Time-integrated determination of labile Sr, Ba, and ⁸⁷Sr/⁸⁶Sr in freshwater using DGT passive sampling

Assessing representative baselines of labile strontium (Sr), barium (Ba), and radiogenic Sr isotope ratios (⁸⁷Sr/⁸⁶Sr) in dynamic freshwaters remains challenging because conventional grab sampling provides only time-specific snapshots. Here, we present a novel diffusive gradients in thin-films (DGT) passive sampling technique using a crown-ether/organophosphorus-functionalized binding layer (TK100 membrane), combined with inductively coupled plasma mass spectrometry (ICP-MS), for the simultaneous, time-integrated determination of these biogeochemical tracers in natural freshwaters.

Laboratory evaluation showed quantitative Sr and Ba uptake by the TK100 membrane (f_u = 100 %), low method detection limits (x_L ≤ 0.07 μg L^-1, t = 7 d), and linear accumulation in 0.45 μm-filtered river water over time (t = 1-7 d). Three-step elution using HCl provided high total Sr and Ba elution efficiencies (f_e ≥ 95 %) and reduced Ca/Sr and Rb/Sr ratios by up to ∼20-fold and ∼250-fold, respectively, relative to grab samples. This enabled ⁸⁷Sr/⁸⁶Sr analysis of DGT eluates by multi-collector ICP-MS without offline preconcentration or matrix separation. Field deployments at two sites in the Mur River, Austria, showed no significant ⁸⁷Sr/⁸⁶Sr bias relative to grab samples and provided time-averaged labile Sr and Ba concentrations and ⁸⁷Sr/⁸⁶Sr ratios over the deployment period (t = 4-7 d).

The results establish TK100 DGT as an integrated passive sampling tool for labile Sr, Ba, and ⁸⁷Sr/⁸⁶Sr, combining time integration, selective uptake, preconcentration, and matrix reduction in a single deployment. The method complements grab sampling and supports representative baseline monitoring and hydrogeochemical tracing of Sr and Ba in the environment.