Dataset: GN01 Dissolved Pb - MIT

For the purpose of intercalibration with the UAF and TAMU labs, we analyzed samples for Pb concentrations from three stations (1, 32 and 57) from this cruise.

Sample storage bottle lids and threads were soaked overnight in 2N reagent grade HCl, then filled with 1N reagent grade HCl to be heated in an oven at 60˚C overnight, inverted, heated for a second day, and rinsed 5X with pure distilled water. The bottles were then filled with trace metal clean dilute HCl (~0.01N HCl) and again heated in the oven for one day on either end. Clean sample bottles were emptied, and double-bagged prior to rinsing and filling with sample.

Trace metal-clean seawater samples were collected using the U.S. GEOTRACES sampling system consisting of 24 Teflon-coated GO-FLO bottles that had been pre-rinsed with a 24+ hour treatment of filtered surface seawater at the beginning of the cruise (see Cutter and Bruland, 2012 for more information on the sampling system). At each station, the bottles were deployed open and tripped on ascent at 3 m/min. On deck, the bottles were kept in a trace metal clean sampling van over-pressurized with HEPA-filtered air, except immediately prior to and following deployments, in which cases they were covered on both ends with shower caps to avoid deck contamination.

Samples were analyzed at least 5 months after acidification over 6 analytical sessions by a resin pre-concentration method. This method utilized a modification of the batch isotope-dilution ICP-MS method described in Lee et al. 2011. The modification consists of replacing the NTA Superflow® resin by pre-concentration onto Nobias Chelate PA1 resin (Sohrin et al., 2008). After elution from the resin, ²⁰⁴Pb and ²⁰⁸Pb are analyzed on a Fisons PQ2+ using a 400uL/min nebulizer. Briefly, samples were poured into 30mL subsample bottles. Then, triplicate 1.5mL polypropylene vials (Nalgene) were rinsed three times with the 30mL subsample. Each sample was pipetted (1.3mL) from the 30mL subsample to the 1.5mL vial. Pipettes were calibrated daily to the desired volume. 25 μl of a ²⁰⁴Pb spike were added to each sample, and the pH was raised to ~4.3 using a trace metal clean ammonium acetate buffer, prepared at a pH of between 7.95 and 7.98. ~0.16mg of cleaned Nobias Chelate PA1 beads were added to the mixture, and the vials were placed on a shaker table for 3 days to allow the sample to equilibrate with the resin. After equilibration, the beads were centrifuged and washed 3 times with pure distilled water, using a trace metal clean siphon tip to remove the water wash from the sample vial following centrifugation. After the last wash, 350 μl of a 0.1M solution of trace metal clean HNO₃ was added to the resin to elute the metals, and the samples were set to shake on a shaker for 1 – 2 days prior to analysis by ICP-MS.

Nobias Chelate PA1 resin was cleaned with 2 methanol rinses, distilled water rinse followed by leaching with ultrapure 6M HCl for 12-24 hours. This procedure was repeated twice, followed by two one-day leaches with ultrapure 3M HNO₃ on a shaker table. The resin was then rinsed six times with distilled water to remove the nitric acid. It was then leached twice with ultrapure 0.1M HNO₃ for one day each. The final 0.1N rinse was checked for Pb blank by ICPMS and the resin only used if the blank was acceptably low. The cleaned resin was stored in pure distilled water to prevent resin degradation. If more than 3 days passed between initial cleaning and use, it was re-cleaned for 12 hours with 0.1M HNO₃ and then rinsed with pure distilled water until the pH was >4.5 just prior to use.

Nalgene polypropylene (PPCO) vials were cleaned by heated submersion for 2 days at 60˚C in 1M reagent grade HCl, followed by a bulk rinse and 4X individual rinse of each vial with pure distilled water. Each vial was then filled with trace metal clean dilute HCl (~0.01M HCl) and heated in the oven at 60˚C for one day on either end. Vials were kept filled until just before usage.

On each day of sample analysis, procedure blanks were determined. Replicates (12) of 300µL of an in-house standard reference material seawater (low Pb surface water) were used, where the amount of Pb in the 300µL has been verified as negligible. The procedural blank over the relevant sessions for resin preconcentration method ranged from 2.0 – 6.7pmol/kg, averaging 3.7 ± 1.8 pmol/kg. Within a day, procedure blanks were very reproducible with an average standard deviation of 0.8 pmol/kg, resulting in detection limits (3x this standard deviation) of 2.4pmol/kg. Fifteen replicate analyses of two different large-volume seawater samples (one with ~11pmol/kg, another with ~24pmol/kg, and a third with ~39pmol/kg) indicated that the precision of the analysis is 3% or 1.3 pmol/kg , whichever is larger.

Triplicate analyses of an international reference standards SAFe D2: 26.6 ± 1.7 pmol/kg (consensus value 27.7 ± 1.5) and SAFe S: 46.3 ± 1.9 pmol/kg (consensus value 48.0 ± 2.2). This standard run was linked into our own long-term quality control standards that are run on every analytical day to maintain long-term consistency.

For the most part, the reported numbers are simply as calculated from the isotope dilution equation on the day of the analysis. For one analytical day, however, quality control samples indicated offsets in the blank used to correct the samples, so for this day the blank was estimated as that giving the best agreement of the quality control samples.

The internal U.S. intercalibration between MIT, UAF, and TAMU was good with analyses scattering about a 1:1 line by ± 2 pmol/kg. However, it should be noted that Pb concentrations within the Arctic are as low as 1 pmol/kg, so there is a relatively large relative uncertainty at the lowest concentrations.

There was an external intercalibration on 6 samples from Canadian-collected samples from GN03 with MIT, UBC and UVic data, with comparable agreement as the internal intercalibration just noted. However, comparison of samples independently collected by the U.S. and Canadian groups (along different sections of the western Arctic) showed some significant differences which may point to spatiotemporal variability.

The MIT Pb measurements were performed by Cheryl Zurbrick.