Dataset: Dissolved Cobalt

Samples were collected during the KM1128 METZYME research expedition (Metals and Enzymes in the Pacific) on the R/V Kilo Moana October 1-25, 2011 from Oahu, Hawaii to Apia, Samoa, with Carl Lamborg and Mak Saito as Chief Scientists.

Dissolved trace metal samples were collected by an internally programmed standard SBE Rosette (Seabird Electronics Inc.) user-modified to serve as a trace metal clean system with 24 8 L X-Niskin bottles (Ocean Test Equipment), 12 X-Niskin bottles were attached to the rosette per deployment) with minimal exposed metal surfaces using 5000 m of non-metallic non-conducting line. Temperature, oxygen, and conductivity sensor data were collected using a SBE19plus V2 system (Seabird Electronics Inc.) attached to a CTD extension stand on the Trace Metal Rosette. All sensors were factory calibrated immediately prior to the expedition. X-Niskins were pressurized with ultra-high purity nitrogen gas and seawater was filtered through cleaned 47 mm 0.2 micron Supor membrane filters within a HEPA filtered cleanroom space aboard the ship. The volume filtered was calculated (X-Niskin volume minus small unfiltered samples) and the filters were stored in cleaned tubes and frozen for particulate metal analysis (see below). Acid-washed 60 mL LDPE bottles were filled entirely, leaving no headspace. Samples were stored at preserved in a heat-sealed bag containing gas absorbing satchels immediately after sample collection and kept at 4C until analysis in lab.

Sample storage and reagent bottles were soaked for >1 week in the acidic detergent Citranox, rinsed thoroughly with 18.2 M-Ohm Milli-Q water (Millipore), filled with 10% HCl to soak for 10 days, and rinsed thoroughly with Milli-Q water adjusted to pH 2 with TM-grade HCl. Reagent purification protocols were identical to those previously published (Saito and Moffett 2001).

Concentrations of total dissolved cobalt were determined using a previously described cathodic stripping voltammetry (CSV) method (Saito and Moffett 2001, Saito et al. 2004). Measurements were made using the Eco-Chemie µAutolabIII systems connected to Metrohm 663 VA Stands equipped with hanging mercury drop electrodes and Teflon sampling cups. Sample preparation was modified slightly to accommodate use of a Metrohm 837 Sample processor, operated with NOVA 1.8 software (Metrohm Autolab B.V.).

For dissolved cobalt analyses, samples were UV-irradiated for 1 h prior to analysis in a Metrohm 705 UV digester to degrade the organic ligands that bind cobalt, enabling full chelation by the added electroactive ligand, dimethylglyoxime (DMG). 11 ml of sample was pipetted into 15ml polypropylene tubes. Recrystallized DMG (0.1M in methanol) was added to a final concertation of 400 uM and purified N-(2-hydroxyethyl)piperazine-N-(3-propanesulfonic acid) (EPPS) buffer (0.5 M in Milli-Q water) was added to a final concentration of 3.8 mM. Tubes were inverted several times before being placed in the autosampler queue, where 8.5 ml of the mixture was dosed into the teflon analysis cup using a 800 Dosino automated burrette (Metrohm). 1.5 ml of purified sodium nitrite (1.5M in Milli-Q water) was added directly to the Teflon cup using a dedicated 800 Dosino burrette. Once loaded, samples were purged with high purity (>99.99%) N2 for 3 min and cobalt concentrations were determined by standard addition, with triplicate measurement of the sample followed by four 25 pM cobalt additions. 5 nM Co stock was diluted from a certified 1ppm reference (SPEX) and added to the analysis cup via a third Dosino burrette.

The analytical blank was determined by analyzing seawater that had been UV-irradiated for 1 h, equilibrated overnight with prepared Chelex 100 resin beads (Bio-Rad), and UV-irradiated a second time to degrade any leached synthetic ligands. Mean blank was 4.6 +/- 0.7 pM, and the detection limit was calculated as triple the standard deviation of the blank, 2.1 pM. A portion of this dataset was published previously in Saito et al. 2014; the blank for those samples was 3.5 pM.

The Saito laboratory has participated in the GEOTRACES intercalibration effort using this electrochemical Co method. Acidified standards were neutralized with concentrated ammonium hydroxide (Seastar), mixing the entire sample between drops, prior to UV digestion. We report our laboratory values for the GEOTRACES and SAFe standard analyses using this electrochemical method, including those conducted during analysis of the EPZT samples to be:
SAFe D1 = 48.5 +/- 2.4 (n=3, at sea),
SAFe D2 = 45.0 +/- 2.7 (n=7),
GEOTRACES GSP = 2.5 +/- 2.0 (n=10),
GEOTRACES GSC = 77.7 +/- 2.4 (n=4).

These results are in good agreement with those from the GEOTRACES intercalibration effort for Co and demonstrate that the methodologies employed to produce this dataset detect concentrations within the standard deviation of current consensus values for UV irradiated samples, which can be found on the International GEOTRACES Program website