A simplified diffusion method for δ15N analysis of dissolved ammonium

Abstract

The δ¹⁵N-NH₄⁺ method presented here is a modified version of the commonly used acidified disk diffusion method and is designed for water samples with an NH₄⁺ concentration of 0.6 mg N/L or greater. Samples with lower NH₄⁺ concentrations can be analyzed using greater sample volumes in larger diffusion jars but optimal conditions for diffusion (e.g., disk size, diffusion time, jar size) have not been finalized yet and will be described separately. In summary,dissolved NH₄⁺ in the sample is converted to NH₃ gas and subsequently trapped on an acidified quartz disk that is enclosed in a gas permeable, hydrophobic membrane. The diffusion apparatus consists of a 60mL wide-mouth jar with tetrafluoroethylene (TFE)-lined cap containing anacidified quartz disk enclosed in a polytetrafluoroethylene (PTFE) membrane. The required sample volume, which contains 12-30μg of NH₄⁺-N, is pipetted into each diffusion jar. Deionized water (DI) is added to each jar so that the total volume is approximately the same (20mL). In order to prevent the PTFE traps from absorbing water and bursting, KCl is added to each diffusion jar to bring the KCl concentration of the solution to approximately 4M. Phenolphthalein pH indicator solution is added to each diffusion jar and the samples are then made basic by the drop-wise addition of a NaOH solution. Immediately after the solution turns pink (pH = 8-9), 2mL of tetraborate pH buffer solution is added to stabilize the pH at about 9.5. A PTFE trap is then placed in the diffusion jar and the jar is capped. The pH is buffered slightly higher than the pKa of NH₄⁺ (9.25; Equation 1) to promote the conversion of NH₄⁺ to NH3 gas without the risk of organic N hydrolysis that could occur at higher pH.
NH₄⁺_(aq)↔ NH_3(g)+ H⁺ _(aq) , pKa 9.25 (Eqn. 1)
Diffusion jars are placed on an orbital shaker for 10 days to ensure complete diffusion of the NH₃ onto the acidified disks. Three internal δ¹⁵N-NH₄ ⁺ isotope standards are processed with each batch of samples. Once diffusion is complete, the PTFE traps are removed and the quartz disks collected and placed in clean vials. The vials are frozen, freeze dried overnight and are then ready for isotopic analysis. δ¹⁵N values of the quartz disks are determined at the Environmental Isotope Lab, University of Waterloo, using a Carlo Erba 1108 Elemental Analyzer interfaced with a Thermo Instruments Deltaplus isotope ratio mass spectrometer (EAIRMS). The precision associated with δ¹⁵N-NH₄⁺ analysis of water samples is generally better than ±0.2‰.