Dissolved organic nitrogen (DON) is usually determined as the difference between total dissolved nitrogen (TDN) and dissolved inorganic nitrogen (DIN). When applying this approach to samples with high DIN concentrations, there is a risk, that small relative errors in TDN and DIN measurements may propagate into high absolute errors of the determined DON concentration. To reduce such errors, two pretreatment methods have been suggested for the removal of DIN prior to the determination of DON: anion-exchange pretreatment (AEP) and dialysis pretreatment (DP). In this study, we tested the suitability of AEP and DP for DIN removal in order to increase DON determination accuracy of freshwater samples. The AEP pretreatment performed well for standard compounds, yielding high dissolved organic carbon (DOC) recovery rates and > 99% removal of nitrate, whereas DON recovery rates varied and no removal occurred for ammonium and nitrite. However, AEP proved not to be suitable for natural samples, as it removed 36 - 74% DOC and up to 83% DON. In contrast, after 72h of DP, 17 - 32% DOC and DON had been removed from the natural samples, while > 98% nitrate was removed in all but one case, and > 87% of the ammonium and nitrite were removed. Moreover, we found that DP results in a significant increase in DON determination accuracy. In conclusion, DP is a useful measure to increase DON determination accuracy in natural freshwater samples with high DIN:TDN ratios, whereas AEP is not recommendable for DON determination of natural freshwater samples.
Environmental Chemistry (online), 2012, Vol 9, Issue 6, p. 529-536