Na 2 SO 3 as H 2 O 2 quencher. In this sense, we further explored this effect by selecting 2 MPs, metoprolol and diclofenac, which had respectively signal suppression and no interference in the LC-MS/MS response. These MPs were analysed before and after addition of H 2 O 2 and catalase or Na 2 SO 3 in reaction vials, using: (i) different detectors coupled to LC, namely LC-MS/MS with ESI under PI and negative ionization (NI) modes, LC-FD and LC-DAD; (ii) different environmental matrices (SW, drinking water, wastewater) and ultrapure water; and (iii) different magnitude levels (0.1–10 mg L −1 ). The results demonstrated a remarkable signal suppression in LC-MS/MS analyses under PI mode for those compounds with p K a higher than 5.9, confirming the interfering effect of H 2 O 2 /Na 2 SO 3 . To the best of our knowledge, the analytical interference in the LC-MS/MS analysis, after adding Na 2 SO 3 to quench H 2 O 2 in AOTs experiments was never reported before and the results presented herein support the recommendation to use catalase instead of Na 2 SO 3 as quencher in AOTs studies.