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

Abstract

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.Graphical abstract