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

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Soil Science Society of America Journal



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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 (<0.8 mg L−1) has not been determined. We evaluated a diffusion technique where NH3‐N is trapped on acidified filter paper disks that are sealed between two strips of Teflon [polytetrafluoroethylene (PTFE)] tape. Because the PTFE protects the acid trap from neutralization, large volumes of solution can be diffused in plastic specimen containers and the sample can be vigorously mixed during the diffusion period. Six days of diffusion at 22°C resulted in >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 NO3‐N by residual NH+4; however, recovery of NO3 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 NO3 was not required to obtain accurate estimates of 15N enrichment; however, when recovery was <100%, the method of blank correction was critical. When the mass of N contamination measured in diffusion blanks was used to blank‐correct 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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