Abstract. CCQM-P48 was a Pilot Study of the Inorganic Analysis Working Group of the Comité Consultatif pour la Quantité de Matière (CCQM) and was organized by the Institute for Reference Materials and Measurements (IRMM) of the European Commission. It was the first CCQM comparison dedicated to isotope ratio measurements.

It involved a rather rare combination of 5 national metrology institutes (NMIs), representing at the Comité International des Poids et Mesures (CIPM) 3 Member States of the Metre Convention and 2 international organisations, and 10 invited laboratories (5 'nuclear experts', 5 'geochemistry/mass spectrometry experts') selected outside CIPM. The aim was to compare capabilities to measure the n(²³⁴U)/n(²³⁸U), n(²³⁵U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U) ratios in 4 simulated biological/environmental materials prepared at a nominal uranium mass fraction of 5 µg g^-1 (series A) and, optionally, in 4 simulated biological/environmental materials prepared at a nominal uranium mass fraction of 5 ng g^-1 (series B). In addition, a diluted solution of IRMM-184, a natural uranium isotopic certified reference material (ICRM), had also been distributed for rehearsal as well as for calibration purposes. Participants, however, were free to apply the measurement strategy of their choice.

The test material can be assimilated to a biological/environmental type of sample as the matrix simulated approximately the composition of urine, from the combination in ~ 3% HNO₃ solution of purified inorganic salt (aquarium seawater salts to 18 ± 2 g kg^-1) and urea (to 17 ± 2 g kg^-1). Four uranium isotopic mixtures were produced by mixing uranium ICRMs in the gas phase as UF₆. Fractions of these mixtures, following their transformation under the form of solid UO₃, were dissolved individually into ~ 3% HNO₃ to get separate ~ 1000 mg uranium kg^-1 mother solutions. Both series of test materials derived from these solutions (simple dilution into batches of simulated urine), but participants were not informed about this similitude.

The absolute value of the n(²³⁵U)/n(²³⁸U) ratio in each isotopic mixture was established at the earliest stage (i.e. in the gas phase) by calibrated gas source mass spectrometry measurements, and thus could serve as independent reference value (expanded, k = 2, uncertainty U = 0.05%, relative) for results from series A. 'Mixture mode' (MM) median data based on participants' results were proposed as consensus reference values for all the other sets of results produced.

For samples from series A, 6 laboratories implemented a combination of sample digestion and matrix separation steps, whereas 3 laboratories did not report any particular sample treatment (apart possibly from dilution). Only mass spectrometry was employed for isotope ratio measurements, including 9 MC-ICPMS, 3 HR-ICPMS, 2 TIMS and 1 ICP-QMS. For n(²³⁵U)/n(²³⁸U) ratios, results from 11 laboratories were in agreement within stated U with reference values for at least 3 mixtures; stated relative U were always > 0.0120% (except for 1 participant) and always ≤ 0.7% (except for 1 participant with U > 1% for 3 samples). Results on n(²³⁴U)/n(²³⁸U) ratios were almost always within ± 3% around the calculated MM median value. For the n(²³⁶U)/n(²³⁸U) smallest ratio, ~ 5 × 10^-7, results relative to the calculated MM median value were within ± 20% for 8 participants, between 20% and 50% above for 3 other participants, over 50% above for 1 participant, and 3 participants did not report results. The perception of which factor caused the largest contribution to these uncertainty estimations differed among participants because of differences in the analytical methodologies deployed but mostly because of wide differences in concepts of uncertainty estimation.

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The final report has been peer-reviewed and approved for publication by the CCQM IAWG.