Comparison of different types of diffusive gradient in thin film samplers for measurement of dissolved methylmercury in freshwaters

Determination of bioavailable concentrations of methylmercury (MeHg+) in freshwater is key to further understanding its potential risk and toxicity. In this work, two in-house-manufactured mercury-specific diffusive gradients in thin films (DGT) were used in laboratory to assess the lability of MeHg+, and to develop a relationship between chemical lability and bioavailability. After diffusing through the diffusive gel, the MeHg+ accumulated in a thiol functionalised resin gel was extracted using acidic thiourea that was analysed using aqueous-phase propylation followed by headspace solid-phase microextraction (HS-SPME) and gas chromatography (GC) coupled to pyrolysis-atomic fluorescence spectrometry (Py-AFS) detection. The diffusion coefficient (D) at 25 °C in agarose (A-DGT) in the absence and presence of dissolved organic matter (DOM) was obtained. Moreover, these values were experimentally compared against polyacrylamide (P-DGT), which is the most frequently used DGT for mercury to date. Statistically significant differences were observed between D values for A-DGT in the absence (3.15×10−6 cm2 s−1) and presence of DOM (2.68×10−6 cm2 s−1) and also for P-DGT (2.49×10−6 and 1.69×10−6 cm2 s−1). Interestingly, our results show that diffusion of MeHg+ was higher on agarose diffusive gel with and without DOM in comparison with those observed in polyacrylamide. Even with higher diffusion coefficients of MeHg+ in the agarose diffusion layer, however, DGT based on polyacrylamide seems to be a better choice for eutrophic waters, when monitoring very low concentrations of MeHg+, considering its slightly higher uptake capacity.