Diffusion Technique for Preparing Salt Solutions, Kjeldahl Digests, and Persulfate Digests for Nitrogen‐15 Analysis

Abstract

When diffusion techniques are used to prepare samples for ¹⁵N analyses, low or variable N recovery is sometimes observed. The effect of low recovery on estimates of ¹⁵N enrichment is unknown. Also, the suitability of diffusion techniques for use with a variety of salt solutions and digests containing very low N concentrations (92% recovery of 50 µg NH⁺₄‐N from 75 mL of 2 M NaCl, total Kjeldahl digests, or 2 M KCl preserved with acid. Leaving sample containers open for 5 d after diffusion for NH⁺₄ completely eliminated contamination of NO⁻₃‐N by residual NH⁺₄; however, recovery of NO⁻₃ was inversely related to the length of the open period. Recovery of NO₃⁻‐N from 0.5 M K₂SO₄ and alkaline persulfate digests after a 3‐d open period was 90 and 77%, respectively. Complete recovery of NH⁺₄ or NO⁻₃ was not required to obtain accurate estimates of ¹⁵N enrichment; however, when recovery was 15N enrichments, diffused standards often had ¹⁵N enrichments that were significantly different from nondiffused standards; however, when the mass of N in blanks was estimated using an isotope dilution equation, there was excellent agreement between diffused and nondiffused standards, regardless of the degree of recovery.