Improving the measurement of nitrogen stable isotopes in organic materials containing high C:N ratios using a 5A molecular sieve column

Abstract

The nitrogen stable isotope composition (δ¹⁵N) of plant materials has numerous applications. Plant materials like bark can have a very high C:N ratio. Incomplete C combustion in such samples interferes with the δ¹⁵N measurement due to CO production. We modified the standard setup for δ¹⁵N measurement using an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (IRMS) by incorporating a 5A molecular sieve column, which better separates N₂ from CO. We compared this new modified setup and the standard one for the measurement of bark samples. Precision and accuracy for δ¹⁵N in standards with low C:N ratio were equivalent for the two methods. However, for bark the results obtained with the new method had better precision and accuracy than the standard method. Replicates are nevertheless recommended with the new method to ensure confidence in the results.

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  During elemental analysis, incomplete combustion of material with high C:N ratio can lead to CO formation, which interferes with δ¹⁵N IRMS measurements.

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  Here we use a 5A molsieve column to remove the CO interference in δ¹⁵N measurements Precision and accuracy on δ¹⁵N measurements of samples with high C content are significantly improved