Determination of 236U/238U isotope ratio in contaminated environmental samples using different ICP-MS instruments

This paper considers use of the U-236 isotope to monitor the spent uranium from nuclear fallout using inductively coupled plasma mass spectrometry (ICP-MS) in soil samples collected in the vicinity of the Chernobyl Nuclear Power Plant (NPP). Sector field ICP-MS (ICP-SFMS) and quadrupole based ICP-MS without and with hexapole collision cell (ICP-CC-MS) were used for uranium isotope analysis. In addition, a multiple ion collector ICP-MS (MC-ICP-MS) was used for high-precision isotope ratio measurements. A U-238(+) ion sensitivity of 18 GHz ppm(-1), 12.4 GHz ppm(-1) and 16 GHz ppm(-1), respectively, was observed in ICP-SFMS, ICP-CC-MS and MC-ICP-MS with ultrasonic nebulizer. An absolute sensitivity of 3600 counts fg(-1) was achieved for uranium by using a direct injection high efficiency nebulizer for solution introduction in ICP-SFMS. The detection limit for U-236 was in the fg g(-1) range and abundance ratio sensitivity for U-236/U-238 was 5 x 10(-6),3 x 10(-7), 6 x 10(-7), and less than 3 x 10(-7) in ICP-SFMS, MC-ICP-MS, quadrupole ICP-MS and ICP-CC-MS at mass resolution m/Deltam = 300, respectively. Interlaboratory comparison yielded a good accuracy (0.4-1.6%) of U-236/U-238 isotope ratios ranging from 1.5 x 10(-3) to 3.2 x 10(-4) measured in samples containing 100 ng of uranium. The U-236/U-238 isotope ratios and spent uranium fraction were determined in Chernobyl soil samples using single ion detector ICP-MS and multiple ion collector ICP-MS (MC-ICP-MS). In comparison to U-235/U-238 isotope ratio, the U-236/U-238 isotope ratio provided more sensitive and accurate determination of the portion of spent uranium from Chernobyl NPP in spent/natural uranium mixture in soil samples down to 0.1%. The concentration of Chernobyl spent uranium in upper 0-10 cm soil layers in investigated areas in the vicinity of Chernobyl NPP amounts to 2.4 x 10(-9) g g(-1) to 8.1 x 10(-7) g g(-1) depending mainly on the distance to the Chernobyl reactor.