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An improved off-line separation method of vanadium redox species V(V) and V(IV) in seawater using a solid phase extraction with chelating resin Chelex-100 was developed. Column shape, eluting solution, elution volume, pH of sample, and flow rate were tested and optimized to establish a simple offline separation method. Analyses with inductively coupled plasma mass spectrometry (ICP-MS) combining with a Kinetic Energy Discrimination mode (KED) using helium gas achieved precise determination of vanadium redox species in saline matrix solutions. Average recovery of solutions doped with both V(V) and V(IV) was 92% for V(V) and 96% for V(IV). The method detection limit was 0.87 nmol/kg for V(V) and 0.47 nmol/kg for V(IV) obtained from 0.6 mol/kg NaCl solution. Stability tests of redox species V(IV) in seawater with/without an aerobic chamber revealed that V(IV) species were preserved in seawater for …