Predictable Liquid Chromatography Quadrupole Time-of-Flight Mass Spectrometry Fragmentation of Ozone-Reactive N‑Nitrosodimethylamine Precursors Coupled with In Silico Fragmentation and Ion Mobility-Quadrupole Time-of-Flight Facilitates Their Identification in Sewage

This study investigated the liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF/MS) fragmentation of 10 potent model ozone (O3)-reactive N-nitrosodimethylamine (NDMA) precursors bearing (CH3)2N–N or (CH3)2N–(SO2)–N. Fragments (m/z 61.0766, 60.0688 Da loss, and 72.0688 Da loss) were discovered as pertinent diagnostic fragments for precursors bearing (CH3)2N–N, whereas a loss of 108.0119 Da was consistent for precursors bearing (CH3)2N–S­(O2)–N. Using the fragments as structural hints on a sewage fraction with a high concentration of O3-reactive precursors, peaks of precursors sharing m/z 61.0766, a 60.0688 Da loss, or both were flagged. Then, using in silico fragmenters and (CH3)2N–N as a substructure filter on online-chemical structure databases, we identified PubChem’s compound identifier (PCCID) 141210417 and 1,1,1′,1′-tetramethyl-4,4′-(methylene-di-p-phenylene)­disemicarbazide (TMDS). TMDS was confirmed using an authentic standard, and ion mobility (IM)-QTOF/MS confirmed its rider peak as PCCID 141210417. PCCID 141210417 is an isomer of TMDS, and its environmental occurrence is associated with technical-grade TMDS and industrial effluents. The estimated contribution of TMDS to the total NDMA formation potential of the sewage fraction was 20–24%, which was suggestive of the significance of PCCID 141210417 and other precursors.