GEOTRACES radium isotopes interlaboratory comparison experiment

In anticipation of the international GEOTRACES program, which will study the global marine biogeochemistry of trace elements and isotopes, we conducted a multi-lab intercomparison for radium isotopes. The intercomparison was in two parts involving the distribution of: (1) samples collected from four marine environments (open ocean, continental slope, shelf, and estuary) and (2) a suite of four reference materials prepared with isotopic standards (circulated to participants as 'unknowns'). Most labs performed well with Ra-228 and Ra-224 determination, however, there were a number of participants that reported Ra-226, Ra-223, and Th-228 (supported Ra-224) well outside the 95% confidence interval. Many outliers were suspected to be a result of poorly calibrated detectors, though other method specific factors likely played a role (e. g., detector leakage, insufficient equilibration). Most methods for radium analysis in seawater involve a MnO2 fiber column preconcentration step; as such, we evaluated the extraction efficiency of this procedure and found that it ranged from an average of 87% to 94% for the four stations. Hence, nonquantitative radium recovery from seawater samples may also have played a role in lab-to-lab variability.