Development and validation of an online SPE-HPLC-MS/MS for tire-derived pollutants of environmental concern (6PPD and 6PPD-Q): Detection and rainfall-driven dynamics in an urban river-estuary continuum

Microplastics such as tire wear particles and their associated tire-derived chemicals are emerging contaminants in urban surface waters. N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its toxic transformation product 6PPD-Quinone (6PPD-Q) are widely detected in roadway runoff but remain difficult to quantify at environmentally relevant concentrations due to their low abundance, matrix interferences, and susceptibility to degradation during sample processing. Here, we developed and validated an online solid-phase extraction high-performance liquid chromatography tandem mass spectrometry (SPE-HPLC-MS/MS) method for sensitive and robust determination of 6PPD and 6PPD-Q in surface waters. The method provided low detection limits and strong resistance to matrix effects, enabling routine monitoring at trace levels. The method was applied to surface waters from the lower Hillsborough River (Florida, USA) collected across ten sampling events between 2023 and 2024. 6PPD-Q was detected in all samples (0.29-11.60 ng/L; mean 1.82 ± 2.02 ng/L), whereas 6PPD occurred less frequently (ND-1.62 ng L; mean 0.20 ± 0.39 ng/L). Concentrations of 6PPD and 6PPD-Q were strongly correlated (r = 0.66, p < 0.001) and exhibited clear rainfall-driven and seasonal patterns, indicating stormwater-mediated transport and transformation. Observed concentrations were within the lower range of values reported globally for urban surface waters. To our knowledge, this study provides the first dataset of 6PPD and 6PPD-Q in Florida waters. These results demonstrate the utility of online SPE-HPLC-MS/MS for tracking tire-derived contaminants in urbanized watersheds and emphasize the need for continued regional and global monitoring given their environmental persistence and ecotoxicological significance.