Isobaric interference removal for selected radionuclides using nitrous oxide and ammonia with inductively coupled plasma tandem mass spectrometry

Abstract

The determination of long-lived radionuclides by inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) is a well-established approach. However, such determinations can still be hindered by isobaric interferences from stable isotopes of neighbouring elements. As such, investigations towards novel gas cell approaches for removing interfering ions are required in order to improve the reliability of the analysis. Nitrous oxide (N₂O) is a reaction gas that has been well studied for stable isotope analysis. Studies towards its applicability to radionuclide analysis have so far been limited. Here, the use of N₂O, as well as a mixture with ammonia (NH₃), have been evaluated for determinations of 10 radionuclides of interest for nuclear decommissioning: ⁴¹Ca, ⁶³Ni, ⁷⁹Se, ⁹⁰Sr, ⁹³Zr, ⁹³Mo, ⁹⁴Nb, ¹⁰⁷Pd, ¹³⁵Cs, and ¹³⁷Cs. Single element solutions of stable isotope analogues of the radionuclides, as well as solutions of the interfering ions, were used to observe the reactions with the ICP-MS/MS reaction cell gases. Abundance-corrected sensitivities were used to assess the achievable separation factors and sensitivities for the determination of the radionuclides of interest. The N₂O/NH₃ gas mixture was found to provide a significant enhancement in the removal of isobaric interferences, as well as instrument detection limits (given in brackets), compared to N₂O alone for determinations of ⁴¹Ca (0.50 pg g⁻¹ (0.0016 Bq g⁻¹)), ⁷⁹Se (0.11 pg g⁻¹ (5.4 × 10⁻⁵ Bq g⁻¹)), ⁹⁰Sr (0.11 pg g⁻¹ (0.56 Bq g⁻¹)), ⁹³Mo (0.12 pg g⁻¹ (0.0044 Bq g⁻¹)), ¹³⁵Cs (0.1 pg g⁻¹ (7.5 × 10⁻⁶ Bq g⁻¹)), and ¹³⁷Cs (0.1 pg g⁻¹ (0.33 Bq g⁻¹)).