Investigating uranium isotopic distributions in environmental samples using AMS and MC-ICPMS

Major, minor and trace uranium isotopes were measured at Lawrence Livermore National Laboratory in environmentally acquired samples using different instruments to span large variations in concentrations. Multi-collector inductively-coupled plasma mass spectrometry (MC-ICPMS) can be used to measure major and minor isotopes: 238U, 235U, 234U and 236U. Accelerator mass spectrometry (AMS) can be used to measure minor and trace isotopes: 234U, 236U and 233U. The main limit of quantification for minor or trace uranium isotopes is the abundance sensitivity of the measurement technique; i.e. the ability to measure a minor or trace isotope of mass M in the presence of a major isotope at M±1 mass units. The abundance sensitivity for 236U/235U isotope ratio measurements using MC-ICPMS is around ∼2×10−6. This compares with a 236U/235U abundance sensitivity of ∼1×10−7 for the current AMS system, with the expectation of 2–3 orders of magnitude improvement in sensitivity with the addition of another high energy filter. Comparing 236U/234U from MC-ICPMS and AMS produced agreement within ∼10% for samples at 236U levels high enough to be measurable by both techniques.