Preconcentration of Zn, Cu, and Ni Ions from Coffee Infusions via 8-Hydroxyquinoline Complexes on Graphene Prior to Energy Dispersive X-ray Fluorescence Spectrometry Determination.

A simple and effective preconcentration procedure based on dispersive micro solid-phase extraction prior to energy dispersive X-ray fluorescence spectrometric (EDXRF) determination of trace amounts of Ni, Cu, and Zn in coffee infusions was proposed. The method is based on the adsorption of 8-hydroxyquinoline metal complexes on micro amounts of graphene nanoparticles. In order to optimize adsorption process, the influence of some parameters such as pH, graphene mass, concentration of 8-hydroxyquinoline (8-HQ) and Triton X-100, sample volume, and sorption time were examined. At optimal preconcentration conditions, calibration curves were linear from 1 to 150 ng mL for Ni and Cu and from 1 to 200 ng mL for Zn. The recoveries of the metal ions were in the 95-98% range with the precision lower than 4.6%. The obtained detection limits were 0.08 ng mL for Ni and 0.09 ng mL for Cu and Zn. The proposed method was successfully applied to determination of Ni, Cu, and Zn in coffee infusions. Accuracy and repeatability of the proposed procedure were confirmed by the standard addition method and compared to the results obtained by ICP-OES technique.