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

Abstract

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 < 10 μg g−1 to 1% m/m nitrogen mass fractions and δ15N of −0.5 to +19.8‰. 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‰ 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‰, 1s).

Supplemental Material: Hydrothermal recycling of sedimentary ammonium into oceanic crust and the Archean ocean at 3.24 Ga

“…Results were calibrated with USGS-61 and USGS-62 and are expressed in delta notation (δ 15 N) relative to atmospheric air. BHVO-2 was used for quality 36 control and gave a value of +1.9 ± 0.6 ‰, in reasonably good agreement with previous 37 measurements (Feng et al, 2018;Boocock et al, 2020). The results from the EA and sealed-tube combustion method are in good agreement, as shown in Figure A1.…”

“…ratio mass spectrometer at the STAiG labs, University of St Andrews. Sample powders were 32 oxidized in an evacuated quartz glass tube in the presence of CuO at 1000°C for c.4 h, followed 33 by c.2 h at 600°C (Boocock et al, 2020). This technique ensures complete liberation of latticebound ammonium from silicate rocks.…”

Reconstructing Nitrogen Sources to Earth’s Earliest Biosphere at 3.7 Ga

“…For nitrogen abundance and isotopes, analyses were done by offline combustion, which allows accurate isotopic analyses down to low nitrogen abundances (<10 ppm) in hard-tocombust silicate phases (Boocock et al, 2020). Quartz glass tubes were cleaned by combustion at 1,000 • C for >6 h before use.…”

Recommendations for offline combustion‐based nitrogen isotopic analysis of silicate minerals and rocks

“…Compared with other commonly used nitrogen extraction techniques, e.g. Kjeldahl digestion (which may volatize significant amounts of nitrogen during chemical processing) and short‐time combustion in an elemental analyzer (which combusts samples for only a few seconds at relatively low temperatures of 900–1000°C), the sealed‐tube combustion method surpasses them for its capability to carry out combustion at higher temperatures (up to 1200°C) and longer time (for hours) while retaining all released nitrogen intact inside a tube 7–11 …”

“…For example, Busigny et al 7 employed a preheating step at 450°C for 12 h with 1 h under an O 2 environment to remove possible organic contamination and absorbed air in samples, whereas other laboratories employed a lower temperature (e.g. 150°C) 10,16 or no preheating at all 8 . The effect of preheating on nitrogen yields and isotopic compositions has not been assessed, which makes inter‐laboratory data comparison difficult.…”