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

Abstract

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 abundances and concentrations of nitrate, nitrite, and ammonium in aqueous solutions without any sample preparation. The minimum N concentration for an accurate determination of theN 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 N-labeled NH and NO from soil extracts with the denitrifier method and the SPIN-MIMS system shows a good agreement between both methods.

Nitrate uptake and carbon exudation – do plant roots stimulate or inhibit denitrification?

“…The extracts were analyzed colorimetrically for the concentrations of NO 3 − and NH 4 + using a San ++ continuous flow Analyzer (Skalar Analytical B.V., Breda, The Netherlands). 15 N concentration in NH 4 + and NO 3 − was analyzed using an automated sample preparation unit for inorganic nitrogen coupled to a membrane inlet quadrupole mass spectrometer (QMS, GAM 200, InProcess, Bremen, Germany) as described in detail by Eschenbach et al (2017Eschenbach et al ( , 2018. In parallel subsamples, soil water content was determined by oven drying at 105°C.…”

Evidence of Reactivity in the Membrane for the Unstable Monochloramine during MIMS Analysis

“…It has gained interest since it was first reported in 1987 [1] as it allows real-time analysis of water and air samples by mass spectrometry [2] and generally preconcentration of targeted compounds. Its use spans from detection of organics in water to the more recent analysis of nitrites and nitrates [3]. MIMS technology is based on the separation of the analyte stream and the mass spectrometer inlet by a membrane.…”

Nitrogen isotope analysis of aqueous ammonium and nitrate by membrane inlet isotope ratio mass spectrometry (MIRMS) at natural abundance levels

“…We can exclude this difference being due to an N 2 blank, because this would lead to a linear relationship for the reciprocal sample amounts 19 . As it has been shown that the reaction yield is not affected by the sample/reagent ratio, 12 we speculate that the offset and amount dependency observed here could be a result of fractionation due incomplete gas transfer in the membrane inlet, with lower yields and thus increasing apparent fractionation for higher sample amounts.…”

“…While the setup has been specifically developed for the analysis of natural abundance samples, it can also be used for the analysis of labelled samples. For labelled samples, nitrite can also be analysed after conversion into nitrous monoxide (NO) by KI (see Eschenbach et al 12 ) while the NO + fragment ion that is produced from the N 2 O reference gas in the ion source can be used for referencing.…”

“…Numerous approaches have been published and are being used to determine the isotopic composition of ammonium and nitrate, mostly applying some kind of chemical conversion 3–14 or microbiological methods 15 to convert nitrate or ammonium into gaseous analytes suitable for isotope analysis.…”