Molybdenum is widely used in steel,petroleum,chemical,electroplating,nuclear and other industries.The molybdenum contained in the waste water and residue may come into water environment through various routes.High intake of molybdenum will exert hazardous effects on human health.Thus the molybdenum content in water should be monitored.Conventional analytical methods such as atomic absorption spectrometry(AAS) and inductively coupled plasma atomic emission spectrometry(ICP-AES) can only detect the total amount of molybdenum,rather than the level of toxic Mo(Ⅵ).Therefore,it is necessary to develop sensitive and accurate analytical techniques for determination of Mo(Ⅵ).Proper sample preparation procedures for Mo(Ⅵ) are necessary prior to its instrument analysis due to its low concentration in water and the complexity in matrix composition.Meanwhile,polyethyleneimine(PEI) has lots of amino groups,part of which will be protonated under acidic condition.Hence,PEI could interact with Mo(Ⅵ) through coordination and electrostatic actions.In this work,polyethyleneimine modified magnetic nanomaterials(PEI-SiO2-Fe3O4) were prepared by heating and stirring,with polyethyleneimine and silicon-coated ferrosoferric oxide as the reactants.The PEI-SiO2-Fe3O4 was characterized by infrared spectroscopy(FTIR),field emission scanning electronic microscopy(FE-SEM) and vibrating sample magnetometry(VSM).Then the PEI-SiO2-Fe3O4 nanomaterial was used as the adsorbent for magnetic solid phase extraction of Mo(Ⅵ).The optimum SPE conditions were as follows:a solution pH value of 5.0,an adsorption time of 20 min and a sample volume no more than 300 mL.The adsorbed Mo(Ⅵ) was completely eluted with 2 mL of 20 g/L Na3PO4 for 2 min.Coupled with flame atomic absorption spectrometry,a new analytical method for trace Mo(Ⅵ) in water was developed.Under the optimized conditions,the calibration curve was linear over the concentration range of 10-200 μg·L-1 with a determinative coefficient(r2) of 0.999 8.The limit of detection for Mo(Ⅵ) was 1.3 μg·L-1.The precision of the method was examined by performing six replicate experiments with the same Mo(Ⅵ) concentration(50 μg·L-1),which yielded an intra-day RSD of 2.7% and an inter-day RSD of 3.4%.The average recovery for Mo(Ⅵ) was found to be 98.9%.The enrichment factor(EF) evaluated by the ratio of the slopes of the calibration curves with and without the MSPE procedure was 172.In addition,the common coexisting anions(Cl-,NO-3,SO2-4,Cr2O2-7 and PO3-4) and cations(K+,Na+,Ca2+ and Mg2+) did not interfered with the determination.The proposed method was applied to the detection of Mo(Ⅵ) in environmental water,drinking water and industrial effluent samples.The spiked recoveries ranged from 91.0% to 110% with the RSDs of 0.90%-6.1%.The developed method is rapid,sensitive and accurate,and thus is suitable for determination of trace amounts of Mo(Ⅵ) in various water samples.