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

• The use of custom-made solid-phase microextraction (SPME) fibers coated with a perfluorosulfonated ionomer, Nafion®, was investigated for nitrogen isotopic analysis of ammonium in aqueous solutions. Aqueous ammonium was converted to ammonia by addition of a base, followed by absorption from the headspace, desorption in the injection port of a gas chromatograph, and analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Fibers coated with a Nafion tubing were chosen due to a higher fiber-gas distribution constant and a higher Nafion thickness than fibers coated with Nafion solution, both leading to a higher amount of ammonia absorbed at equilibrium. The Nafion membrane-coated fiber absorbed approximately 20 times more than a commercial polydimethylsiloxane (PDMS) fiber. The isotopic fractionation between fiber and gas was 1.0117 ± 0.0009 (standard deviation, SD, of all measurements) at an initial ammonia gas concentration of 21-210 M. At 390 M initial gas concentration it was slightly lower. When sampling from liquid samples, an ammonium concentration of 10 mM was needed to obtain a sufficient amount of ammonia absorbed. Modeling of the absorption at different temperatures showed that the absorption was approximately constant in the temperature range suitable for SPME experiments. Absorption at room temperature was therefore used for simplicity. A pilot study was conducted in which absorption was achieved from a single 9 L droplet of sample. The preliminary results showed that 15N analysis was possible for only 0.4-0.5 mol of ammonium with a SD of 0.8 (n = 5).

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