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

Oxygen stable isotopes (i.e., (16)O, (17)O, (18)O) of nitrite (NO(2)(−)) 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(2)(−) due to the lack of internationally recognized NO(2)(−) reference materials with a well-known Δ((17)O) value. Here we have adopted a combination of methodologies to develop a technique for measuring Δ((17)O) of NO(2)(−) by reducing nitrate (NO(3)(−)) materials with internationally recognized Δ((17)O) values to NO(2)(−) using activated cadmium catalyzed by chloride in a basic solution while conserving Δ((17)O). The NO(3)(−) reference materials reduced to NO(2)(−) and sample NO(2)(−) unknowns are converted to N(2)O using sodium azide/acetic acid reagent and decomposed to O(2) 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((17)O) quantification. The adapted method involves the following main points: • NO(3)(−) reference materials with internationally recognized oxygen isotope composition are reduced to NO(2)(−) under high pH conditions that conserve Δ((17)O) values. • The NO(3)(−) reference materials reduced to NO(2)(−) and sample NO(2)(−) with unknown Δ((17)O) values are reduced to N(2)O using chemical methods involving sodium azide/acetic acid. • The product N(2)O is extracted, purified, decomposed to O(2), and analyzed for its isotope composition using a continuous flow isotope ratio mass spectrometer for Δ((17)O) quantification. The Δ((17)O) of NO(2)(−) samples are calibrated with respect to the NO(3)(−) reference materials with known Δ((17)O) values.