A new era of plutonium assay and isotope ratio measurements at ultra-low levels in IAEA safeguards environmental swipe samples by isotope dilution MC-ICP-MS using a 244Pu spike

We present a new analytical method for accurately measuring femtogram amounts of Pu in environmental swipe samples using a high-purity Pu-244 spike and multi-collector inductively-coupled-plasma mass spectrometry. The method was validated by comparative measurements of samples that were previously analysed using a Pu-242 spike. The Pu-244 spike allows direct normalization of all Pu isotopes to both Pu-244 and Pu-239, along with online monitoring of PbAr interferences, resulting in highly-sensitive and accurate Pu assay and isotope ratio measurements. Expanded uncertainties were typically < 2.5% for Pu amounts larger than 20 fg, and < 1% for Pu isotope ratios larger than 0.08. Detection thresholds (L-c) were well below 1 fg for all Pu isotopes.