Intercalibration studies of ²¹⁰Po and ²¹⁰Pb in dissolved and particulate seawater samples

Documented is an intercalibration (IC) exercise for both ²¹⁰Po and ²¹⁰Pb in seawater aliquots distributed between up to eight international laboratories that followed individual protocols. Dissolved and particulate samples were provided by GEOTRACES during two IC cruises at baseline stations in the North Atlantic and North Pacific oceans. Included were surface and/or deep dissolved and particulate samples at each site, plus complete profiles analyzed by the laboratory of the lead author. An unspecified solid phase standard was also distributed with ²¹⁰Po and ²¹⁰Pb in secular equilibrium to confirm spike calibrations. The ²¹⁰Po activities reported n = 8) for the standard were very similar with a relative standard deviation (RSD) of 3.6% and mean value indistinguishable from the certified value, confirming accurate calibration of Po spikes. For seawater samples, the agreement was strongly dependent for both nuclides on the activity of the samples. The agreement was relatively good for dissolved seawater samples (RSD = 9% to 29%, n = 4), moderate for the particulate samples (RSD = 12% to 80%, n = 8), and poor for particulate dip blanks (RSD = 50% to 200%, n = 8). Noted is the higher apparent affinity of ²¹⁰Po versus ²¹⁰Pb for polysulphone filter material. Some lack of reproducibility between labs may have been caused by unspecified differences in individual lab protocols and calculations. A minimum sample activity of 0.1 dpm for both nuclides is recommended for an adequate reproducible sample activity. It is suggested that a consistent set of procedures and calculations be used to optimize future ²¹⁰Po and ²¹⁰Pb analyses in seawater samples.