Dialysis is superior to anion exchange for removal of dissolved inorganic nitrogen from freshwater samples prior to dissolved organic nitrogen determination
Daniel Graeber A D , Björn Gücker B , Elke Zwirnmann C , Brian Kronvang A , Christoph Weih C and Jörg Gelbrecht CA Department of Bioscience, Aarhus University, Vejlsøvej 25, DK-8600 Silkeborg, Denmark.
B Department of Biosystems Engineering, Federal University of São João del-Rei, Campus Tancredo Neves, 36301-160 São João del-Rei, Minas Gerais, Brazil.
C Central Chemical Laboratory, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 301, D-12587 Berlin, Germany.
D Corresponding author. Email: dgr@dmu.dk
Environmental Chemistry 9(6) 529-536 https://doi.org/10.1071/EN11170
Submitted: 10 January 2012 Accepted: 26 November 2012 Published: 20 December 2012
Environmental context. Aquatic ecosystem health may be adversely affected by dissolved organic nitrogen pollution, and accurate analytical techniques are needed to assess these effects. Our study shows that dialysis is the best sample pre-treatment technique to increase the accuracy of dissolved organic nitrogen determination. It will improve analysis and understanding of the role of dissolved organic nitrogen in the nitrogen cycle of affected aquatic ecosystems.
Abstract. 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 pre-treatment methods have been suggested for the removal of DIN before the determination of DON: anion-exchange pre-treatment (AEP) and dialysis pre-treatment (DP). In this study, we tested the suitability of AEP and DP for DIN removal in order to increase the accuracy of DON determination of freshwater samples. The AEP 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 72 h of DP, 17–32 % DOC and DON had been removed from the natural samples, whereas >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-to-TDN ratios, whereas AEP is not recommended for DON determination of natural freshwater samples.
Additional keywords: anion exchange pre-treatment, dialysis pre-treatment, measurement error, natural samples, standard compounds.
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