Determination of total dissolved cobalt in UV‐irradiated seawater using flow injection with chemiluminescence detection

Abstract

A sensitive flow‐injection method with chemiluminescence detection (FI‐CL) for the determination of dissolved cobalt in open ocean samples, suitable for shipboard use has been developed. To date, FI methods for dissolved cobalt have been used only in coastal and estuarine waters. Therefore, significant modifications to existing methods were required, including (1) the use of a commercially available iminodiacetate (IDA) resin (Toyopearl AF‐chelate 650M) in place of resin immobilized 8‐hydroxyquinoline for online preconcentration and matrix removal, (2) the introduction of acidified ammonium acetate (pH 4) as a column‐conditioning step before sample loading and rinse steps, and most importantly, (3) UV irradiation of acidified seawater samples to determine total dissolved cobalt, rather than an operationally defined fraction. This method had a detection limit of 4.5 pM (3σ of the blank). The accuracy of the method was evaluated by determining total dissolved cobalt in acidified North Pacific deep seawater (1000 m) samples from the Sampling and Analysis of Iron (SAFe) program and NASS‐5. The method yields a mean (± SD) value of 40.9 ± 2.6 pM ( n = 9), which is in excellent agreement with the SAFe consensus value of 43 ± 4 pM, and 208 ± 30 pM for NASS‐5 (certified value 187 ± 51 pM). This study demonstrates that UV irradiation is an essential step for the determination of total dissolved cobalt in seawater by FI‐CL. The method was applied to vertical profiles from the Sargasso Sea, indicating that total dissolved cobalt is influenced by both biological and physical processes.