Magnetite (Fe3O4) nanoparticles were coated with tannic acid to give nanoparticles (NPs) of the type Fe3O4@TA and are shown to be a viable sorbent for preconcentration of Cd2+, Co2+ and Cr3+. The size, morphology, composition, and properties of the Fe3O4@TA NPs were characterized by field emission scanning electron microscopy, energy-dispersive X-ray analysis, vibrating sample magnetometery and FTIR. They were applied to the solid-phase extraction of the metal ions from environmental water samples prior to their quantitation by flow injection inductively coupled plasma-optical emission spectrometry. The effects of sample solution, extraction and desorption times, kind of eluent and quantity of sorbent were optimized. The calibration plots are linear in the concentration ranges from 0.5 to 100 μg L−1 (for both Cd and Co) and from 0.2 to 100 μg L−1 (for Cr). The limits of detection are between 0.1 and 0.2 μg L−1. The intra-day relative standard deviations based on four replicates are in the range of 6.1 to 7.1 %. The method was successfully applied to the determination of the three metal ions in (spiked) tap water, mineral water, and river water. Recoveries varied in the range from 90 to 109 %, this confirming the good performance of the method.
