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

U-236 in the environment mainly originates from human nuclear activities. Based on the unique properties of uranium, U-236 can be used as a powerful tracer for investigation of oceanographic and environmental processes. This requires sensitive measuement of U-236 in various environmental samples. Due to the ultra-low radioactive level of U-236 in the environment, its measurement is only possible by mass spectrometry. Because of the low atomic ratio of 2(36)U/U-235 down to 10(-7)-10(-5) in the environment, the interferences of (UH+)-U-235-H-1 and peak tailings of U-235 and U-238 are critical challenges in the measurement of U-236 by ICP-MS. This work developed a sensitive ICP-MS/MS method for measurement of ultra-low U-236 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 x 10(-7). A minimum detectable U-236/U-238 ratio of 3.0 x 10(-1)0 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 U-236 (236UO(+)) was improved to 7.5 x 10(6) 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 x 10(-1)6 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. U-236/U-238 ratios down to 9 x 10(-10) were measured in these samples, and the sources of U-236 in different sits were discussed