Determination of the 15N/14N ratio of ammonium and ammonia in aqueous solutions by equilibrium headspace-gas chromatography-combustion-isotope ratio mass spectrometry

Abstract

A method for determination of the 15N/14N ratio of total ammoniacal nitrogen (TAN; ammonium and ammonia) in aqueous solutions was developed, primarily intended for use with soil extracts, which have a high TAN level, e.g. from recently fertilised agricultural soils. Ammonium was converted to ammonia by addition of NaOH, followed by nitrogen isotopic analysis of the headspace by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) where complete separation of TAN from the matrix was not necessary. The ammonia concentration in the gas phase was maximised by increasing the temperature and salt concentration and by decreasing the gas liquid ratio in the headspace vials. Isotopic equilibrium was reached after less than 1 h at 80 degrees C. The measured isotopic ratio was constant for solutions containing 30-200 mM NH4-N, corresponding to 950-7000 ng NH3-N detected with the IRMS. The integrated area response at m/z 28 increased linearly with the ammonium ion concentration in the interval 10-200 mM NH4-N. The fractionation factor between the liquid and gas phases was 1.0054 +/- 0.0007 within the linear range, which is in agreement with values reported in the literature, but with a higher precision. Changes in temperature, gas:liquid ratio or salt concentration did not affect the measured ratio, demonstrating the robustness of the developed method.

Carbon, nitrogen, and sulfur isotope changes and hydro-geological processes in a saline lake chain

“…At pH of 8.4 (Station 1), NH 4 (lipid) :NH 3 (gas) calculated by Emerson et al (1975) was 10.4:1, while was 3.2:1 at pH of 8.9 (Station 4). The nitrogen isotope fractionation from NH 4 (lipid) to NH 3 (gas) is 1-7&, and not influenced by the pH and temperature of water (Norlin et al, 2002). Thus, the isotope fractionation for degassing of ammonium is lower than those for NH 3 volatilization, The degassing of ammonium at high pH would also cause the increase in the d 15 N value of POM through the water flow, although the isotope fractionation is lower.…”

Nitrogen isotope analysis of ammonium in aqueous solutions using a perfluorosulfonated ionomer membrane for solid‐phase microextraction

“…The evaluation of the mass spectrometric data is briefly described below. A more detailed description can be found in Norlin et al 16 The sample NH 3 peaks were quantified for N from a calibration curve, where integrated ion current (A·s) was plotted against injected amount of NH 3 ‐N from a standard gas. Reference peaks were arranged to bracket the analyte peak in each chromatogram and the isotopic ratio was calculated from the slopes by comparing the slope for the regression line of the signal at m/z 29 against that at m/z 28 for the analyte and reference N 2 peaks, respectively.…”

“…Recently, we presented a novel method for δ 15 N analysis of ammonium species in aqueous solutions based on conversion of ammonium ions to ammonia, followed by isotopic analysis of the headspace ammonia gas by GC/C/IRMS 16. The method showed a detection limit of 30 mM for δ 15 N‐[NH 4 ] + analysis, which is insufficient for boreal forest soil extracts.…”

“…The objective of this paper was to show the potential of combining headspace‐GC/C/IRMS analysis (see previous work16) with SPME extraction using Nafion‐coated fibers for δ 15 N analysis of ammonium in aqueous solutions. An extension towards analysis of aqueous soil extracts from boreal forests is the long‐term goal of the project.…”

In Situ Soil Phosphorus Monitoring Probe Compared with Conventional Extraction Procedures

“…In that context, the in situ soil analysis using ion-selective electrodes reveals itself as an alternative to modern agriculture, which demands continuous information of chemical and physical soil parameters (Norlin, Irgum, and Ohlsson 2002). Scarce examples of in situ soil analyses can be found in the literature (Norlin, Irgum, and Ohlsson 2002;Artigas et al 2001).…”