Data-Driven Discovery of Unknown Precursors of N-Nitrosodimethylamine in Nontargeted Liquid Chromatography–High Resolution Mass Spectrometry Analysis

N-Nitrosodimethylamine (NDMA) is a carcinogenic byproduct formed from nitrogen-containing organics during water disinfection. Identifying its precursors in source water is necessary for reducing or eliminating NDMA formation. Previous studies focused on model precursors with a dimethylamine group (DMA: −N(CH 3 ) 2 ), missing precursors without DMA that can form NDMA during chloramination. Here, we aimed to develop a new approach enhancing the discovery of various NDMA precursors. By analyzing MS/MS spectra of 39 model NDMA precursors, we identified three diagnostic fragment ions: [C 2 H 6 N] +, [C 2 H 7 N] •+, and [C 2 H 8 N] + . Using these fragments to query the NIST23 MS/MS library revealed 763 candidate compounds. Among 21 commercially available candidates, seven were experimentally confirmed as NDMA precursors through chloramination─four with DMA moieties and three without. Additionally, five drugs lacking DMA were newly identified as NDMA precursors. To facilitate large-scale screening, we developed Nitr os A mine Pre cursor e X plorer ( NitraPreX ) for nontargeted LC-HRMS analysis. When applied to analyze Suwannee River Humic Acid reference material and real local water samples, NitraPreX tentatively identified over 200 potential NDMA precursors and confirmed two non-DMA-containing compounds, hydroxychloroquine and N,N-bis(2-hydroxyethyl)dodecanamide, as NDMA precursors. This approach provides a robust framework for uncovering a wider range of NDMA precursors.