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

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.