Gold-Coated Silica as a Preconcentration Phase for the Determination of Total Dissolved Mercury in Natural Waters Using Atomic Fluorescence Spectrometry

A novel solid-phase preconcentration method is reported, using in-house gold-coated silica adsorbent packed in a microcolumn, for the determination of dissolved mercury in natural waters by atomic fluorescence spectrometry (AFS). The adsorbent was prepared by chemical reduction of a Au(III) solution with hydroxylamine in the presence of suspended silica particles. The resulting Au nanoparticles on the silica surface were highly efficient for adsorbing different mercury species from acidified waters without additional reagents. The acidified aqueous samples were passed over the microcolumn, either incorporated in a fully automated flow injection (FI) system directly coupled to the AFS or as part of a portable FI system for in situ preconcentration. After rinsing and drying of the column, Hg(0) was released by heating and directed to the AFS cell for quantification. The method offers significant advantages because no reagents are needed for species conversion, preconcentration, sample storage, or desorption and therefore the risk of contamination is minimized and blank values are lowered. This results in a low detection limit of 180 pg L(-1) using a sample volume of only 7 mL and good reproducibility, with relative standard deviations <3.2% (n = 10, [Hg] = 5 ng L(-1)). Recoveries were all > 90% in spiked river waters (spiked [Hg] = 0, 1, 5, 10 ng L(-1)), and the experimental value for the certified reference material ORMS-4 (elevated mercury in river water) was 22.3 +/- 2.6 ng Hg L(-1) which was in good agreement with the certified value of 22.0 +/- 1.6 ng Hg L(-1) (recovery = 101%). The method was successfully applied to seven different natural waters and wastewaters ([Hg] 0.5-4.6 ng L(-1)) from south west England.