Determination of vanadium redox species V(IV) and V(V) in seawater using chelating resin and inductively coupled plasma mass spectrometry

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 over 24 h and possibly further 14 days in normal oxidizing atmosphere. We developed a method that does not require pH adjustment of sample solution, which reduces the risk of changing the natural fractions of both redox species. The proposed method was applied to selected samples from open ocean and coastal seawater collected in the South-East Atlantic Ocean during GEOTRACES research cruise GA08. Concentrations of 29–37 nmol/kg of V(V) and 1.5–2.2 nmol/kg of V(IV) in samples off-shore the Namibian coast (43–2997 m water depth) were found. Higher concentrations of V(IV) were detected in near-shore coastal seawater characterized by very low oxygen content (2.4 nmol/kg at 200 m water depth) relative to those from off-shore seawater. The newly obtained results suggest that not only dissolved oxygen concentration but also biogeochemical parameters (e.g. biological activity) may play an important role to explain the distribution of redox species of V in seawater.