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 the measurement of plutonium isotopes. However, 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 presented a new method to solve this problem using ICP- MS with two tandem quadrupole separators and dynamic collision/reaction cell combined with chemical separation. The interference of uranium hydrides (238U1H+ and 238U1H2+) was effectively eliminated using CO2 as reaction gas to convert hydrides to oxides of uranium ions (UO+/UO2+), but still keep the intensity of 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 less than 1E-8, which is 3 orders of magnitude better than the conventional ICP-MS. Combined with chemical separation using for a decontamination factor of uranium of E5, an overall elimination of uranium interference for up to E12 was achieved. The developed method was demonstrated to enable accurate determination <E-15 g/g level plutonium isotopes in the environmental samples even uranium debris sample with a U/Pu atomic ratio up to E12. The developed method was validated by the analysis of spiked solution and certified reference materials of soil.