Diffusion and Automated Nitrogen‐15 Analysis of Low‐Mass Ammonium Samples

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Abstract
Conventional steam‐distillation techniques for concentrating inorganic N in soil extracts prior to 15N analysis are time‐consuming and labor‐intensive procedures. Diffusion procedures have been proposed as alternatives to steam distillation, but incomplete recovery and isotope dilution have been reported. We evaluated a procedure for diffusing low masses of 15NH4‐N from 2 M KCl solution to determine the effects of NH4 and 15N concentrations on recovery and isotope‐ratio analysis of diffused N. A series of solutions were prepared containing from 0.5 to 2.5 mg N L−1 at 1.1461 atom % 15N. Forty‐milliliter aliquots were placed in plastic containers, equipped with acidified glass‐fiber disks, and allowed to diffuse for either 6 d at room temperature or 5 d at 55 °C. Nitrogen recovery in glassfiber disks averaged 100.5% of the added N. Quantitative recovery of added N was also obtained in a second study in which 30‐mL aliquots of standard solutions containing from 25 to 100 µg N at 15N concentrations varying from 0.1030 to 1.1461 atom % 15N were diffused for 10 d at 55 °C. Significant dilution of 15N concentration for low‐mass N samples occurred. Differences in 15N concentrations between diffused and nondiffused samples were attributed to background N at a 15N concentration of natural abundance. Close agreement was obtained between diffused and nondiffused samples when an isotope‐dilution equation was used to calculate the atom % 15N concentration of the sample N. These studies indicate that the diffusion method is suitable for preparing low‐mass N samples for 15N analysis if appropriate blanks are determined and used to correct for background N.
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