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A new method for collection of nitrate from fresh water and the analysis of nitrogen and oxygen isotope ratios

S. R. Silva·C. Kendall·D.H. Wilkison·A.C. Ziegler·Cecily C.Y. Chang·R.J. Avanzino
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Abstract

A new method for concentrating nitrate from fresh waters for δ 15 N and δ 18 O analysis has been developed and field-tested for four years. The benefits of the method are: (1) elimination of the need to transport large volumes of water to the laboratory for processing; (2) elimination of the need for hazardous preservatives; and (3) the ability to concentrate nitrate from fresh waters. Nitrate is collected by, passing the water-sample through pre-filled, disposable, anion exchanging resin columns in the field. The columns are subsequently transported to the laboratory where the nitrate is extracted, converted to AgNO 3 and analyzed for its isotope composition. Nitrate is eluted from the anion exchange columns with 15 ml of 3 M HCl. The nitrate-bearing acid eluant is neutralized with Ag 2 O, filtered to remove the AgCl precipitate, then freeze-dried to obtain solid AgNO 3 , which is then combusted to N 2 in sealed quartz tubes for δ 15 N analysis. For δ 18 O analysis, aliquots of the neutralized eluant are processed further to remove non-nitrate oxygen-bearing anions and dissolved organic matter. Barium chloride is added to precipitate sulfate and phosphate; the solution is then filtered, passed through a cation exchange column to remove excess Ba 2+ , re-neutralized with Ag 2 O, filtered, agitated with activated carbon to remove dissolved organic matter and freeze-dried. The resulting AgNO 3 is combusted with graphite in a closed tube to produce CO 2 , which is cryogenically purified and analyzed for its oxygen isotope composition. The 1 σ analytical precisions for δ 15 N and δ 18 O are ±0.05‰ and ±0.5‰, respectively, for solutions of KNO 3 standard processed through the entire column procedure.

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S. R. Silva, C. Kendall, D.H. Wilkison, A.C. Ziegler, Cecily C.Y. Chang, R.J. Avanzino. 2000. A new method for collection of nitrate from fresh water and the analysis of nitrogen and oxygen isotope ratios. https://doi.org/10.1016/s0022-1694(99)00205-x

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