Analytical Techniques for Quantifying 15N/14N of Nitrate, Nitrite, Total Dissolved Nitrogen and Ammonium in Environmental Samples Using a Gas Chromatograph Equipped with a Quadrupole Mass Spectroscope

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The measurement of 15N concentrations in environmental samples requires sophisticated pretreatment devices and expensive isotope-ratio mass spectrometry (IRMS). This report describes the use of a gas chromatograph equipped with a quadrupole-type mass spectrometer (GC/MS) to measure 15N concentrations of ammonium, nitrate, nitrite, and total dissolved nitrogen (TDN) in distilled water, a 2 M KCl solution and a 0.5 M K2SO4 solution. The system measures nitrous oxide (N2O) that is ultimately converted from the target N compound, requiring no special apparatus such as a purge-and-trap pretreatment device. It uses a denitrifier lacking N2O reductase, which produces N2O from nitrate. Persulfate oxidation was applied to convert TDN to nitrate, while additional pretreatment with ammonia diffusion was required for ammonium prior to the persulfate oxidation. Up to 100 samples can be measured daily using the system. We can generally run 15N measurements with only 1-10 mL of sample for each chemical species of N, a volume 1/10-1/100 times smaller than the amount necessary for conventional methods. Our method is useful for measuring 15N with GC/MS, offering greater convenience than IRMS.