Journal of Chromatography A, 1569 (2018) 193–199 Contents lists available at ScienceDirect Journal of Chromatography A j o ur na l ho me page: www.elsevier.com/locate/chroma Determination of total dissolved nitrogen in seawater by isotope dilution gas chromatography mass spectrometry following digestion with persulfate and derivatization with aqueous triethyloxonium Enea Pagliano a,∗ , Beatrice Campanella b , Lisa Shi a , Marie-Pier Thibeault a , Massimo Onor b , Steven Crum c , Jeremy E. Melanson a , Zoltán Mester a a National Research Council of Canada, 1200 Montreal Road, K1A 0R6 Ottawa, Ontario, Canada b Consiglio Nazionale delle Ricerche (CNR), Istituto di Chimica dei Composti Organometallici, UOS di Pisa, Via Moruzzi 1, 56124 Pisa, Italy c QUASIMEME, NL-6700 EC Wageningen, Bornsesteeg 10, 6721 NG Bennekom, The Netherlands a r t i c l e i n f o Article history: Received 28 March 2018 Received in revised form 13 July 2018 Accepted 17 July 2018 Available online 18 July 2018 Keywords: Total dissolved nitrogen Seawater Isotope dilution Gas chromatography mass spectrometry Persulfate Triethyloxonium derivatization a b s t r a c t In this study, we propose a novel approach for the determination of total dissolved nitrogen (TDN) in seawater combining high-precision isotope dilution GC–MS with persulfate digestion. A 2 mL sample aliquot was digested with an alkaline solution of persulfate to convert nitrogen containing compounds to nitrate. Digested samples were spiked with 15 NO 3 - internal standard and treated with aqueous tri- ethyloxonium to convert the analyte into volatile EtONO 2 . This derivative was readily separated from the matrix under gaseous form and could be sampled from the headspace before GC–MS analysis. The resulting chromatograms showed a stable flat baseline with EtONO 2 as the only eluting peak (retention time 2.75 min on a DB 5.625 column). Such an approach provides specificity and obviates the shortcom- ings of current detection methods employed to analyze seawater samples after digestion with persulfate. In negative chemical ionization mode, the method reached a detection limit of 0.5 mol/kg TDN (7 ng/g N) and could be applied to quantify seawater samples with 1–25 mol/kg TDN. On the upper end of the range, quantitation could be repeated within 1%, whereas on a 6 mol/kg TDN sample repeatability was 2.3% on eight measurements. The method was employed in two proficiency testing exercises providing results in agreement with consensus values. We investigated the impact of reagent blank and we imple- mented a blank-matching optimal design to account for such contribution. Finally, we performed a study on the yield of persulfate oxidation for organic and inorganic nitrogen compounds typically present in seawater. Whilst nitrite and ammonium are fully converted to nitrate, more complex organic molecules showed recoveries varying from 70% to 100%. Crown Copyright © 2018 Published by Elsevier B.V. All rights reserved. 1. Introduction Within current monitoring schemes, accurate and precise deter- mination of total dissolved nitrogen (TDN) in seawater is required for understating the biogeochemistry of a marine ecosystem [1,2]. Historically, such determination has always been a challenging task [3] and today’s strategies still rely on methods which have limi- tations. Measurement of TDN requires quantitative conversion of N-compounds to a single measurable species [4]. Kjeldahl acid digestion was employed in early studies for con- version of organic nitrogen to NH 4 + [5,6]. This method yielded total ∗ Corresponding author. E-mail addresses: enea.pagliano@nrc-cnrc.gc.ca, enea.pagliano@outlook.com (E. Pagliano). organic nitrogen and ammonia, therefore separate determination of nitrite and nitrate was required to estimate TDN. A second wet digestion approach was introduced in the late sixties by Koroleff who converted N-compounds to nitrate in aqueous alkaline per- sulfate (S 2 O 8 2- ) [7]. Despite complete orthogonality, early studies demonstrated comparability between these two strategies [5,6]. Another methodology for TDN was later introduced with high- temperature (catalytic) combustion systems (HTC) [8–12]. In this case, N-compounds were converted into NO 2 * which could be detected with a nitric oxide chemiluminescent detector. A few studies demonstrated fair comparability between HTC and aque- ous oxidation with S 2 O 8 2- [10,11,13], but a recent investigation highlighted that the persulfate method yields higher recovery of nitrogen respect to HTC [12]. https://doi.org/10.1016/j.chroma.2018.07.055 0021-9673/Crown Copyright © 2018 Published by Elsevier B.V. All rights reserved.