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Abstract No.: TuP-LB6
Session: LATE-BREAKING/Gas Phase Ion Chemistry
Presentation date: Tue, Aug 29, 2006
Presentation time: 14:30 – 16:00

Feasibility of using 18O/16O Amount Ratio for Fractionation Correction during Isotopic Analysis of U by Thermal Ionization Mass Spectrometry

Suresh Kumar Aggarwal1, Devanathan Alamelu1, A. R. Parab1

1 Bhabha Atomic Research Centre, Mumbai, India

Correspondence address: A. R. Parab, Bhabha Atomic Research Centre, Fuel chemistry Division, Trombay, Mumbai, 400 085 India.

Keywords: Ionization; Ionization, Surface; Mass Spectrometry, Isotope Ratio; Mass Spectrometry, Magnetic Sector.

Novel aspect: Use of oxygen isotope ratio to correct for isotope fractionation in uranium during TIMS.


Isotope fractionation in thermal ionization mass spectrometry (TIMS) is recognized as a source of variable systematic uncertainty and this limits the accuracy of isotope ratio data. A number of approaches have been advocated to minimize/eliminate this uncertainty. These include (i) total evaporation and ion current integration (ii) internal normalization and (iii) double spike. Amongst these, the only methodology applicable to elements having only two isotopes is the total evaporation and ion current integration (TE-ICI). This methodology is based on monitoring and integrating, using multi Faraday cup detection system, the signal of all the isotopes of the most abundant ion (atomic/molecular) observed in TIMS. However, it is known that the evaporation and ionization in a thermal ionization source may lead to the formation of atomic and molecular ions simultaneously, which can contribute to some uncertainty even when the TE-ICI methodology is employed to overcome the isotope fractionation effect.

In view of the above, it was considered worthwhile investigating the approach of using 18O/16O isotopes amount ratio for internal normalization to correct for isotope fractionation. To study the feasibility of utilizing this approach for U isotope fractionation correction in TIMS, the two certified isotopic reference materials of U (NIST-CRM-U-010 and NIST-CRM-U-500) were analyzed mass spectrometrically for 235U/238U isotope ratio using U+ as well as UO+ ions in the static mode of multicollection. The effects of varying the heating temperatures of ionization and vaporization filaments on the formation of U+ and UO+ were also studied. The fractionation correction per atomic mass unit (amu) derived from 18O/16O ratio observed by monitoring 238UO+ ions at m/q of 256 and 254 was correlated with the fractionation correction per amu for uranium isotopes derived from 238UO+ /235UO+ at m/q of 254 and 251. A linear correlation was observed between the fractionation correction factors (K-factors). In some cases where the filaments were allowed to degas for more than 30 minutes, the K-factor for uranium obtained from UO+ ion was found to remain constant while the K-factor for oxygen isotopes continued to increase with time of analysis. The results are encouraging. There exists a definite possibility to use oxygen isotope ratio as an internal normalization to correct for isotope fractionation during TIMS analysis of uranium. This approach can also be used for elements (e.g. Boron) having only two isotopes.