Isotope reference materials IRMM-3636 was prepared by gravimetrically mixing of solutions from highly enriched 99.96% 233U (IRMM-3630) and highly enriched 99.97% 236U (IRMM-3660) in order to obtain an isotopic ratio n(233U)/n(236U) close to unity. IRMM-3636a and IRMM-3636b were prepared from IRMM-3636 by gravimetrical dilution. The primary solution, IRMM-3636 is for the time being not yet available for external users. The certified isotope content values for IRMM-3636a of 4.27988(54)*10-7 mol U per g of solution and for IRMM-3636b of 2.21802(61)*10-8 mol U per g of solution and the certified isotope amount ratios have been established by mass metrology, subsequently the primary solution IRMM-3636 was verified by isotope mass spectrometry. The methodology used in the preparation and certification was similar to that of comparable uranium mixtures made in the past.
The uncertainties contributing to the final uncertainties of the isotopic ratios are the weighing errors, the measured impurities in each starting material, the stoichiometry of the oxides and the isotopic abundances of each of the starting materials. The method for the preparation and mixing is described and the certification and verification procedures are reported.
Verification of IRMM-3636 was performed by TRITON TIMS measurements using Faraday collectors and secondary electron multiplier in combination with an RPQ energy filter for improved abundance sensitivity. The results agreed well with the certified values obtained from the mixing calculations.
By mixing the n(233U)/n(236U) double spike with a sample and applying internal mass fractionation correction, the n(235U)/n(238U) ratio of a given sample can be determined with the highest possible accuracy.
The Isotopic Reference Materials IRMM-3636, IRMM-3636a and IRMM-3636b are part of a systematic IRMM programme to supply Isotopic Reference Materials of various isotopes at different concentrations. The Isotopic Reference Material is supplied in a sealed quartz ampoule containing 1 mL of a 1 M nitric acid solution.JRC.D.4-Isotope measurement
