Accurate measurements of ¹²⁹I concentration by isotope dilution using MC-ICPMS for half-life determination

Abstract

Determining the ¹²⁹I concentration, a long-lived radionuclide present in spent nuclear fuel, is a major issue for nuclear waste disposal purpose. ¹²⁹I also has to be measured in numerous environmental, nuclear and biological samples. To be able to accurately determine the ¹²⁹I concentration, an analytical method based on the use of a multicollector-inductively coupled plasma mass spectrometer (MC-ICPMS) combined with an isotope dilution technique using an ¹²⁷I spike, was developed. First, the influence of different media (HNO₃, NaOH and TMAH) on natural ¹²⁷I signal intensity and stability and on memory effects was studied. Then an analytical procedure was developed by taking into account the correction of blanks and interferences. Tellurium was chosen for instrumental mass bias correction, as no certified standards with suitable ¹²⁷I/¹²⁹I ratio are available. Finally, the results, reproducibility and uncertainties obtained for the ¹²⁹I concentration determined by isotope dilution with a ¹²⁷I spike are presented and discussed. The final expanded relative uncertainty obtained for the iodine-129 concentration was lower than 0.7% (k = 1). This precise ¹²⁹I determination in association with further activity measurements of this nuclide on the same sample will render it possible to determine a new value of the ¹²⁹I half-life with a reduced uncertainty (0.76%, k = 1).