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

Oxygen stable isotopes (i.e., O, O, O) of nitrite (NO) 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 NO due to the lack of internationally recognized NO reference materials with a well-known Δ(O) value. Here we have adopted a combination of methodologies to develop a technique for measuring Δ(O) of NO by reducing nitrate (NO) materials with internationally recognized Δ(O) values to NO using activated cadmium catalyzed by chloride in a basic solution while conserving Δ(O). The NO reference materials reduced to NO and sample NO unknowns are converted to NO using sodium azide/acetic acid reagent and decomposed to O by passing through a heated gold tube and introduced into a continuous flow isotope ratio mass spectrometer for analysis at 32, 33, and 34 for Δ(O) quantification. The adapted method involves the following main points:•NO reference materials with internationally recognized oxygen isotope composition are reduced to NO under high pH conditions that conserve Δ(O) values.•The NO reference materials reduced to NO and sample NO with unknown Δ(O) values are reduced to NO using chemical methods involving sodium azide/acetic acid.•The product NO is extracted, purified, decomposed to O, and analyzed for its isotope composition using a continuous flow isotope ratio mass spectrometer for Δ(O) quantification. The Δ(O) of NO samples are calibrated with respect to the NO reference materials with known Δ(O) values.