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

In inductively coupled plasma mass spectrometry, the most abundant Ca isotope ( 40 Ca) 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 (NH 3 ) has proven to be an effective reaction gas by process of a charge transfer reaction. However, NH 3 is highly corrosive and toxic and cannot remove isobaric 40  K. Therefore, this work proposes the use of nitrous oxide (N 2 O) to mass shift the target analyte 40 Ca to 40 Ca 16 O + as a non-corrosive and non-toxic alternative. Instrument performance testing demonstrated that N 2 O was capable of reaching equivalent detection limits (0.015 ng g -1 ) and background equivalence concentrations (0.041 ng g -1 ) to that of NH 3 and limited by the blank only. Further investigation of matrix interferences with synthetic standards highlighted that the N 2 O 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 NH 3 was found to only support the removal of Mg. This work highlights a clear advantage of N 2 O for low-level Ca determinations with high matrix loads, as well as compatibility with other instrumentation sensitive to corrosion that supports reaction cell technology.