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Diffusion of Nitrogen-15 Kjeldahl Digests for Isotope Analysis¹

ABSTRACT

Steam distillation methods are commonly used to prepare Kjeldahl digests for N stable isotope analysis. Steam distillation requires skilled operation of specialized equipment as well as considerable operator time, and is subject to sample cross contamination unless precautionary washing procedures (which further increase sample preparation time) are adopted. A simple, inexpensive, labor saving diffusion method for Kjeldahl digests using disposable polypropylene specimen containers (128 mL) with threaded lids was devised to overcome problems with steam distillation. Diffused ammonia was collected in a disposable test tube (12 by 75 mm) containing an acid trap. Fractionation of N stable isotopes was <1% for diffused diluted simulated Kjeldahl digests, even when <20% of the initial NH₃ present was diffused. As diffusion becomes more complete isotope fractionation decreases. Direct comparison of aliquots of the same plant tissue Kjeldahl digest by steam distillation and diffusion over a wide range of ¹⁵N enrichments (0.36-43.3 atom %) indicated that deviation of the two methods was about 1%. The ¹⁵N enrichment of diffused samples was consistently slightly less than that of steam distilled samples. The method is recommended for research using ¹⁵N enriched or depleted sources, but not for research measuring natural variation in ¹⁵N abundance, unless complete diffusion is assured.

¹ Contribution from cooperative research of the Dep. of Agron., Univ. of Kentucky, Lexington, KY 40546-0091, the USDA, Agric. Res. Serv., Lexington, KY 40546-0091, and Dep. of Plant and Soil Biology, Univ. of California, Berkeley, CA 94720. Published with the approval of the Kentucky Agric. Exp. Stn. Director as Journal no. 86-3-73.

² Plant Physiologist, USDA, Agric. Res. Serv., and Adjunct Associate Professor, Dep. of Agron., Univ. of Kentucky; Spectroscopist, Dep. of Plant and Soil Biology, Univ. of California, Berkeley; Associate Professor, Dep. of Agron., Univ. of Kentucky, respectively.

Received for publication April 28, 1986.

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