Simultaneous determination of the 235U/ 238U isotope ratio and concentration at nanogram levels of uranium employing a mixed spike in thermal ionization mass spectrometry

Abstract

A method for the simultaneous determination of the 235U/ 238U isotope ratio (at 235U abundances close to that of natural uranium) and the concentration of uranium in the sample at nanogram levels of uranium using a commercially available thermal ionization mass spectrometer with secondary electron multiplier (SEM) detector is presented. The method depends on the use of a spike of 233U + 235U with about 4 at.% of 235U which is added to the unknown sample solution. This increases the ion current of 235U + in the mass spectrometer, thereby facilitating the use of the peak integration option with peak centring each time during data acquisition for the 235U peak. The 235U/ 238U isotope ratio and the concentration or total amount of uranium in the sample are calculated from the data obtained on the 238U/ 233U and 235U/ 233U isotope ratios in the spiked mixture. An accuracy of better than 1% is demonstrated for the determination of the 235U/ 238U isotope ratio. This methodology will be useful for determining the abundances of the minor isotopes of different elements using the recently available multi-Faraday cup detector system. From the analyses of isotopic reference materials of uranium, the overall mass discrimination factor determined using a Faraday cup detector is 0.09% per mass unit which is due to the mass-dependent isotope fractionation in the ion source using the double rhenium filament assembly. The overall mass discrimination determined using SEM in the current mode at an operating voltage of 1.87 kV is 0.33% per mass unit, which leads to a contribution of 0.24% (= 0.33−0.09)% per mass unit due to the electron multiplier.