Systematic bias in the ²⁴⁰Pu/²³⁹Pu isotopic ratio of plutonium isotopic standard CRM138

Plutonium certified reference materials (CRMs) 136, 137, and 138 were produced, characterized, and certified in the period 1966 through 1971 by the National Bureau of Standards. The thermal ionization mass spectrometry (TIMS) instruments and the analytical methodologies evolved significantly since the characterization of these Pu isotopic standards. Given the excellent precision of modern TIMS and inductively coupled plasma instruments, systematic biases at the major and minor isotope ratios of these plutonium (Pu) isotopic standards (compared to the certified values) are to be expected. Studies on the major and minor isotope ratios of uranium (U) CRMs indicated systematic bias in the certified ratios of several of these standards. We present a systematic investigation of the ²⁴⁰Pu/²³⁹Pu major isotope ratio of Pu isotopic CRM138. The ²⁴⁰Pu/²³⁹Pu isotope ratio is shown to be biased by ~ 0.07–0.08%, larger than the bias seen in the major isotope ratio of any of the U CRM from New Brunswick Laboratory (NBL).