Measuring N-15 Abundance and Concentration of Aqueous Nitrate, Nitrite, and Ammonium by Membrane Inlet Quadrupole Mass Spectrometry

An automated sample preparation unit for inorganic nitrogen (SPIN) coupled to a membrane inlet quadrupole mass spectrometer (MIMS) was developed for automated and sensitive determination of the N-15 abundances and concentrations of nitrate, nitrite, and ammonium in aqueous solutions without any sample preparation. The minimum N concentration for an accurate determination of the N-15 abundance is 7 mu mol/L for nitrite and nitrate, with a relative standard deviation (RSD) of repeated measurements of <1%, and 70 mu mol/L with an RSD < 0.4% in the case of ammonium. The SPIN-MIMS system provides a wide dynamic range (up to 3500 mu mol/L) for all three N species for both isotope abundance and concentration measurements. The comparison of parallel measurements of N-15-labeled NH4+ and NO3- from soil extracts with the denitrifier method and the SPIN-MIMS system shows a good agreement between both methods.