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

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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.

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