Total dissolved nitrogen analysis: comparisons between the persulfate, UV and high temperature oxidation methods

Abstract

We compared the persulfate (PO), ultraviolet (UV), and high temperature oxidation (HTO) methods used to analyze total dissolved nitrogen (TDN) concentrations in aquatic samples to determine whether the three methods differed in terms of standard parameters (blanks, limits of detection and linearity, and precision) or in oxidation efficiency of standard compounds and field samples of varying salinity. The TDN concentrations of several N-containing standard compounds, as well as a humic mixture and a suite of field samples collected from the Sargasso Sea, Chesapeake Bay and an aquaculture pond were determined with the three methods. The PO method had the highest percent recoveries for the range of labile and refractory standard compounds tested (93±13). The HTO method yielded recoveries of 87±14; recoveries increased to 91±10 under optimized conditions. The standard UV method, with 30% H2O2 as the oxidant, was found to be highly variable, producing the lowest percent recoveries (71±21); the oxidation efficiency of the UV method increased substantially in subsequent trials (91±12), when the PO reagent was used in place of H2O2. In the field sample comparison, the PO, UV with PO reagent, and HTO method produced similar results (slopes of the Model II regression lines comparing them ranged from 1.00 to 1.05 with r2≥0.99). The standard UV method, however, produced concentrations 5% to 40% lower than the other methods. Analysis of the spectra emitted by the UV lamp used in this study suggests that variations in the UV spectra reaching the sample may have caused the reduced efficiencies. The poor recovery of some standard compounds with each of the methods suggests that concentrations of TDN, and subsequently DON, measured in the field with any of the methods will likely be underestimated to some degree depending on the composition of the TDN pool at that time. With careful attention to detail, however, the PO and HTO methods can provide reproducible results consistent with each other. The standard UV method, however, was found to be highly unpredictable in practice.