Online measurement of denitrification rates in aquifer samples by an approach coupling an automated sampling and calibration unit to a membrane inlet mass spectrometry system

Abstract

Aquifers within agricultural catchments are characterised by high spatial heterogeneity of their denitrification potential. Therefore, simple but sophisticated methods for measuring denitrification rates within the groundwater are crucial for predicting and managing N-fluxes within these anthropogenic ecosystems. Here, a newly developed automated online (15)N-tracer system is presented for measuring (N(2)+N(2)O) production due to denitrification in aquifer samples. The system consists of a self-developed sampler which automatically supplies sample aliquots to a membrane-inlet mass spectrometer. The developed system has been evaluated by a (15)N-nitrate tracer incubation experiment using samples (sulphidic and non-sulphidic) from the aquifer of the Fuhrberger Feld in northern Germany. It is shown that the membrane-inlet mass spectrometry (MIMS) system successfully enabled nearly unattended measurement of (N(2)+N(2)O) production within a range of 10 to 3300 µg N L(-1) over 7 days of incubation. The automated online approach provided results in good agreement with simultaneous measurements obtained with the well-established offline approach using isotope ratio mass spectrometry (IRMS). In addition, three different (15)N-aided mathematical approaches have been evaluated for their suitability to analyse the MIMS raw data under the given experimental conditions. Two approaches, which rely on the measurement of (28)N(2), (29)N(2) and (30)N(2), exhibit the best reliability in the case of a clear (15) N enrichment of evolved denitrification gases. The third approach, which uses only the ratio of (29)N(2)/(28)N(2), overestimates the concentration of labelled denitrification products under these conditions. By contrast, at low (15)N enrichments and low fractions of denitrified gas, the latter approach is on a par with the other two approaches. Finally, it can be concluded that the newly developed system represents a comprehensive and simply applicable tool for the determination of denitrification in aquifers.

Nitric oxide and nitrous oxide turnover in natural and engineered microbial communities: biological pathways, chemical reactions, and novel technologies

“…Recently, it was coupled with an automated sampling and calibration unit (ASCU), and was tested in a long-term 15 N-NO − 3 tracer experiment over 7 days. It was confirmed that 15 N measurements of N 2 and N 2 O, detected as N 2 at m/z 28, 29, and 30 (N 2 O was reduced to N 2 in an elemental copper furnace prior to analysis), are in good agreement with IRMS-based analysis (Eschenbach and Well, 2011). …”

An enhanced technique for automated determination of 15N signatures of N2, (N2+N2O) and N2O in gas samples

“…The 15 N gas flux method has long been used for determining denitrification in terrestrial and aquatic environments and for quantifying the associated N 2 O and N 2 emissions. [1][2][3][4][5][6][7][8][9] Denitrification is the microbial process of nitrate reduction where nitrous oxide (N 2 O) is produced as an intermediate and molecular nitrogen (N 2 ) is the final product of the reaction chain. Denitrification is a key N transformation process in the biosphere and plays an important role in the closure of the global N cycle.…”

Conquering the Outdoors with On-site Mass Spectrometry

“…[1] Membrane inlet mass spectrometers (MIMS), for instance, have been used successfully for the on-site quantification of dissolved gases in surface waters [2][3][4][5][6][7] and in groundwater. [8,9] While MIMS easily provide the sensitivity required to quantify the targeted gases, they usually lack the long-term robustness and stability that is required if they are to be used at field sites for longer than a few days. The main reason for this lack of stability is that the sensitivity of the MS and the gas permeability of the membrane inlet are both dependent on temperature.…”

“…1A) employ a membrane to separate the water (or other liquid sample matrix) from the high vacuum of the ion source. [3][4][5][6][7][8][9] The membrane is permeable only to gases, blocking liquid water. The efficiency of gas transfer depends on the material of the membrane, the temperature of the water, the rate of flow of the water along the membrane, and the physico-chemical properties of the gas species (e.g.…”