Accurate Ultra-Trace Measurements of ²³⁶U Using the NexION 5000 ICP-MS

Uranium-236 (²³⁶U) is the product of thermal neutron capture of ²³⁵U, but the natural occurrence of ²³⁶U is very rare. The presence of this isotope in a sample of uranium indicates that the sample originates from a nuclear reactor. Compared to naturally occurring uranium isotopes like ²³⁴U and ²³⁵U, ²³⁶U has the advantage of high sensitivity in detecting the small anthropogenic U inputs to the large reservoir of environmental U.

ICP-MS is becoming the technique of choice for the measurement of long-lived radionuclides, including ²³⁶U, thanks to its affordability and ease of use in comparison with other techniques. However, accurate trace-level analysis of the ²³⁶U/²³⁸U ratio using ICP-MS requires proper interference management.

This work presents a method for accurate ultra-trace analysis of ²³⁶U using the NexION^® 5000 multi-quadrupole ICP-MS, the ideal system for this application. It is equipped with two full-length quadrupole mass analyzers (Q1 and Q3), which ensure that a typical < 0.7 amu mass resolution (with custom resolution < 0.3 amu) can be achieved for both analyzers while also ensuring high transmission efficiency. For the analysis of ²³⁶U in Mass Shift mode, only ²³⁶U⁺ and ²³⁵U¹H⁺ are selected in the first analyzer quadruple and transmitted to the Universal Cell (Q2), while ²³⁵U is rejected, and therefore, will not interfere with the downstream reaction and analysis.