[en] A plutonium nitrate solution containing the isotopes ²³⁹Pu, ²⁴⁰Pu and ²⁴²Pu in a 1:1:1 atomic ratio has been produced at Harwell for use as a reference material for thermal ionisation mass spectrometric analysis. This reference material was prepared by blending weighed quantities of stock solutions of plutonium of high isotopic enrichment and accurately known plutonium concentration, established by controlled-potential coulometric measurement. The calculated atom ratios together with the associated uncertainties at the 95% confidence level are ²⁴⁰Pu to ²³⁹Pu 0.9999 ± 0.0006 and ²⁴²Pu to ²³⁹Pu 0.9999 ± 0.0010. (author)