A comparison between high-temperature catalytic oxidation and persulfate oxidation for the determination of total nitrogen in freshwater

The determination of total nitrogen (TN) in freshwater is of prime importance, both in monitoring water quality and in environmental research. TN in freshwater samples (lakes, rivers and atmospheric deposition) have been routinely analysed at the laboratory of the CNR Institute of Ecosystem Study in Italy by persulphate oxidation (PO) with UV determination of nitrate at 220 nm since the 1980s. In 2004, high temperature catalytic oxidation (HTCO) with chemiluminescence detector (CLD) began to be used for TN analysis. A mixed catalyst of cobalt-chromium and cerium oxide was selected, as it gave the best recoveries for ammonium and organic N. To assess the comparability of the new method with the reference one, almost 800 samples were analysed for TN using both methods. The average difference between paired data was low, both as absolute and relative values (0.06 mg N L-1 and 3%, respectively, in the range of 0.1-7.0 mg N L-1). The results of our study confirmed HTCO as a reliable method for TN determination in freshwater samples. Compared to PO, HTCO proved to be faster and required less sample handling. From the point of view of linearity and repeatability, results by HTCO were comparable to those by PO UV 220. The LOD of HTCO calculated in this study was lower than that of PO UV 220 (0.04 with respect to 0.09 mg N L-1).