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Effect of chloride on the chemical conversion of nitrate to nitrous oxide for δ 15 N analysis
Author(s) -
Ryabenko Evgenia,
Altabet Mark A.,
Wallace Douglas W.R.
Publication year - 2009
Publication title -
limnology and oceanography: methods
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.898
H-Index - 72
ISSN - 1541-5856
DOI - 10.4319/lom.2009.7.545
Subject(s) - chemistry , halide , chloride , nitrogen , sodium , cadmium , nitrous oxide , seawater , inorganic chemistry , mass spectrometry , nuclear chemistry , nitrate , analytical chemistry (journal) , environmental chemistry , chromatography , oceanography , organic chemistry , geology
We investigate the influence of chloride concentration on the performance of the chemical reduction method for measurement of the nitrogen isotopic ratio (δ 15 N) in NO 3 − in natural waters (McIlvin and Altabet, 2005). In this method, NO 3 − is first reduced to NO 2 − using activated cadmium metal, with further reduction to N 2 O using sodium azide in an acetic acid buffer. N 2 O 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 NO 3 − to NO 2 − 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 δ 15 N determination. Addition of 5 M NaCl to all samples resulted in rapid (<4 h) and quantitative (>99%) reduction of NO 3 − to NO 2 − as well as stable δ 15 N 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 Cd 2+ produced over the course of the reaction due to formation of CdCl 2 .