For Cr(III): 100 mL of aqueous sample solution were put into a glass beaker and then adjusted to pH 9.1 with 1 mol L−1
HCl or 27%(w/w) of aqueous solution of ammonia. 20 mg of Zincon–Si–MNPs was subsequently thrown into the solution and the mixture was ultrasonicated for 3 min to facilitate adsorption of the metal ions on the nanoparticles. Then the absorbent were gathered together and collected at the bottom of the beaker by an external magnetic field and the supernatant was discarded. Then, 1 mL 2 mol L−1HCl was added into the beaker as eluent and the new mixture was
ultrasonicated for 3 min again. Finally, the magnetic absorbent was removed through an external magnetic field again and the supernatant was collected for Cr determination by GFAAS. For total Cr: The MSPE procedure for total Cr was nearly the same as that of Cr(III) except the pH of sample solution was adjusted to 6.5 prior to extraction. Highly pure deionized water was selected as the blank solution and carried out magnetic SPE procedure described above. The determined values for Cr(III) and total Cr were obtained after substracting the blank values