Determination of ultra-low ²³⁶U in environment samples using ICP-MS/MS measurement and chemical separation

²³⁶U in the environment mainly originates from human nuclear activities. Based on the unique properties of uranium, ²³⁶U can be used as a powerful tracer for investigation of oceanographic and environmental processes. This requires sensitive measuement of ²³⁶U in various environmental samples. Due to the ultra-low radioactive level of ²³⁶U in the environment, its measurement is only possible by mass spectrometry. Because of the low atomic ratio of ²³⁶U/²³⁵U down to 10^-7-10^-5 in the environment, the interferences of ²³⁵U¹H⁺ and peak tailings of ²³⁵U and ²³⁸U are critical challenges in the measurement of ²³⁶U by ICP-MS. This work developed a sensitive ICP-MS/MS method for measurement of ultra-low ²³⁶U by employing reaction cell technique and sequential quadrupole mass separators. By using 0.6 mL min^-1 CO₂ - 7 mL min^-1 helium as collision/reaction gas to convert U⁺ and UH⁺ to UO⁺, the interferences of UH⁺ (UOH⁺/UO⁺ ratio) were significantly reduced to less than 2.4 × 10^-7. A minimum detectable ²³⁶U/²³⁸U ratio of 3.0 × 10^-10 was achieved, which is one order of magnitude better than reported values. By using collision focusing with helium in the reaction cell and APEX sample introduction system, the measurement sensitivity for ²³⁶U (²³⁶UO⁺) was improved to 7.5 × 10⁶ cps ppb^-1. In combination with an effective chemical separation of uranium from sample matrix and interferences using total borate fusion following extraction chromatography with UTEVA resin, a detection limit of 7.2 × 10^-16 g g^-1 for ²³⁶U was achieved. The developed method was verified by analysis of certified reference materials and by comparison with AMS measurement method. Soil samples collected from Northwest China were successfully analyzed. ²³⁶U/²³⁸U ratios down to 9 × 10^-10 were measured in these samples, and the sources of ²³⁶U in different sits were discussed.