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

Abstract

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. 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. 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. 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- .