Effect of chloride on the chemical conversion of nitrate to nitrous oxide for δ15N analysis

We investigate the influence of chloride concentration on the performance of the chemical reduction method for measurement of the nitrogen isotopic ratio (delta N-15) in NO3- in natural waters (McIlvin and Altabet, 2005). In this method, NO3- is first reduced to NO2- using activated cadmium metal, with further reduction to N2O using sodium azide in an acetic acid buffer. N2O is introduced into an isotope ratio mass spectrometer (IRMS) for isotopic measurement. Previously, it was recognized that the presence of halides was necessary for the speed and efficiency of the second step but not thought to be important for the first step. Whereas quantitative Cd reduction of NO3- to NO2- had been noted for seawater samples, here we report, for freshwater and low-salinity (S < 30) samples, a variable conversion efficiency (both under-and overreduction were observed) and significant variation in delta N-15 determination. Addition of 5 M NaCl to all samples resulted in rapid (< 4 h) and quantitative (>99%) reduction of NO3- to NO2- as well as stable delta N-15 values that closely matched expected values for standards (within 0.3% of standard value). The positive effect of NaCl is likely due to a decrease in free Cd2+ produced over the course of the reaction due to formation of CdCl2.