A technique to facilitate diffusions for nitrogen-isotope analysis by direct combustion

A technique was developed that permits the use of boric acid-indicator solution in carrying out diffusions for inorganic ¹⁵N analysis of soil extracts by direct combustion, so as to eliminate the need for a separate quantitative determination. In this technique, the titrated sample is acidified with 2.5 M KHSO₄ and subsequently treated with methanol to remove H₃BO₃. The NH₄⁺-N is dissolved in 1 mL of deionized water, concentrated after transfer to a microcentrifuge tube, and freeze-dried in a tin capsule. Isotope-ratio analyses were accurate to within 5% when diffusions were performed on 100 to 1000 μg of NH₄⁺-N (0.2-10 atom % ¹⁵N), whereas errors of 8 to 17% occurred with 25 to 50 μg of labeled N, even after correction for isotopic dilution by atmospheric NH₃. The technique described permits ¹⁵N analysis of any form of N that can be recovered by H₃BO₃ diffusion of soil, soil extracts, soil hydrolysates, Kjeldahl digests, water, or wastewater.