Optimization of dissolved organic nitrogen (DON) measurements in aqueous samples with high inorganic nitrogen concentrations

Since dissolved organic nitrogen (DON) concentrations in aqueous samples can only be determined by the subtraction of three independently measured concentrations (total dissolved nitrogen (TDN), nitrate and ammonium), analytical errors are compounded in the calculation of DON concentration. Several methods were tested to increase the recovery and precision of the DON determinations in aqueous samples with high inorganic nitrogen concentrations. The first step was the objective selection of the most accurate TDN analysis method. The persulfate oxidation (PO) method had a significantly higher recovery than the high-temperature catalytic oxidation (HTCO) method. The second step involved the lowering of the DIN (dissolved inorganic N)/TDN ratio by applying three sample pretreatment procedures: conventional dialysis, dialysis against a buffered acceptor solution and concentration of the samples combined with the buffered dialysis. Despite the fact that DIN was only partially removed, conventional dialysis pretreatment resulted in higher precision and recovery of the DON determination compared to analysis of untreated samples. Dialysis of the samples against a buffered acceptor solution gave additional improvements in precision, recovery, and reproducibility. The concentration of aqueous samples by lyophilization, however, did not lead to higher recovery rates in the DON measurements.