Improved speciation of dissolved organic nitrogen in natural waters: amide hydrolysis with fluorescence derivatization.

The objective of this study was to improve primary-amine nitrogen (1 degree-N) quantification in dissolved organic matter (DOM) originating from natural waters where inorganic forms of N, which may cause analytical interference, are commonly encountered. Efforts were targeted at elucidating organic-N structural criteria influencing the response of organic amines to known colorimetric and fluorescent reagents and exploring the use of divalent metal-assisted amide hydrolysis in combination with fluorescence analyses. We found that reaction of o-phthaldialdehyde (OPA) with primary amines is significantly influenced by steric factors, whereas fluorescamine (FLU) lacks sensitivity to steric factors and allows for the detection of a larger suite of organic amines, including di- and tri-peptides and sterically hindered 1degree-N. Due to the near quantitative recovery of dissolved peptides with the FLU reagent and lack of analytical response to inorganic nitrogen, we proposed that FLU be utilized for the quantification of primary amine nitrogen. In exploring the application of divalent metal promoted peptide hydrolysis to the analysis of organic forms of nitrogen in DOM, we found that Zn(II) reaction increased the total fraction of organic-N detectable by both OPA and FLU reagents. Zn-hydrolysis improved recovery of organic-N in natural waters from < 5% to 35%. The above method, coupled with standard inorganic-N analyses, allows for enhanced resolution of dissolved organic nitrogen (DON) speciation in natural waters.