Removal of nitrite with sulfamic acid for nitrate N and O isotope analysis with the denitrifier method

In environmental water samples that contain both nitrate (NO ³_-) and nitrite (NO₂^-), isotopic analysis of nitrate alone by all currently available methods requires pretreatment to remove nitrite. Sulfamic acid addition, used previously for this purpose (Wu JP, Calvert SE, Wong CS. Deep-Sea Research Part I - Oceanographie Research Papers 1997; 44:287), is shown here to be compatible with the denitrifier method for both N and O isotope analysis of nitrate. Sulfamic acid at a pH of ∼1.7 reduces nitrite to N₂. Samples are then neutralized with base prior to isotope analysis, to alleviate the buffering demands of the bacterial media and as a precaution to prevent modification of nitrate during storage with the residual sulfamic acid at low pH. Under appropriate reaction conditions, nitrite is completely removed within minutes. Sulfamic acid treatment does not compromise the completeness of the conversion of nitrate into N₂O or the precision and accuracy of N and O isotope measurements by the denitrifier method. Nitrite concentrations upwards of 7 times the ambient nitrate can be removed without affecting the isotope composition of nitrate. The method is applied to analyses of the coupled N and O isotopes of nitrate and nitrite in waters of the Mexican Margin, to illustrate its efficacy and utility when employed either in the field upon sample collection or in the lab after months of frozen sample storage.