A double-spike MC TIMS measurement procedure for low-amount Ca isotopic analysis of limited biological tissue samples

Low-level 40Ca determinations using nitrous oxide with reaction cell inductively coupled plasma-tandem mass spectrometry

In inductively coupled plasma mass spectrometry, the most abundant Ca isotope (⁴⁰Ca) suffers from isobaric interference with argon, hindering the potential for low detection limits of Ca. A powerful approach is to remove the interference by using a reaction gas in a reaction cell. Ammonia (NH₃) has proven to be an effective reaction gas by process of a charge transfer reaction. However, NH₃ is highly corrosive and toxic and cannot remove isobaric ⁴⁰ K. Therefore, this work proposes the use of nitrous oxide (N₂O) to mass shift the target analyte ⁴⁰Ca to ⁴⁰Ca¹⁶O⁺ as a non-corrosive and non-toxic alternative. Instrument performance testing demonstrated that N₂O was capable of reaching equivalent detection limits (0.015 ng g⁻¹) and background equivalence concentrations (0.041 ng g⁻¹) to that of NH₃ and limited by the blank only. Further investigation of matrix interferences with synthetic standards highlighted that the N₂O approach supports the separation of potassium (K) and magnesium (Mg)–based interferences at tested concentrations of more than 600 times and almost 800 times higher than Ca respectively, whereas NH₃ was found to only support the removal of Mg. This work highlights a clear advantage of N₂O for low-level Ca determinations with high matrix loads, as well as compatibility with other instrumentation sensitive to corrosion that supports reaction cell technology.