Particle isolation for analysis of uranium minor isotopes in individual particles by secondary ion mass spectrometry

A new technique to measure 234U/238U and 236U/238U isotope ratios for individual particles in environmental samples was developed, which was a combination of particle isolation under scanning electron microscope (SEM) and secondary ion mass spectrometry (SIMS). The technique was verified by measuring 234U/238U and 236U/238U isotope ratios in individual particles in a simulated environmental sample containing uranium standard (NBL CRM U010) and Pb metal particles. When the uranium particles were not isolated, the relative deviations of the measured isotope ratios from the reference values increased with increasing the signal intensity ratio of 208Pb to 238U, which was due to the molecular ion interferences by the Pb particles co-existing in the sputtered area. By the isolation of individual uranium particles, the interferences were eliminated and the measured isotope ratios were in good agreement with the reference values. The maximum relative deviations among 20 particles were 8.9% for 234U/238U and 13.1% for 236U/238U isotope ratios, respectively. The technique was also successfully applied to the analysis of a real swipe sample containing various kinds of elements.