Measuring ¹⁵N 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 15N 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 15N abundance is 7 µmol/L for nitrite and nitrate, with a relative standard deviation (RSD) of repeated measurements of <1%, and 70 µmol/L with an RSD < 0.4% in the case of ammonium. The SPIN-MIMS system provides a wide dynamic range (up to 3500 µmol/L) for all three N species for both isotope abundance and concentration measurements. The comparison of parallel measurements of 15N-labeled NH4 + and NO3 - from soil extracts with the denitrifier method and the SPIN-MIMS system shows a good agreement between both methods.