Degradation by-products of pharmaceuticals in a tropical Estuary: Identification and ecotoxicological insights via DIMS and molecular modeling.

Organic pollutants are seldom found in aquatic environments in their original molecular forms; they are naturally transformed into degradation by-products through multiple pathways. Identifying these contaminants via their transformation products remains a major analytical challenge and demands innovative strategies. Here, we combined Direct-Infusion Mass Spectrometry (DIMS) with computational molecular modeling to identify pharmaceutical residues and their degradation by-products in a tropical estuary in Paraíba, Brazil. We detected six pharmaceuticals (caffeine, ibuprofen, paracetamol, lidocaine, venlafaxine, and ractopamine) and twenty-two associated by-products. The analysis enabled the reconstruction of probable degradation pathways and the estimation of physicochemical parameters, including the octanol-water partition coefficient (logP), topological polar surface area (tPSA), and molecular electrostatic potential, linked to toxicity and bioaccumulation. Notably, 13 of the 22 by-products (approximately 59 %) became more lipophilic and less polar than their parent compounds, 6 (approximately 27 %) became more hydrophilic, and 3 (approximately 14 %) remained essentially unchanged, potentially heightening ecotoxicological risks even when the parent compounds are not observed. This integrative approach proved effective for tracing contamination sources and assessing potential impacts on aquatic ecosystems.