In situ measurement of trace sulfide concentrations in marine coastal waters using diffusive gradient in thin film passive samplers

The diffusive gradient in thin film technique (DGT) represents an in situ passive sampling method designed to preconcentrate various compounds, including sulfides, for detection at low concentrations. While DGT applications for sulfides have been studied in freshwater, this research extends its use to marine environments. A detailed methodology is presented for synthesizing, assembling, calibrating, and field-deploying DGT samplers to measure sulfides in the low micromolar range in marine waters. The in-house DGT samplers developed in this study demonstrated improved performance, with more homogeneous binding gels and smaller silver iodide particles (0.51 +/- 0.34 mu m) compared to commercial alternatives. Grayscale imaging enabled accurate quantification of sulfide accumulation in the gels, confirming the method's reliability for detecting trace-level sulfides in marine environments. Comparative analysis showed in-house and commercial samplers performed similarly in estimating sulfide concentrations. Field deployments along the Peruvian coast revealed significant vertical and spatial sulfide gradients. In the Callao coastal area (12 degrees S) (July-August 2022), concentrations ranged from 0.03 to 0.45 mu M across a 35 m depth profile. In Paracas bay (13.8 degrees S) (March-April 2023), a shallower coastal station, concentrations ranged from 1.17 to 6.46 mu M, reflecting increased benthic production. These results highlight the utility of DGT samplers as cost-effective tools for biogeochemical monitoring, enabling studies of the ocean sulfur cycle. The findings emphasize the growing application of DGTs in marine and coastal water column research.