Environmental sample analysis is a pillar of the modern international nuclear safeguards and nuclear nonproliferation missions, and frequently requires ultra-trace actinide isotopic and assay measurements of elements like plutonium (Pu) and neptunium (Np). At the lowest analyte concentrations, measurements are performed using highly sensitive thermal ionization mass spectrometry (TIMS) techniques, which have been shown to detect Pu and Np at the femtogram level. At such low analyte concentrations there is a persistent concern about accurately defining the lowest quantity or concentration of a component that can be reliably detected, otherwise called the limit of detection (LOD). Here we report on efforts to calculate robust LOD values for both Pu and Np measurements by thermal ionization mass spectrometry multiple ion-counting total evaporation (TIMS-MICTE). To achieve this, representative environmental samples of Pu and Np were used to assess the full method detection limits (MDL); including ashing, dissolution, purification, chemical recovery, and mass spectrometry analysis. Single femtogram level ^242-239Pu and ²³⁷Np from CRM126A (Pu) and SRM 4341a¹(²³⁷Np) were used for the tests and employed to calculate LOD values using several common approaches. In addition, we report on efforts to develop a TIMS specific internal LOD approach and compare it against Environmental Protecftion Agency MDL, the international standard LOD, and an uncertainty-based approach LOD. The internal TIMS LOD resulted in a ²³⁹Pu detection limit of 0.154 fg and 0.259 fg for ²³⁷Np. We believe this firmly establishes the detection limits that can be achieved for Pu and Np by the TIMS-MICTE technique.

Comparison of individual 1 fg swipe assay results (K=2 uncertainty) and internal TIMS LOD equation.

Joel Maassen

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