A sensitive method for determining nitrogen-15 isotope in urea

A method is presented in which the N-15 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 N-2 by reaction with hypobromite iodine. The isotopically labeled N-2 thus formed is mixed with the N-2 initially present in the sample, and the N-15 at.% of urea is calculated from the relative amounts of (NN)-N-14-N-15 and (NN)-N-15-N-15 by means of the isotope pairing principle. The presented method for N-15- urea analysis proved to be precise (SE < 0.4 at.%, n = 5), when applied on both marine and freshwater samples with an N-15 enrichment >1 at.%, and interference was found only from volatile methyl amines.