Dataset: GT10-11 - Dissolved Po Pb
Deployment: KN204-01

Sampling and Analytical Methodology for Dissolved NAST 210Po and 210Po Sample

Currently, the recommended assay of both dissolved and particulate sea water samples are outlined in the GEOTRACES procedures manual at:
http://www.obs-vlfr.fr/GEOTRACES/science/intercalibration/222-sampling-and-sample-handling-protocols-for-geotraces-cruises

Specific methods based on this cruise follow.

The dissolved samples were drawn onboard from 30 liter Niskin bottles deployed on the rosette and at the designated depths.  The samples were filtered (0.45 micron) seawater using Acropac capsules with Supor membranes, and acidified (pH2 with Optima HCl). The twenty liter cubitainers were rinsed, filled and sealed dark at sea and transferred to cold storage after shipping and before analysis ashore.  The samples were spiked with a well-calibrated 209Po tracer solution (T1/2 =102 y), together with a stable lead carrier added to monitor the losses of Po and Pb during sample processing. For the dissolved samples, Po and Pb were pre-concentrated from large volumes of seawater via co-precipitation with Fe(OH)3 (Thomson and Turekian 1976; Nozaki 1986). The Po nuclides are then plated by spontaneous deposition onto a silver disc (Flynn 1968). Silver discs, 1-2 cm in diameter, were be obtained with greater than 99.99% purity from A.F. Murphy Die and Machine Co, Inc. USA. They were first cleaned with a commercial silver polish and then using water and ethanol. One side of the disc was covered by rubber cement, so that Po was plated on only one side. The Po activities were measured after deposition by alpha spectrometry (Ortec Inc, Alpha Quad; Canberra Inc., Alpha Analyst). Any remaining 209Po and 210Po in solution was removed by using anion exchange resin AG-1X8 (Sarin et al. 1992). After separation, the final eluate containing the 210Pb was re-spiked with 209Po and stored for greater than 6 months to allow in-growth of 210Po from 210Pb. At that time, the 210Pb activity of the sample was determined by replating the eluate on a new silver disc and measuring the in-growth of 210Po. The determination of the initial activities of 210Po and 210Pb in the sample at the time of collection accounted for decay and ingrowth between the time of collection and processing, with corrections for chemical recoveries (Rigaud, et al., in revision). The error analysis accounts for the relative uncertainty for each parameter considered in the calculation of final 210Po and 210Pb activities. For the cruise, the method allows estimating 210Pb activity with a relative uncertainty of about 7%. However for 210Po activities, the final relative uncertainty is more variable and depends on the 210Po/210Pb activity ratio in the initial sample and the time elapsed between sampling and sample processing.  For samples from this cruise, the 210Po uncertainty is not better than 6%.

References

Fleer, A. P., and M. P. Bacon. 1984. Determination of 210Pb and 210Po in seawater and marine particulate matter. Nucl. Instrum. Methods Phys. Res. 223: 243-249.

Flynn, W. W. 1968. The determination of low levels of polonium-210 in environmental materials. Anal. Chim. Acta 43: 221-227.

Nozaki, Y. 1986. 226Ra-222Rn-210Pb systematics in seawater near the bottom of the ocean. Earth Planet. . Earth Planet. Sci. Lett. 32: 304-312.

Sarin, M. M., R. Bhushan, R. Rengarajan, and D. N. Yadav. 1992. The simultaneous determination of 238U series nuclides in seawater: results from the Arabian Sea and Bay of Bengal. Indian J. Mar. Sci. 21: 121-127.

Thomson, J., and K. K. Turekian. 1976. 210Po and 210Pb distributions in ocean water profiles from the Eastern South Pacific. Earth Planet. Sci. Lett. 32: 297-303.