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

This work aimed to investigate the possible effect of 2 quenchers commonly used in HO-based advanced oxidation technologies (AOTs), i.e. catalase and sodium sulphite (NaSO), 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/HO of 26 micropollutants (MPs) from a spiked surface water (SW), for which the residual HO in the samples was quenched by NaSO, triggered the need of understanding these effects and thus catalase was used as comparative quencher. From the 26 MPs having a wide range of polarity and pK, those monitored after electrospray ionization (ESI) under positive ionization (PI) mode and presenting a pK higher than 5.9 revealed a great signal suppression, but only when using NaSO as HO 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 HO and catalase or NaSO 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). The results demonstrated a remarkable signal suppression in LC-MS/MS analyses under PI mode for those compounds with pK higher than 5.9, confirming the interfering effect of HO/NaSO. To the best of our knowledge, the analytical interference in the LC-MS/MS analysis, after adding NaSO to quench HO in AOTs experiments was never reported before and the results presented herein support the recommendation to use catalase instead of NaSO as quencher in AOTs studies.