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 (δN) 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 δN measurement due to CO production. We modified the standard setup for δN measurement using an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (IRMS) by incorporating a 5A molecular sieve column, which better separates N from CO. We compared this new modified setup and the standard one for the measurement of bark samples. Precision and accuracy for δN 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. •During elemental analysis, incomplete combustion of material with high C:N ratio can lead to CO formation, which interferes with δN IRMS measurements.•Here we use a 5A molsieve column to remove the CO interference in δN measurements Precision and accuracy on δN measurements of samples with high C content are significantly improved.