The Isobar Separator for Anions for Accelerator Mass Spectrometry
A Short History
Professor Emeritus Ted Litherland from the IsoTrace Laboratory of University of Toronto, one of the pioneer of AMS, once noted that, in theory, highly specific chemical reactions involving common gases such as NO2 were selective enough to resolve chlorine from sulphur at the desired separation level. In practice however, the high energy environment in AMS instruments was not conducive to this type of low energy ion-molecule chemistry. What was needed was to first slow down the high energy negative ions produced by the AMS source, make them react with a gas chosen for its physical and chemical properties, and reaccelerate them into the AMS injection path.
Designed by RFQ specialists of Isobarex using a combination of high precision machined quadrupole rods and high precision RF quadrupole power supplies, the ISA demonstrated that it creates the ideal reactive environment for the elimination of 36S from 36Cl. Soon, other applications were found and today atomic interferences of 41Ca, 90Sr, 135/137Cs can be addressed through the use of standard reaction gases such as NO2, H2 and Ar.
How the Isobar Separator Works
The Isobar Separator incorporates a high efficiency ion transfer optic system and a low energy RFQ ion guide. At the heart of the Isobar Separator, a gas reaction cell allows ions to react with a neutral gas at low energy (~1 eV). The differential rate of reaction between the isotope of interest and its interfering species provides a significant stage of interference reduction prior to entering the accelerator. In the 36Cl application, the isobar 36S is reduced by a proven seven orders of magnitude by use of NO2 as reaction gas, providing an impressive example of the high selectivity now accessible on AMS of all size.
More generally, the Isobar Separator creates optimal energy conditions for a variety of ion-molecule reactions to occur in the AMS ion beam, offering a powerful research platform for investigating new separation strategies involving other reaction gases. Up to now, the acceleration power of the AMS instrument was the main factor determining which radiotracers could be detected on a given system. Since many of the radiotracers currently in use require very high energy accelerators to be separated from their respective isobars, only the rarest and most expensive AMS instruments could be used to detect them.
The Isobar Separator is opening a new era where smaller and more affordable AMS systems will be able to measure a larger variety of radiotracers, even possibly radiotracers never used before. The cost and size of AMS systems has always constituted a significant barrier that prevents a more extensive use of key radiotracers in many scientific disciplines. By putting the capabilities of smaller AMS systems on par with the largest ones, the Isobar Separator will improve access to AMS technology and new applications. As well, its use on current very large AMS systems should push limits of detection for many radiotracers to unprecedented low levels.
The ISA technology is patented in Canada, United States, Japan and major European countries.
J. Eliades et al, “Cl / S isobar separation using an on-line reaction cell for 36Cl measurement at low energies”, Nuclear Instruments and Methods B268 (2010) 839–842
J-F Alary et al, “A Design Study for the Analysis of 90Sr and 135,137Cs by ISA-AMS”, Proceedings of the 22nd International Conference on the Applications of Accelerators in Research and Industry, Fort Worth TX, August 2012, AIP Conf. Proc. 1525 (2013) 436; DOI: 10.1063/1.4802365
W. E. Kieser et al, “Fluoride Sample Matrices and Reaction Cells – New Capabilities for Isotope Measurements in Accelerator Mass Spectrometry”, Proceedings of ER2010 - Environmental Radioactivity New Frontiers and Developments, Vol 104, eds W. Plastino, P.P. Povinec, EPJ Web of Conferences 24 07007 (2012); DOI: 10.1051/epjconf/20122407007
X.-L. Zhao et al, “A Study of KF3- Attenuation in an RFQ Gas Cell for 41Ca AMS”, Radiocarbon 55 (2013), 268-281
C. MacDonald et al, “Determination of 135Cs by accelerator mass spectrometry”, Nuclear Instruments and Methods B361 (2015), 554-558; DOI: 10.1016/j.nimb.2015.03.008
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