Diffusion of Kjeldahl Digests for Automated Nitrogen-15 Analysis by the Rittenberg Technique

Nitrogen-isotope analysis by the Rittenberg technique requires N in the form of NH₄. Conversion of organic N to NH₄-N is normally accomplished by the Kjeldahl method, which involves digestion with concentrated H₂SO₄. A diffusion method recently developed for isotope-ratio analysis of inorganic N in soil extracts was modified for use with Kjeldahl digests. An aliquot of the digest (to 20 mL) containing 50 to 150 µg of N was neutralized by overturning a vial with 2 mL of 20 M NaOH in a 118-mL specimen container sealed with a screw-top lid, and the NH₃ liberated was collected for 6 d at room temperature in a 7-mm-diam. glass-fiber disk acidified with 10 µL of 1 M H₂SO₄. For automated Rittenberg analysis of the diffused NH₃-N by mass spectrometry, the glass disk was tranferred to a plastic sample tray (96 samples per tray), treated with 10 µL of 28.9 M (concentrated) HF, and dried in a desiccator over anhydrous CaSO₄. Studies showed that diffusion was complete (>97%) and that, when correction was made for isotopic dilution by natural-abundance N in reagents, isotope-ratio analyses of diffused N (0.2–4.2 atom % ¹⁵N) were in close agreement with analyses using conventional steam-distillation and concentration techniques.