Triple oxygen stable isotope analysis of nitrite measured using continuous flow isotope ratio mass spectrometry.

ABSTRACT

Oxygen stable isotopes (i.e., 16O, 17O, 18O) of nitrite (NO2-) are useful for investigating chemical processes and sources contributing to this important environmental contaminant and nutrient. However, it remains difficult to quantify the oxygen isotope compositions of NO2- due to the lack of internationally recognized NO2- reference materials with a well-known Δ(17O) value. Here we have adopted a combination of methodologies to develop a technique for measuring Δ(17O) of NO2- by reducing nitrate (NO3-) materials with internationally recognized Δ(17O) values to NO2- using activated cadmium catalyzed by chloride in a basic solution while conserving Δ(17O). The NO3- reference materials reduced to NO2- and sample NO2- unknowns are converted to N2O using sodium azide/acetic acid reagent and decomposed to O2 by passing through a heated gold tube and introduced into a continuous flow isotope ratio mass spectrometer for analysis at m/z 32, 33, and 34 for Δ(17O) quantification. The adapted method involves the following main points•NO3- reference materials with internationally recognized oxygen isotope composition are reduced to NO2- under high pH conditions that conserve Δ(17O) values.•The NO3- reference materials reduced to NO2- and sample NO2- with unknown Δ(17O) values are reduced to N2O using chemical methods involving sodium azide/acetic acid.•The product N2O is extracted, purified, decomposed to O2, and analyzed for its isotope composition using a continuous flow isotope ratio mass spectrometer for Δ(17O) quantification. The Δ(17O) of NO2- samples are calibrated with respect to the NO3- reference materials with known Δ(17O) values.