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 degrees C overnight, rotated, 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 & 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.
During sampling in the clean van, the GO-FLO bottles were pressurized to ~0.4 atm with HEPA-filtered air, and their spigots were fitted with an acid-cleaned piece of Bev-a-Line tubing that fed into an Acropak-200 Supor capsule filter (0.2 µm pore size made of polyethersulfone). Before use, this filter had been filled with filtered surface seawater that had been acidifed to pH 2 with trace metal clean HCl and left overnight to rinse. Before collecting any subsamples, at least 500 mL of seawater was passed through the filter (while eliminating air bubbles in the capsule reservoir). Subsamples were taken into acid-cleaned (see above) Nalgene HDPE bottles after a double rinse with the sample. Acropak filters were used for at most 3 casts before a new filter was used, and they were stored empty in the refrigerator while not in use. GEOFish surface samples were taken using an all-plastic "towed fish" pumping system as described in Bruland et al. 2005 at approximately 2 m depth and were subsampled identically.
All samples were acidified at sea to pH 2 with trace metal clean 6N HCl. Samples were analyzed at least 1 month after acidification over 27 analytical sessions by the isotope-dilution ICP-MS method described in Lee et al. 2011, which includes pre-concentration on nitrilotriacetate (NTA) resin and analysis on a Fisons PQ2+ using a 400 µL/min nebulizer. Briefly, samples were poured into 1.5 mL polyethylene vials (Eppendorf AG) in triplicate. Each sample was pipetted to 1.3 mL with the remaining solution. Pipettes were calibrated daily to the desired volume. 25 µL of a Pb-204 spike were added to each sample, and the pH was raised to 5.3 using a trace metal clean ammonium acetate buffer, prepared at a pH of between 7.9 and 7.98. ~2400 beads of NTA Superflow resin (Qiagen Inc., Valencia, CA) were added to the mixture, and the vials were set to shake on a shaker for 4 days to allow the sample to equilibrate with the resin. After 4 days, the beads were centrifuged and washed 3 times with pure distilled water, using a trace metal clean syphon tip to remove the water wash from the sample vial following centrifugation. After the last wash, 250 µL of a 0.1N solution of trace metal clean HNO3 was added to the resin to elute the metals, and the samples were allowed to sit for 2 days prior to analysis by ICP-MS.
NTA Superflow resin was cleaned by batch rinsing with 0.1N trace metal clean HCl for a few hours, followed by multiple washes until the pH of the solution was above 4. Resin was stored at 4 degrees C in the dark until use, though it was allowed to equilibrate to room temperature prior to the addition to the sample.
Eppendorf polyethylene vials were cleaned by heated submersion for 2 days at 60 degrees C in 1N 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 (~0.01N HCl) and heated in the oven at 60 degrees C for one day on either end. Vials were kept filled until just before usage.
On each day of sample analyses, procedure blanks were determined using 12 replicates of 300 µL of an in-house standard reference material seawater (low Pb surface water), where the amount of Pb in the 300 µL was verified as negligible. The procedure blank over the relevant sessions ranged from 1.4-6.9 pmol/kg, averaging 2.9 pmol/kg. Within a day, procedure blanks were very reproducible with an average standard deviation of 0.4 pmol/kg, resulting in a detection limit (3x this standard deviation) of 1.2 pmol/kg. Replicate analyses of two large-volume seawater samples (one with ~35 pmol/kg and the other with ~63 pmol/kg) indicates that the precision of the analysis is 3% or 0.5 pmol/kg, whichever is larger.
Triplicate analyses of international reference standards linked into the investigators' lab standard reference frame gave:
SAFe S1: 46.0 +/- 2.2 pmol/kg
SAFe D1: 22.9 +/- 1.2 pmol/kg
US-GT-IC1-GS: 26.8 +/- 2.2 pmol/kg
US-GT-IC1-GD: 41.8 +/- 1.7 pmol/kg
For the most part, the reported numbers are simply as calculated from the isotope dilution equation on the day of the analysis. For some stations, however, small offsets between samples run on different days or quality control samples indicated offsets, so further re-runs were used to determine which day was correct. For USGT11-05 (KN204-01 station 5) and USGT11-17 (KN204-01 station 17) the quality control samples indicated that the daily numbers were 5% high compared to the long-term average, and the investigators applied a -5% correction to the Pb concentrations from that day. For USGT11-11, USGT11-13, and USGT11-15 the quality control samples indicated that the investigators' daily numbers were 4% high compared to their long-term average, and they applied a -4% correction to their Pb concentrations from that day. For USGT10-12 (KN199-04 station 12), the investigators applied a constant offset correction of -2.5 pmol/kg to the data from the day on which most samples were run. For USGT11-10, the quality control samples indicated that the investigators' daily numbers were 3% low compared to their long-term average, and they applied a +3% correction to their Pb concentrations from that day (this also improved the agreement with the Bruland and Middag numbers). With these corrections, the overall internal comparability of the Pb collection should be better than 1-2%.
In addition, Drs. Ken Bruland and Rob Middag analyzed separate samples from US-GT-NAT-2011 stations 10, 12, and 20 using a different analytical scheme and the agreement between the two labs was within the 3% precision quoted above before any corrections were applied, and within 1-2% after the corrections noted above were applied. The investigators have also used this method to re-analyze samples collected by their lab and analyzed many years ago by a different analytical method (ID-Mg(OH)2 coprecipitation) and the analyses agreed within 5%. The investigators expect this level of precision will apply between the present dataset and their previous data analyzed by ID-Mg(OH)2 coprecipitation.
Most of the US GT-10 cruise samples were analyzed by Yolanda Echegoyen-Sanz, and all of the US GT-11 samples were analyzed by Abigail Noble, with no significant difference between them for the low concentration reference sample (both analysts averaged 34.8 pmol/kg).
References:
Lee, J.-M., Boyle, E.A., Echegoyen-Sanz, Y., Fitzsimmons, J.N., Zhang, R., and Kayser, R.A. 2011. Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batch nitrilotriacetate resin extraction and isotope dilution inductively coupled plasma mass spectrometry. Analytica Chimica Acta 686: 93-101. doi: 10.1016/j.aca.2010.11.052
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