Progress in the Preparation of a New Generation of Isotopic Reference Materials of Uranium

The need for reference materials of the highest quality (= lowest uncertainty) has been reiterated many times. The analytical techniques employed nowadays in field laboratories can measure isotopic ratios with high precision and therefore reference material with very low uncertainties are required to support such measurements. IRMM-072 has been the isotopic reference material of choice for calibrating mass-spectrometers as it has ratios certified to 0.03% relative uncertainty (k=2). This material now needs to be replaced and preparations are well advanced so that we expect the first members of a replacement series (designated as IRMM-074) to be ready within the next year. Other uranium reference materials are still needed for calibrating mass-spectrometers. For instance, the well-known NBS U CRM series, with ratios of 235U to 238U to cover the abundance range from 10 to 90% 235U enrichment, should be augmented or replaced with materials having lower uncertainties. There is also a recognised need for certified isotopic reference materials for the minor isotopes of uranium, 234 U and 236 U, especially for environmental samples. On-going work in the preparation of suitable materials will be described