Investigations for isobaric interference of Pu-238 at U-238 during thermal ionization mass spectrometry of uranium and plutonium from the same filament loading

Three different methodologies are reported for circumventing the isobaric interference of Pu-238 at U-238 during simultaneous thermal ionization mass spectrometric analysis of U and Pu. These are as follows: (i) obtaining the U-235/U-238 amount ratio at lower evaporation filament current and hence eliminating the need for any isobaric interference correction due to Pu-238 at U-238; (ii) determining the Pu-240/Pu-239 amount ratio at lower evaporation filament current during the optimization of the ion current of U+ and Pu+ and calculating the Pu-238/Pu-239 amount ratio of the unknown sample; (iii) optimizing the evaporation filament heating current for simultaneous mass spectrometric analysis of U and Pu and using the Pu-240/Pu-239 amount ratio obtained from the first 2-3 blocks to arrive at the Pu-238/Pu-239 atom ratio required in the interfering element correction (IEC) approach. The three methodologies described are useful for either (a) completely eliminating the need to know the Pu-238/Pu-239 amount ratio or (b) determining the Pu-238/Pu-239 amount ratio from the same filament loading prior to performing simultaneous mass spectrometric analysis. The drawbacks and advantages of each of the methodologies are discussed with respect to the accuracy of the data and the time of analysis. It is shown that the last methodology along with the IEC approach provides a universal solution to correct for the isobaric interference of Pu-238 at U-238 in all the unknown samples. The studies were carried out by using the conventional method of loading nitrate solutions of U and Pu, a double rhenium filament assembly, a multicollector Faraday cup detector system and employing synthetic mixtures of SRM-200-U and SRM-947-Pu with varying U/Pu ratios.