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

作者：John M. Stark, Stephen C. Hart · 发表于：Soil Science Society of America Journal · 年份：1996 · DOI：10.2136/sssaj1996.03615995006000060033x · 被引用次数：450 · 研究领域：Groundwater and Isotope Geochemistry、Soil and Water Nutrient Dynamics、Groundwater flow and contamination studies

Abstract When diffusion techniques are used to prepare samples for 15 N analyses, low or variable N recovery is sometimes observed. The effect of low recovery on estimates of 15 N 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 NH 3 ‐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 NO − 3 ‐N by residual NH + 4 ; however, recovery of NO − 3 was inversely related to the length of the open period. Recovery of NO 3 − ‐N from 0.5 M K 2 SO 4 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 15 N enrichment; however, when recovery was <100%, the method of blank correction was critical. When the mass of N contamination measured in diffusion b...