A case study to demonstrate the importance of analyte-tracer equilibrium for the quantitative analysis of plutonium by isotope dilution

This case study from the in-vitro bioassay program of the Los Alamos National Laboratory (LANL) demonstrates the importance of analyte-tracer equilibrium when determining plutonium (Pu) using isotope dilution methods. The program utilizes externally produced urine QC materials spiked with known quantities of Pu isotopes to validate performance. In early 2019, we started observing a significant low bias between measured and expected results for the QC samples, and as part of an investigation into the root cause, conducted a study into tracer—analyte equilibrium chemistry. During the study, we also uncovered that a contributing cause of the low bias was the use of a new urine donor that had a high EDTA diet, which was likely complexing Pu in the urine QC samples. A modified radiochemistry procedure that more effectively destroys the EDTA-Pu complex and ensures equilibrium between tracer and analyte Pu was validated.