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 (NO2 -) are useful for investigating chemical processes , 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 Delta(17O) value. Here we have adopted a combination of methodologies to develop a technique for measuring Delta(17O) of NO2 - by reducing nitrate (NO3 -) materials with internationally recognized Delta(17O) values to NO2 - using activated cadmium catalyzed by chloride in a basic solution while conserving Delta(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 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 , 34 for Delta(17O) quantification. The adapted method involves the following main points:center dot NO3 - reference materials with internationally recognized oxygen isotope composition are reduced to NO2 - under high pH conditions that conserve Delta(17O) values.center dot The NO3 - reference materials reduced to NO2 - and sample NO2 - with unknown Delta(17O) values are reduced to N2O using chemical methods involving sodium azide/acetic acid.center dot The product N2O is extracted, purified, decomposed to O 2 , analyzed for its isotope com-position using a continuous flow isotope ratio mass spectrometer for Delta(17O) quantification. The Delta(17O) of NO2 - samples are calibrated with respect to the NO3 - reference materials with known Delta(17O) values.