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 mu g cm(-2), while neither P+v/(up to 10 mg L-1) nor As+v(up to 1 mgL(-1)) impacted the DGT-P+III measurement. This novel method's functionality stems from the herein confirmed speciation preservation and double preconcentration 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. (C) 2018 Elsevier Ltd. All rights reserved.