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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 15 N 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 15 N 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 15 N-labeled NH 4 + and NO 3 - from soil extracts with the denitrifier method and the SPIN-MIMS system shows a good agreement between both methods.