Measuring sulfide accumulation in diffusive gradients in thin films by means of purge and trap followed by ion-selective electrode.

A procedure for measuring in situ sulfide concentrations by coupling diffusive gradients in thin films (DGT) to solid-state ion-selective electrodes (ISE) is described and evaluated. Laboratory tests were performed to evaluate the coupling of these techniques, and these results were compared to the previously used methods of computer imaging densitometry (CID) and methylene blue. An average elution efficiency of 89%, as detected by ISE, was determined for a series of solutions containing amounts of sulfide ranging from 0.009 to 2.50 micromol. The validity of the standard mass-transfer equation for sulfide accumulation by DGT probes and subsequent detection by ISE was tested by measuring the mass of sulfide collected over time and with respect to varying diffusive gel thicknesses. Regressions of the sulfide mass accumulation versus the independent variables of time and inverse diffusive thickness proved to be linear. The use of DGT coupled to ISE provides a preconcentration method for sulfide that improves the detection limit (DL) over other techniques. This alternative method provides quantitative measurements of DGT-captured sulfide from solutions with a detection limit of at least 0.1 micromol/L for a 24-h deployment. The DGT technique also provides potential advantages in understanding sulfide speciation over existing methods.