Diffusion of Nitrogen‐15 Kjeldahl Digests for Isotope Analysis

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 NH 3 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 15 N enrichments (0.36‐43.3 atom %) indicated that deviation of the two methods was about 1%. The 15 N enrichment of diffused samples was consistently slightly less than that of steam distilled samples. The method is recommended for research using 15 N enriched or depleted sources, but not for research measuring natural variation in 15 N abundance, unless complete diffusion is assured.