Nitrogen Mass Fraction and Stable Isotope Ratios for Fourteen Geological Reference Materials: Evaluating the Applicability of Elemental Analyser Versus Sealed Tube Combustion Methods

Thirteen commercially available silicate reference materials (RM) and one in-house reference material, eleven of which have no previously published values, were analysed for nitrogen mass fraction and isotopic ratios with an Elemental Analyser (EA), and a Sealed Tube Combustion line, coupled to a continuous flow isotope ratio mass spectrometer (IRMS). These materials ranged from m/mnitrogen mass fractions and delta N-15 of -0.5 to +19.8 parts per thousand. Existing nitrogen RM BHVO-2, MS#5 and SGR-1b were used to assess the accuracy of the data from the sealed tube combustion line, which was found to be in good agreement with existing published values. In contrast, the EA-IRMS failed to fully liberate nitrogen from all silicate rocks and minerals (achieving a mean of 44 +/- 10% nitrogen yield) resulting in kinetic fractionation of isotope values by -1.4 parts per thousand on average. Therefore, sealed tube combustion is better suited for analyses of silicate-bound nitrogen. The EA worked reliably for organic samples, but care should be taken when using the EA for silicate nitrogen research. Moving forward, it is recommended that BHVO-2, Biotite-Fe, FK-N and UB-N be used as quality control materials as they appear to be most reproducible in terms of nitrogen mass fraction (relative error < 10%, 1s), and isotopic composition (< 0.6 parts per thousand, 1s).