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

Agricultural Research Dep., National Fertilizer and Environmental Research Center/Tennessee Valley Authority, Muscle Shoals, AL 35660

D. C. Ditsch and M. M. Alley

Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061

*Corresponding author.

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.

Received for publication July 26, 1990.

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