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The correction for 238 U isobaric effect was investigated to determine the 238 Pu isotope content. It is generally difficult to get rid of uranium effect on the measurement of 238 Pu by mass spectrometry because plutonium cannot be purely separated from spent nuclear fuel and uranium exists on filament and in reagents as the impurities. The effect of 238 U was corrected by measuring Pu + and UO + in the sample spiked with 233 U and subtracting the uranium effect from the measured values. We controlled the heating currents of vaporization and ionization filaments to get the different ions of U + , Pu + , UO + and PuO + using thermal ionization mass spectrometry. For the Pu-U mixed solution, uranium was detected at lower filament temperature compared to plutonium, and the UO + peak was larger than the U + peak. The 238 U/ 233 U ratio was obtained by measuring the 238 UO + / 233 UO + ratio at low temperature in a mixed solution of plutonium and 233 U (> 98 atom %) and then the 238 Pu/ 239 Pu ratio was obtained by measuring the ( 238 Pu+ 238 U)/ 239 Pu and 233 U/ 239 Pu ratios at high temperature. For the plutonium standard solution, New Brunswick Laboratory-Certified Reference Material, Plutonium oxide (NBL CRM No. 122-PuO2), the 238 Pu isotope composition obtained after 238 U correction was approximately 15 % lower than the measured value before the correction. The 238 Pu isotope composition after the 238 U correction for the plutonium solution that separated from the spent nuclear fuel was 30 % lower than the measured value.

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