Determination of Femtogram-Level Plutonium Isotopes in Environmental and Forensic Samples with High-Level Uranium Using Chemical Separation and ICP-MS/MS Measurement

ICP-MS is becoming a competitive technique for measurement of plutonium isotopes. Besides the abundance sensitivity (tailing of ²³⁸U to m/z = 239 and 240), isobaric and polyatomic ions interferences (e.g., ²³⁸U¹H⁺) are the most critical challenges for determination of low-level plutonium in high uranium samples. This work presents a new method to solve this problem using ICP-MS with two tandem quadrupole separators and a dynamic collision/reaction cell combined with chemical separation. The interference of uranium hydrides (²³⁸U¹H⁺ and ²³⁸U¹H₂⁺) was effectively eliminated using CO₂ as reaction gas by converting hydrides to oxides of uranium ions (UO⁺/UO₂⁺) but still keeping the intensity of the Pu⁺ signal. The tailing interference of ²³⁸U⁺ (abundance sensitivity) was intensively eliminated by significantly suppressing the ²³⁸U⁺ signal using CO₂ as reaction gas and using two tandem quadrupole mass separators in the ICP-MS/MS. With these approaches the overall interference of uranium was reduced to <1 × 10^–8, which is 3 orders of magnitude better than the conventional ICP-MS. Combined with chemical separation with a decontamination factor of 10⁵ for uranium, an overall factor of 10¹² for elimination of uranium interference was achieved. The developed method was demonstrated to enable accurate determination of <10^–15 g/g level plutonium isotopes in environmental samples even in a uranium debris sample with a U/Pu atomic ratio of up to 10¹². The developed method was validated by analysis of a spiked solution and certified reference materials of soil.