Effect of Laboratory Reagents for Cleaning Modern Contamination on the δ15N Integrity of Nitrogen-Poor Samples

Abstract

Rationale

Nitrogen isotope values have been widely used for tracing ancient biogeochemical processes, but contaminant-sensitive materials, such as Precambrian sedimentary rocks, meteorites, or returned extraterrestrial samples, often have low nitrogen levels and are thus vulnerable to modern organic contamination. However, the polluting effects of laboratory solvents commonly used for organic decontamination of such materials remain unquantified for nitrogen contents and isotopic ratios.

Methods

Four pre-treatment protocols using the common organic solvents dichloromethane (DCM), n-hexane, and ethanol were applied to powdered quartz controls to evaluate their pollution of nitrogen concentrations and isotopic compositions. The least polluting protocol was then applied to chert samples with low nitrogen contents to assess its effectiveness in recovering primary nitrogen signatures.

Results

Comparison of total nitrogen contents before and after pre-treatments indicates that DCM introduced the most nitrogen pollution, followed by ethanol, with n-hexane causing the least. DCM-only and DCM with ethanol pre-treatments showed a large deviation of δ¹⁵N values compared to untreated controls, while the ethanol-only pre-treatment showed a positive shift in δ¹⁵N values. Both n-hexane-only and n-hexane with ethanol pre-treatments showed consistent δ¹⁵N values with the untreated controls. While n-hexane-only pre-treatment had the least effect on total nitrogen content and the nitrogen isotopic composition of the untreated controls, the use of double-solvent pre-treatment with differing polarities is recommended to address the broad range of modern contaminants found in nature.

Conclusions

The n-hexane followed by ethanol pre-treatment provided the optimal outcome for pre-treatment and can effectively remove modern nitrogen contamination in samples with as little as 9 ppm total nitrogen without indigenous isotopic values being obscured by reagent-derived nitrogen pollution.