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

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)).