Direct immersion thin-film microextraction method based on the sorption of pyrrolidine dithiocarbamate metal chelates onto graphene membranes followed by total reflection X-ray fluorescence analysis

This work reports on the development of a direct immersion thin-film microextraction procedure for the simultaneous preconcentration of different metal ions present in water samples using graphene membranes as novel sorptive platforms. The method relies on the sorption of metal ions as pyrrolidine dithicarbamate (APDC) chelates prior to determination by Total Reflection X-Ray Fluorescence technique (TXRF). Easy-to-manipulate and flexible graphene membranes were prepared by thermal reduction of graphene oxide and drop-casting onto a glass support. The sorption of Co(II), Ni(II), As(III), Se(IV), Au(III), Hg(II), Pb(II) and Bi(III) as their APDC chelates onto the graphene membranes was carried out under the following optimal conditions: reduction time for the synthesis of graphene membranes, 8 min; pH 3; 0.04% (w/v) APDC concentration; 50 mL sample volume; 900 rpm stirring rate; 10 min extraction time; room temperature conditions. Detection limits in the range of 0.2-0.6 mu g/L were achieved, which were much below the maximum contaminant levels in waters established by the different regulations. In order to assess the trueness of the method, two certified reference materials (i.e. QC1014 and QC1187) were analyzed. The method is simple, very sensitive and solvent free, being suitable for the determination of metal ions at trace levels in different types of water samples.