Nitrogen Mass Fraction and Isotope Determinations in Geological Reference Materials Using Sealed‐Tube Combustion Coupled with Continuous‐Flow Isotope‐Ratio Mass Spectrometry

Abstract

A method using a high‐temperature sealed‐tube combustion technique coupled with a continuous‐flow isotope‐ratio mass spectrometer (CF‐IRMS) was developed in this study to accurately determine the nitrogen mass fractions and δ15N of silicate rocks with a low nitrogen mass fraction of < 200 μg g−1. Analysis of reference material (HBa3) demonstrated that the nitrogen mass fraction and δ15N results using the developed method were in agreement with data obtained using classical sealed‐tube combustion coupled to a dual‐inlet MS. Given the lack of commercially accessible reference materials usable for interlaboratory comparisons, two igneous reference materials (BCR‐2 and BHVO‐2) obtained from the United States Geological Survey (USGS) were analysed for nitrogen mass fractions and δ15N using the new method. Due to their homogenous characteristics, they can potentially be used as test materials for inter‐laboratory comparisons. The sealed‐tube combustion technique yielded higher N mass fractions than the elemental analyser, suggesting that previous nitrogen mass fractions measured using EA‐IRMS were underestimated. Thus, the nitrogen mass fractions and δ15N of BCR‐2 and BHVO‐2 may not have been correctly determined using EA‐IRMS.

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

“…This average value was subtracted from all standard and sample data. To assess analytical accuracy, we analyzed three aliquots of BHVO-2 and obtained an isotopic value of +2.3 ± 0.3 and a total nitrogen abundance of 20.8 ± 0.9 ppm, which agrees well with previous studies (Feng et al, 2018;Boocock et al, 2020).…”

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.…”

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

“…The analytical line (Figure 1) used at the University of Alberta (Edmonton, Canada) includes a Thermo MAT 253 isotope ratio mass spectrometer (Thermo Fisher Scientific, Bremen, Germany), a GasBench II interface (Thermo Fisher Scientific) and a custom‐made ultrahigh‐vacuum metal line. The setup is similar to the ones at Lehigh University 8 and the Chinese Academy of Sciences, 16 but improved in two aspects: (1) the tube breaker was expanded from one to a set of six, which allows six samples to be run in a batch without breaking the system vacuum to air for sample reloading; and (2) six extra ports were added for cryogenic sample collection, which allow CO 2 gas from the same sample (if there is any) to be collected for additional carbon isotopic analysis.…”

“…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.…”