In situ sampling and speciation method for measuring dissolved phosphite at ultratrace concentrations in the natural environment.

Phosphite (P) is of emerging chemical interest due to its importance within the global phosphorus cycle. Yet, to date, precise/accurate measurements of P are still lacking due to the inherent analytical challenges linked to its instability/ease of oxidation and ultra-trace concentration. Here, we present the first in-situ sampling and speciation analysis method, for dissolved P, using the diffusive-gradients-in-thin-films (DGT) technique, combined with capillary-column-configured-dual-ion-chromatography (CC-DIC). Method optimization of the DGT elution regime, to simultaneously maximize desorption efficiency and CC-DIC sensitivity, along with the characterization of diffusion coefficients for P, were undertaken before full method validation. Laboratory-performance testing confirmed DGT-P acquisition to be independent of pH (3.0-10.0) and ionic strength (0-500 mM). The capacity for P was 45.8 μg cm, while neither P (up to 10 mg L) nor As (up to 1 mg L) impacted the DGT-P measurement. This novel method's functionality stems from the herein confirmed speciation preservation and double pre-concentration of P, resulting in quantification limits as low as 7.44 ng L for a 3-day deployment. Applications of this method in various terrestrial/aquatic environments were demonstrated and simultaneous profiles of P and P across a sediment-water interface were captured at mm resolution in two contrasting redox-mesocosm systems.