Certification of a new series of gravimetrically prepared synthetic reference materials for n(236U)/n(238U) isotope ratio measurements

A new series of gravimetrically prepared synthetic isotope reference materials, the so-called IRMM-075 series, with n(236U)/n(238U) isotope ratios varying from 10−4 to 10−9 has been prepared and certified. This series is suited for calibration of n(236U)/n(238U) measurements made by various mass spectrometric techniques such as TIMS (Thermal Ionization Mass Spectrometry), ICPMS (Inductively Coupled Plasma Mass Spectrometry), AMS (Accelerator Mass Spectrometry) and RIMS (Resonance Ionization Mass Spectrometry).Natural uranium with low 236U isotope abundance and highly enriched 236U were both purified using well proven chemical methodology. The oxides were calcined into U3O8 in an oven installed in a glove-box with controlled humidity. Primary solutions of the same concentration were prepared by dissolving the oxides of 236U and natural U. From the 236U solution a series of four dilutions was made by weighing. Weighed amounts of the dilutions of 236U and weighed amounts of natural U were mixed to form a set of n(236U)/n(238U) mixtures at a concentration of 1 mg U g−1.Verification of the mixtures IRMM-075/1–6 was performed by TIMS measurements using Faraday collectors and a secondary electron multiplier in combination with an energy filter for improved abundance sensitivity. The results agreed well with the certified values obtained from the gravimetrical mixing calculations.The uncertainties of the calculated isotopic ratios are significantly lower compared to typical uncertainties from mass spectrometric techniques such as TIMS and ICPMS and in particular to AMS and RIMS. The methods for the preparation and mixing of the IRMM-075/1–6 series are described and the certification and verification procedures are reported.