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

Abstract

Rationale Due to isotope fractionations during partial nitrogen release from minerals and rocks, the complete extraction of nitrogen for analysis is crucial to ensure high‐quality nitrogen isotopic data. However, the appropriate nitrogen extraction conditions (e.g. temperature, duration) have not been established for most silicate minerals and rocks. Methods Nitrogen in a number of common minerals and rocks was extracted using the most robust sealed‐tube offline combustion techniques, purified and quantified in a custom‐made metal manifold, and carried by helium gas to an isotope ratio mass spectrometer for isotopic measurement at nanomolar nitrogen level. Each mineral or rock was combusted in a variety of temperature and duration conditions to compare the nitrogen yields and isotopic compositions. Results The nitrogen yields and isotopic compositions of minerals and rocks are strongly affected by combustion temperature and duration. The optimal combustion temperature is lowest for cyclosilicate minerals, followed by phyllosilicate, tectosilicate and inosilicate minerals. Preheating of samples can induce significant nitrogen loss and δ15N shift. Heating of samples above their optimal temperatures may cause nitrogen re‐assimilation by the residual mineral or rock. Conclusions Each mineral or rock has a characteristic optimal temperature and duration for complete nitrogen release. Preheating, under‐heating or over‐heating can cause nitrogen loss and isotopic shift. Therefore, we recommend using the offline combustion techniques and the optimal combustion conditions obtained in this study for nitrogen quantification and isotopic analysis of silicate minerals and rocks.

N2 in deep subsurface fracture fluids of the Canadian Shield: Source and possible recycling processes

“…The 15 N values are reported relative to the atmospheric N2. Repeat analyses of two standards (Low Organic Content Soil and High Organic Content Soil) from Elemental Microanalysis Ltd gave 2 analytical errors better than 0.2‰ (Li et al, 2021a).…”

“…Nitrogen isotopic ratios of rocks were measured at University of Alberta following the method described by Li et al (2021a). In brief, after removal of the surface by sawing, rock chips from drill cores were ground into fine powders (< 200 mesh).…”

“…For the graphite-bearing lens samples, nitrogen was measured on both bulk-rock samples and refractory reduced carbon residues following the extraction protocol of Eigenbrode and Freeman (2006). The powder samples were weighted and loaded into pre-cleaned one end-sealed quartz tube together with CuO reagents, pumped in a metal manifold, sealed under high vacuum, and combusted first at 1200 ºC for 30 min to 60 min (depending on lithology; see details in Li et al, 2021a) and then at 900 ºC overnight to completely release the fixed nitrogen in minerals. The sample tube was then cracked under high vacuum to release N2, which was further cryogenically purified, quantified by a capacitance manometer, and carried by an ultrahigh-purity helium gas to a Thermo Finnigan MAT 253 for nitrogen isotopic measurements.…”

Oceanic serpentinites: A potentially critical reservoir for deep nitrogen recycling

“…Following Li et al (2021), N concentrations and δ 15 N values of serpentinites were obtained using the offline sealed-tube combustion and extraction method coupled with continuous-flow isotope ratio mass spectrometry at the University of Alberta. Briefly, 150 -200 mg sample powder, ~400 mg CuO reactants and pre-cleaned (1200 ºC for 2 hours) quartz wool were sequentially loaded into a pre-cleaned one-end sealed quartz tube.…”

“…Subsequently, the sample tube was loaded back to the metal manifold, cracked to release N2 gas, which was purified, quantified by a capacitance manometer, and analyzed for isotopic ratio by a Thermo Finnigan MAT 253 isotope ratio mass spectrometer. While adsorption of tiny amounts of atmospheric components (e.g., nitrogen) by sample powders is possible, the impact of atmospheric N2 adsorption has been demonstrated to be minor by blank tests and comparison among N released at heating under different temperatures in Bebout et al (2007) and Li et al (2021). The δ 15 N value is reported relative to the atmospheric N2 (or Air) standard.…”