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

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