Characterizing Limits of Precision for Dissolved Organic Nitrogen Calculations

Saunders, James; Yu, Yun; McCutchan, James; Rosario-Ortiz, Fernando

The usefulness of dissolved organic nitrogen (DON) data to environmental science depends on the precision of each calculated concentration. The precision of a calculated DON concentration is based on propagation of error from the measured nitrogen fractions used to calculate the DON concentration. We present an improved approach for calculating DON precision that strengthens conventional error propagation by developing empirical relationships between precision and concentration for each of the measured nitrogen fractions used to calculate DON concentration. Because the concentration and relative importance of each measured nitrogen fraction differ among samples, DON precision and the corresponding detection limit are likely to be different for each sample. Case studies from two different research efforts, a synoptic study of surface water DON and an experimental assessment of DON leached from heat-treated soils, demonstrate how the proposed approach can be tailored to the analytical methods and concentration ranges expected for any research project. In addition, current recommendations for pretreatment (e.g., dialysis) to improve DON precision should be modified to include consideration of analytical precision. Improved characterization of DON precision facilitates a more realistic assessment of sample-specific detection limits, and the results can be generalized to support decisions about pretreatment and statistical analysis of DON data.