Using a quadrupole mass spectrometer as versatile detector to study N transformations and fluxes in soils and aquatic systems

Research on the nitrogen (N) cycle in agricultural ecosystem is key to better understand and manage N nutrition of crops and N losses to the environment. Stable isotope tools have been extensively used to identify and quantify N pathways and processes, but suitable methods typically require sophisticated and expensive instrumentation which is not always available and is rarely suitable for in situ analysis. A quadrupole mass spectrometer (GAM200, InProcess, Bremen) was modified to study ¹⁵N enrichment in N species and N₂ in water and air in the lab and in the field. To establish a membrane inlet mass spectrometer (MIMS) a silicone tubing inlet was added to enable online analysis of dissolved gases. Four applications were established:

 

• The MIMS was used to analyze N₂ and Ar in groundwater samples to determine excess-N₂ from denitrification. In situ online analysis in the Fuhrberger Feld aquifer was conducted at multilevel groundwater monitoring wells, clearly identifying the steep rise in denitrification upon appearance of sulfides.
• To study N₂ production by denitrification the in situ ¹⁵N push pull method [1] was used, where ¹⁵N labelled NO₃^- solution is injected to groundwater and subsequently samples containing ¹⁵N labelled N₂ are analyzed, in this study by the MIMS. This method was automated and tested in lab mesocosms [2].
• An automated sample preparation unit for inorganic nitrogen (SPIN) was coupled to the MIMS for automated and sensitive determination of the ¹⁵N abundances and concentrations of nitrate, nitrite, and ammonium in aqueous solutions. It was based on the principle of the SPIN-MAS [3] but with the advantage to analyze samples online. It provides a wide dynamic range for all three N species for both isotope abundance and concentration measurements [4, 5]. We propose to use this method in conjunction with online sampling of dissolved N species in soil using dialysis membranes [6] which had not been performed until now to our knowledge.
• The improved ¹⁵N gas flux method to measure N₂ fluxes from soils under N₂ depleted atmosphere has been applied int the field [7] but was complicated by the difficulty to maintain stable background concentrations [8]. A capillary inlet was added to the GAM 200 and used for in situ monitoring of background N₂ concentrations in flux chambers.

 

We conclude the used quadrupole mass spectrometer has been proven as a versatile, economic and easy to use detector for a wide range of applications in N cycle research and is promising for future applications.

 

References:

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• Inselsbacher, E., et al. 2011, doi.org/10.1016/j.soilbio.2011.03.003
• Well, R., et al 2019 doi.org/10.1002/rcm.83638.
• Eckei, J., et al., 2024. DOI: 10.1007/s00374-024-01806-z