Determination of the isotopic composition of ammonium-nitrogen at the natural abundance level from estuarine waters



A method was developed to measure the stable nitrogen isotope ratio of dissolved ammonium (NH4+) at the natural abundance level from estuarine waters. This method employed rapid steam distillation with collection of ammonium on zeolite via ion-exchange. The steam distillation step had a recovery of 103±5%; subsequent exchange of the ammonium on zeolite had a yield of 96.4±1.6%. The zeolite with exchanged ammonium was converted to N2 in quartz tubes at 910°C with CuO and Cu and the isotopic composition of the gas was measured in an isotope ratio mass spectrometer. When analyzing 200 μg of N the accuracy using isotopic standards was within 4% of the true ratio, with an overall precision of ±0.5%. A benefit of this method is that samples can be distilled and preserved onboard ship, thereby minimizing storage artifacts. This method was used in a seasonal study of the isotopic composition of dissolved ammonium from the Delaware Estuary.

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Marine Chemistry



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