A method for nitrite removal in nitrate N and O isotope analyses

Julie Granger, Daniel M. Sigman, Maria G. Prokopenko, Moritz F. Lehmann, Philippe D. Tortell

Limnol. Oceanogr. Methods 4:205-212 (2006) | DOI: 10.4319/lom.2006.4.205

ABSTRACT: We describe a method to remove nitrite from seawater and freshwater samples to determine the nitrogen (N) and oxygen (O) isotopic composition of nitrate in aqueous samples (15N/14N and 18O/16O, respectively). This method is simple, inexpensive, and effective in the removal of nitrite without compromising the natural N and O isotopic composition of nitrate. Moreover, the method is nontoxic and compatible with the “denitrifier method” for nitrate N and O isotope analysis. Nitrite is removed from solution by reduction to nitric oxide (NO) gas using ascorbic acid at a pH of ~3.5. The NO produced is continually degassed with an inert gas during the reaction, preventing O2 from reacting with NO to form new nitrate. Nitrate N and O isotope ratios were measured with the denitrifier method. The precision of isotope measurements of nitrate by the denitrifier method after nitrite removal averaged ±0.34‰ for 15N/14N and ±0.39‰ for 18O/16O.