Diffusion and automated nitrogen-15 analysis of low-mass ammonium samples
1991
Kelley, K.R. | Ditsch, D.C. | Alley, M.M.
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 degrees C. Nitrogen recovery in glass-fiber 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 micrograms N at 15N concentrations varying from 0.1030 to 1.1461 atom % 15N were diffused for 10 d at 55 degrees C. Significant dilution of 15N concentration for low-mass N samples occurred. Differences in N 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 bum are determined and used to correct for background N.
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