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

Abstract

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.