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

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 (N-15/N-14 and O-18/O-16, 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 similar to 3.5. The NO produced is continually degassed with an inert gas during the reaction, preventing O-2 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 parts per thousand for N-15/N-14 and +/- 0.39 parts per thousand for O-18/O-16.