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This International Standard describes three simple and fast procedures to prepare plutonium sources and a procedure to measure the activity ratio of 238Pu to (239Pu + 240Pu) by á spectrometry. The á spectrometry method is used for the determination of isotopic abundance of 238Pu in combination with mass spectrometry and eliminates the possible interferences of 238U in the latter method. It applies to the analysis of purified solutions of plutonium in 2 mol/l to 4 mol/l nitric acid containing 50 µg to 200 µg of plutonium per millilitre, as may result from the chemical treatment and purification preceding plutonium isotopic analysis by mass spectrometry. This International Standard applies to plutonium solutions free from 241Am and those containing less than 10 % of other non-volatile impurities relative to the plutonium content. Anion exchange chromatography in accordance with ISO 8299:1993 [12], Subclauses 7.2 and 7.3, or solvent extraction chromatography in ISO 15366 [13] are suitable purification methods. The methods given in this International Standard are intended for use in conjunction or in parallel with mass spectrometry for the isotopic analysis of plutonium in spent-fuel solutions or nuclear-grade plutonium products.