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 U to / = 239 and 240), isobaric and polyatomic ions interferences (e.g., UH) 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 (UH and UH) was effectively eliminated using CO as reaction gas by converting hydrides to oxides of uranium ions (UO/UO) but still keeping the intensity of the Pu signal. The tailing interference of U (abundance sensitivity) was intensively eliminated by significantly suppressing the U signal using CO 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, which is 3 orders of magnitude better than the conventional ICP-MS. Combined with chemical separation with a decontamination factor of 10 for uranium, an overall factor of 10 for elimination of uranium interference was achieved. The developed method was demonstrated to enable accurate determination of <10 g/g level plutonium isotopes in environmental samples even in a uranium debris sample with a U/Pu atomic ratio of up to 10. The developed method was validated by analysis of a spiked solution and certified reference materials of soil.