Influence of nitrogen speciation on the TDN measurement in fresh waters by high temperature catalytic oxidation and persulfate digestion

Analytical methodologies for the determination of Total Dissolved Nitrogen (TDN) in waters are based on a conversion step able to transform selectively all the nitrogen species into a compound that is then quantified. A crucial requirement to meet accuracy is the quantitative recovery of all organic and inorganic nitrogen species during the conversion step. In this work, the N recoveries of two widely employed methodologies that use different conversion steps (High Temperature Catalytic Oxidation, HTCO, and Persulfate Digestion, PD) were assessed on a set of organic nitrogen compounds, representative of the structures of both DOM and anthropogenic contaminants. Low recoveries are due to poor selectivity during the conversion step, with the formation of nitrogen-compounds other than nitrogen oxide (HTCO) and nitrate (PD). The results show that in many instances the TDN measurements give systematically low results depending on N speciation. PD could give lower results than HTCO even for samples containing only DOM of biological origin. In particular i) low N recovery was always observed with compounds having two or more contiguous N atoms; ii) the HTCO method is very effective for TDN quantification in the presence of s-triazine rings while PD method did not yield satisfactory N recovery; iii) a full N recovery was observed with compounds having amido or amino groups or nitrogen atoms in imidazole, indole, pyrimidine rings; iv) the N recoveries for purine derivatives are almost complete with HTCO, but give systematically low results by PD. Finally, the estimation of DON fluxes and pools from TDN measurements can be affected by uncertainties larger than previously thought as a consequence of i) the lower N recovery for some nitrogen-compounds and ii) the differences in the N recovery as a function of the adopted analytical methods.