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 Spectrometer

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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.