Experimental Evaluation and Optimisation of Sealed-Tube Combustion Methods for Silicate Nitrogen Extraction.

RATIONALE: Nitrogen (N) abundance and isotopic compositions in silicate rocks are commonly measured by sealed-tube continuous-flow isotope ratio mass spectrometry (CF-IRMS), but combustion protocols vary widely. It remains uncertain how the quality of data obtained from different published methods compares for differing sample types.

METHODS

To address this uncertainty, nitrogen abundance ([N]) and isotopic composition (δ15N) were measured in granite, plagioclase, alkali feldspar and biotite using tube-cracker CF-IRMS with seven different combustion protocols: One followed a combined tube- and box-furnace method, whereas the other six used a simplified box-furnace-only approach, generally involving longer heating durations at lower temperatures.

RESULTS

For granite, plagioclase, alkali feldspar and biotite, we successfully identified a method that does not require a 1200°C heating step in the tube furnace without negatively impacting N yield. For granitic rocks, in cases where the highest yield methods are unsuitable (i.e., due to low [N], high sample mass requirements or lower sample melting point), we propose a suitable alternative that requires corrections on the final measured value to account for incomplete N extraction.

CONCLUSIONS

For sealed tube combustion paired with continuous-flow isotope ratio mass spectrometry, lower combustion temperatures typically reduce N extraction from silicate materials. However, increasing the combustion duration at temperatures equal to or below 1000°C can reduce, and in some cases eliminate, this yield difference.