Dissolved Nitrogen Isotopes
Overview of the 15N diffusion method
We use the ammonia diffusion method to extract natural-abundance nitrogen for isotopic characterization. The process involves increasing the pH of the dissolved sample, converting the ammonium to gaseous ammonia, which is captured on an acidified filter in the headspace (Holmes et al. 1998). Nitrate-specific 15N is quantified by first boiling-off the volatile ammonia, adding a reducing agent to convert oxidized N to NH4, then proceeding with the standard diffusion and ammonia capture on an acidified filter (Sigman et al. 1997). The filter is then analyzed as a typical solid sample on our Carlo Erba IRMS. Download 15N-Diffusion Protocol HERE.
Sample collection and preparation for 15N diffusion
Estimate the minimum expected concentration of the target N-forms. Calculate the volume of sample needed to contain at least 10µg of target N. Collect at least this volume for each N-analyte. If samples need to be stored before shipping, filter with Pall A/E or Whatman GF/F grade glass fiber filters and freeze. Ship samples over night with adequate dry ice to keep frozen for 36 hours.
Sigman, D. M., M. A. Altabet, R. Michener, D. C. McCorkle, B. Fry, and R. M. Holmes. 1997. Natural abundance-level measurement of nitrogen isotopic composition of oceanic nitrate: an adaptation of the ammonia diffusion method. Marine Chemistry 57:227-242.
Holmes, R. M., J. W. McClelland, D. M. Sigman, B. Fry, and B. J. Peterson. 1998. Measuring 15N-NH4 in marine, estuarine and fresh waters: an adaption of the ammonium diffusion method for samples with low ammonium concentrations. Marine Chemistry 60:235-243.