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Modified Method for the Analysis of Nitrogen Isotopic Composition of Oceanic Nitrate at Low Concentration

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Abstract

A method for the determination of the δ15N of nitrate in seawater described by Cline and Kaplan (1975) has been modified for application to low-level nitrate samples. We have minimized the reagent blank problem by replacing the Devarda's alloy with an aluminum reagent, and have also established a procedure that yields quantitative (93 ± 2%) extraction of nitrogen even at low nitrate levels. Though the amounts and the δ15N of the blank N varied from one reagent set to another, with these modifications, an overall N blank was reduced to approximately 0.80 ± 0.33 µmole N having an estimated δ15N value of −1.8‰. After blank and yield corrections, the measured isotopic composition of nitrate differed by approximately 0.1‰ from the actual value while the precision was within ±0.2‰ at the 1.25 µM level. The modified procedure was applied to seawater samples collected from the equatorial Pacific in order to compare the N blanks in field samples with those derived from laboratory experiments. The results support the suitability of the modified approach for isotopic analysis of oceanic nitrate in shallow water.

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  1. Institute for Hydrospheric and Atmospheric Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan

    Tomoyuki Tanaka & Toshiro Saino

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  1. Tomoyuki Tanaka
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  2. Toshiro Saino
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Tanaka, T., Saino, T. Modified Method for the Analysis of Nitrogen Isotopic Composition of Oceanic Nitrate at Low Concentration. Journal of Oceanography 58, 539–546 (2002). https://doi.org/10.1023/A:1021210625853

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