The determination of palladium (Pd) in environmental samples by ICP-MS is challenging as all its isotopes are extensively interfered due to isobaric (e.g. 110Cd on 110Pd, 106Cd on 106Pd), polyatomic (e.g. 92Mo16O on 108Pd, 89Y16O on 105Pd) and doubly-charged (e.g. 208Pb2+ on 104Pd) species formed in the plasma from elements usually present at concentrations several orders of magnitude higher. As a result, the determination of Pd in natural waters is extremely scarce despite is has been proven that this metal is subject to a significant anthropogenic impact mainly linked to its use in catalytic converters in motor vehicles. In order to overcome this situation, we have developed an ultra-trace interference-free methodology for the determination of Pd in natural waters by ICP-MS after on-line matrix separation and preconcentration. The method is based on the strong affinity of Pd towards a commercially-available carboxymethylated polyethylenimine resin, which also has the ability to retain most of the transition metals. However, Pd is not eluted from the resin at typical elution conditions (e.g. 2 M HNO3, which removes all the interference-forming metals), but this can be attained by passing a diluted thiourea solution (10−3 M). Therefore, the interference-free on-line determination of Pd in natural waters was successfully achieved using a two-step elution procedure. Procedural blank values were 0.012 ± 0.003 ng kg−1 (n = 6), which results in a detection limit of 0.010 ng kg−1, allowing the determination of dissolved Pd in natural samples at low, ambient concentrations. The optimized methodology was applied to determine the concentrations of Pd in the Gironde estuary, which represents the first dissolved Pd profile along an estuarine salinity gradient and one of the first dataset of Pd concentrations in natural waters at ambient levels in almost 4 decades. The simplicity of the preconcentration setup and the possibility for its automation offers new analytical opportunities, which will be useful for future studies aiming to improve our understanding of the behavior of Pd in natural waters.Read less <