Optimization of dissolved organic nitrogen (DON) measurements in aqueous samples with high inorganic nitrogen concentrations Jeroen Vandenbruwane a, ⁎ , Stefaan De Neve a , Robert G. Qualls b , Joost Salomez a , Georges Hofman a a Department of Soil Management and Soil Care, Division of Soil Fertility and Nutrient Management, Ghent University, Coupure Links 653, 9000 Ghent, Belgium b Department of Natural Resources and Environmental Sciences, University of Nevada, Reno, NV 89557, USA Received 2 February 2007; received in revised form 22 May 2007; accepted 16 June 2007 Abstract 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Dialysis; Total nitrogen; Dissolved organic nitrogen; HTCO; Persulfate oxidation; DIN removal 1. Introduction During the last decade, dissolved organic matter (DOM) received increasingly greater attention in biogeochemical studies of natural ecosystems. Leaching of DOM gives rise to the loss of available nutrients (Qualls and Haines, 1991) and eutrophication of ground- and surface waters. Perakis and Hedin (2002) reported that nitrogen is lost from unpolluted South American forests mainly via dissolved organic com- pounds. Despite the growing interest in dissolved organic nitrogen (DON) cycling in natural ecosystems, the analytical methods still face serious limitations. The DON concentration in water samples cannot be quantified directly, but must be calculated by subtracting Science of the Total Environment xx (2007) xxx – xxx + MODEL STOTEN-10076; No of Pages 11 www.elsevier.com/locate/scitotenv ⁎ Corresponding author. Tel.: +32 9 264 60 55; fax: +32 9 264 62 47. E-mail address: Jeroen.Vandenbruwane@UGent.be (J. Vandenbruwane). 0048-9697/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2007.06.025 ARTICLE IN PRESS Please cite this article as: Vandenbruwane J et al. Optimization of dissolved organic nitrogen (DON) measurements in aqueous samples with high inorganic nitrogen concentrations. Sci Total Environ (2007), doi:10.1016/j.scitotenv.2007.06.025