Determining 15N enrichment of dissolved organic nitrogen in environmental waters by gas chromatography/negative-ion chemical ionization mass spectrometry

Toshihiro Miyajima, Yasuaki Tanaka, Isao Koike

Limnol. Oceanogr. Methods 3:164-173 (2005) | DOI: 10.4319/lom.2005.3.164

ABSTRACT: To minimize the required sample volume and simplify the overall procedure for determining 15N enrichment of dissolved organic nitrogen (DON) in environmental waters, we employed derivatization of NO3– with pentafluorobenzyl bromide and gas chromatography/negative-ion chemical ionization mass spectrometry (GC-NICI-MS). The pretreatment processes to remove inorganic nitrogen were optimized to ensure recovery of labile DON molecules, and the derivatization procedure for NO2– and N3– was optimized for environmental samples. The required sample volume is 10 mL or less and the detection limit is about 0.015 µmol 15N L–1, or 0.3 atomic percent excess of 15N for water samples containing 5 µmol DON L–1. This method can also be applied to 15N enrichment analysis of particulate organic nitrogen. We applied the developed method to measure DON production rate by the hermatypic coral Porites cylindrica during incubation with 15N-labeled NO3–.