Modifications to the azide method for nitrate isotope analysis.

Abstract

The azide method for measuring the stable isotope ratios of nitrate (NO3- ) is easy to set up. However, the method requires spongy cadmium (Cd) or activated Cd powder which are not easy to prepare, and a toxic azide buffer is used. We aimed to use Cd powder directly to simplify preparation and to substantially reduce the azide dose. The reaction conditions were optimized in order to maximize the NO3- reduction yield. The original azide buffer was diluted by 10- to 10000-fold with or without addition of sodium acetate to reduce O-exchange between nitrite (NO2- ) and H2 O. The isotope ratios of the produced nitrous oxide (N2 O), used to examine the overall reaction performance, were measured using a purge and cryogenic trap system coupled to an isotope ratio mass spectrometer. It was found that Cd powder could be directly used to reduce NO3- to NO2- . A 100-fold diluted azide buffer could be used to reduce NO2- to N2 O when only the δ15 N value was measured, and the diluted azide buffer with sodium acetate when both δ15 N and δ18 O values were measured. Using the modified method, the standard deviations of the δ15 N and δ18 O measurements of international NO3- standards were 0.1 to 1.0‰ and often better than 0.3‰ (3 replicates). Compared with the original azide method, the techniques described here can reduce preparation time by using Cd powder without activation in the first reaction step and substantially (by >60-fold) reduce the dose of extremely toxic reagents containing azide by incorporating sodium acetate in the second reaction step. Our modified method is suitable for samples with small volume (5 mL), being different from previous methods in which 50 or 70 mL samples were used. Copyright © 2016 John Wiley & Sons, Ltd.