A chelating sorbent which employs magnetic nanoparticles (MNPs) functionalized with 1,5-bis(di-2-pyridil)methylene thiocarbohydrazide (DPTH-MNPs) was used to develop a simple method for the analysis of trace amounts of metal ions (As, Sb and Hg) present in environmental samples combining on-line magnetic solid phase extraction (MSPE) with atomic spectrometry. The application of hydride generation with high resolution continuum source electrothermal atomic absorption spectrometry (HR CS ETAAS) improves the sensitivity of As, Sb and Hg determination. This method has proven to have a high sensitivity with detection limits of 0.25 μg L−1, 0.003 μg L−1 and 0.22 μg L−1 for As, Sb and Hg, respectively. Besides, the proposed method was applied in the sequential determination of the 3 elements mentioned above with a sample throughput of about 16.7 h−1, thereby, reducing the time of analysis and the volume of reagents and sample required. This method was applied to determine the target elements in a TMDA 54.5 and TM 24.3 certified reference materials, and seawater collected from Malaga Bay. The results showed good agreement with the certified values. •The on-line DPTH-MNPs/HR CS ETAAS system is promising for routine analysis.•It is a low cost and environmentally friendly tool for elemental studies.•It has proved useful for determination of Sb, As and Hg in environmental waters (seawater).