A simple determination of trace mercury concentrations in natural waters using dispersive Micro-Solid phase extraction preconcentration based on functionalized graphene nanosheets

In this work, we developed an innovative analytical method for the trace and ultra-trace determination of total mercury (Hg) concentration in natural water samples (fresh and seawaters). In this method, Dispersive Micro-Solid Phase Extraction (DMSPE) is applied using graphene nanosheets to quantitatively preconcentrate dissolved Hg from natural water samples, before its direct analysis by commercially available pyrolysis gold amalgamation and atomic absorption spectroscopy (AAS). In this new methodology, only two easy steps are necessary, saving time and effort. First, the operator has to add 500 mu L of nanoparticles suspension containing graphene, Ammonium Pyrrolidine DithioCarbamate (APDC) and Triton-X-100 in the water sample. This solution is filtered under vacuum and the Hg complex on the functionalized graphene can be simply collected on a membrane filter (Polyethersulfone PES, 0.2 mu m). The filter obtained can then be analysed back at the laboratory by direct pyrolysis of the PES filter using a commercial mercury analyser. Different parameters have been tested to optimize this preconcentration procedure, such as the sample volume, the amount of nanoparticles suspension and the extraction time. The stability conditions of the Hg preconcentrated on PES filters during storage and before analysis has also been investigated. The influence of the occurrence of marine salts (sodium chloride), natural organic matter or competing metals (calcium) in the sample has also been evaluated to prevent possible matrix effects. This method is fully operational after application to real water sample matrices and exhibits suitable limit of detection, as low as 0.38 ng L-1 using 200 mL of the water sample, and excellent reproducibility (< 5% as RSD).