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

Phosphite (P^+III) is of emerging chemical interest due to its importance within the global phosphorus cycle. Yet, to date, precise/accurate measurements of P^+III 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^+III, 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^+III, were undertaken before full method validation. Laboratory-performance testing confirmed DGT-P^+III acquisition to be independent of pH (3.0–10.0) and ionic strength (0–500 mM). The capacity for P^+III was 45.8 μg cm⁻², while neither P^+V (up to 10 mg L⁻¹) nor As^+V (up to 1 mg L⁻¹) impacted the DGT-P^+III measurement. This novel method's functionality stems from the herein confirmed speciation preservation and double pre-concentration of P^+III, resulting in quantification limits as low as 7.44 ng L⁻¹ for a 3-day deployment. Applications of this method in various terrestrial/aquatic environments were demonstrated and simultaneous profiles of P^+III and P^+V across a sediment-water interface were captured at mm resolution in two contrasting redox-mesocosm systems.