Abstract
A method is presented in which the 15N at.% of urea is determined with high precision on liquid samples containing as little as 10 nmol of urea. The method involves removal of interference from NH4 + initially present in the sample by cation exchange. Urea in the sample is subsequently hydrolyzed to NH4 + by the enzyme urease. Liberated NH4 + is separated from the alkaline sample by diffusion as NH3 through a helium gas phase where it is finally oxidized to N2 by reaction with hypobromite iodine. The isotopically labeled N2 thus formed is mixed with the N2 initially present in the sample, and the 15N at.% of urea is calculated from the relative amounts of 14N15N and 15N15N by means of the isotope pairing principle. The presented method for 15N-urea analysis proved to be precise (SE < 0.4 at.%, n = 5), when applied on both marine and freshwater samples with an 15N enrichment >1 at.%, and interference was found only from volatile methyl amines.
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Received: 19 September 1996 / Accepted: 25 October 1996
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Rysgaard, S., Risgaard-Petersen, N. A sensitive method for determining nitrogen-15 isotope in urea. Marine Biology 128, 191–195 (1997). https://doi.org/10.1007/s002270050082
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DOI: https://doi.org/10.1007/s002270050082