Rapid and precise determination of the 238Pu/239Pu isotope ratio using thermal ionization mass spectrometry†

The combination of thermal ionization mass spectrometry and alpha spectrometry is employed for the routine isotope ratio measurements of plutonium. A rapid and reliable method based only on TIMS has been proposed for the isotope ratio analysis of plutonium including ²³⁸Pu/²³⁹Pu for the routine chemical quality control of nuclear fuel samples. The use of a membrane for sample loading and systematic study of ion source chemistry of uranium and plutonium resulted in a reduction in overall sample preparation and isotopic analysis time from about 15 h to 3–4 h. N,N′-Dioctyl-α-hydroxyacetamide (DOHA) in a room temperature ionic liquid (RTIL) viz. 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₆mim][NTf₂] immobilized in a polypropylene membrane was used for the purification of plutonium from nuclear fuel samples with U : Pu amount ratios up to 1000. The methodology is based on the isotope ratio measurements of ²⁴⁰Pu/²³⁹Pu, ²⁴¹Pu/²³⁹Pu and ²⁴²Pu/²³⁹Pu at low vaporization filament currents and ²³⁸Pu/²³⁹Pu at high vaporization filament currents for the elimination of isobaric interference at m/z 238 a.m.u., due to uranium. The accuracy and precision of ²³⁸Pu/²³⁹Pu obtained by the proposed TIMS based method are better than 0.8%, which is comparable to those obtained by alpha spectrometry.