I am planning series of experiments in which I’d like to measure concentrations of N-15 labelled ammonia and urea in solution. I basically know that it is possible to measure concentrations of these compounds, and that it is possible to measure N-15 signature for all dissolved N-compounds. However, is it possible to measure this for each compound separately? If so, how do you do this? Could you recommend some literature?
You can use alkaline hypobromite to convert NH4+ to N2 of which the N-isotopic composition can easily be measured using GC/IRMS. However, having both 15NH4+ and 15N-urea in your samples complicates the whole thing a little bit as hypobromite will also oxidize other reduced N-compounds, such as urea. There is a so-called "microdiffusion method" (Risgaard Peterson et al. (1995) Soil. Sci. Soc. Am. J.), where you convert NH4+ to gaseous NH3 which then reacts with the hypobromite. Do you expect any organic 15N-compounds other than urea in your solution?
Thank you for your answer Tim. I'd expect still other simple organic compounds with N, like aminoacids etc
There is another diffusion method for 15N-NH3 where you buffer your sample with MgO to a pH of 9.5 to 10, which also converts the NH4+ to gaseous NH3 which can then be captured on a filter as ammonium sulfate. The filter is acidified beforehand with Sodium Bisulfate, and is made into a "filter packet" that floats on the aqueous phase. Diffusions last from 3 to 10 days depending on the concentration of ammonia, and the filters can then be dried and analyzed via EA-IRMS. Make sure to preserve your samples at pH2 with H2SO4 to prevent any loss of NH3 beforehand (although freezing also works). volatile organic nitrogen compounds can be a problem though
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