Adaptation of the acetone method for the δ¹⁵ N-NO₃ ^- analysis of saline water with a low concentration of nitrate

Rationale: The recently reported acetone method for δ¹⁵ N-NO₃ ^- analysis is a straightforward sample preparation process. However, the applicability of the method to water samples having high salinity and low concentration of nitrates is not good. Therefore, we developed a new method to measure δ¹⁵ N-NO₃ ^- values in a saline sample having low nitrate concentration by improving the acetone method (the adapted method), and verified its usability.

Methods: The adapted method involves (a) reducing a large volume of sample by evaporation, (b) removing NaCl by mixing acetone twice, (c) changing the volume ratio of the acetone/NaI/n-hexane solvent from 21/0.35/10 to 30/0.25/5 mL, (d) using an increased volume of BaI₂ (0.1 M) - from 0.75 to 1 mL, and (e) using an alternative capsule drying process of lyophilization.

Results: The adapted method has provided nearly exact δ¹⁵ N-NO₃ ^- values of standard materials with high precision and accuracy (<0.1‰) at a lower cutoff of 2 μmol of NO₃ ^- in a sample capsule. The recovery of NO₃ ^- in this method was significantly dependent (P < 0.05) on the Cl^- concentration of the samples. However, a minimum recovery of 60% NO₃ ^- , required for the reproducibility of the accurate value of δ¹⁵ N-NO₃ ^- , was possible from the concentrated sample aliquot containing up to 6 g of Cl^- . The application of the adapted acetone method to creek water samples, collected from mangrove watersheds, has shown high consistency in the analysis results.

Conclusions: It was proved that the inexpensive and easy-to-handle adapted acetone method could be used for the δ¹⁵ N-NO₃ ^- analysis of saline water samples when the sample aliquot contained about 4 μmol of NO₃ ^- with less than 6 g of Cl^- .