Introduction
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.