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Evaluation of a Diffusion Method for Preparing Low-Nitrogen Samples for Nitrogen-15 Analysis

USDA-ARS, 119 Keim Hall-East Campus, Univ. of Nebraska, Lincoln, NE 68583

M. P. Russelle

USDA-ARS and Dep. of Soil Science, 439 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108

*Corresponding author (jlory{at}unl.edu).

ABSTRACT

Ammonia diffusion techniques have been advocated to eliminate crosscontamination problems associated with distillation methods for concentrating low-N-content samples for ¹⁵N analysis. Two potential problems with diffusion are nonquantitative N recovery and isotopic fractionation. These are most likely to have significant effects for low-N-content samples. We evaluated the ability of a diffusion method to accurately prepare low-N (<1000 µg N), high-volume (55 mL) simulated acidic digest samples and quantified sinks for N in the diffusion system. This diffusion system trapped N on acid-impregnated glass fiber disk suspended above the sample solution. The ¹⁵N/¹⁴N isotopic ratio of N recovered on the disk declined linearly as the quantity of N in the original sample solution decreased below 500 µg N. Nitrogen-15 values adjusted for contamination detected in reagent blanks still differed significantly from nondiffused reagent blanks. Significant sinks of N other than the acidified disk existed despite efforts to minimize them. The method produced unexplained changes in isotopic composition of the sample. These results emphasize the importance of thoroughly evaluating any NH₃ diffusion technique with representative sample types in each laboratory before adoption of the technique for routine use. In addition, we recommend standard curves as a more robust method for adjusting ¹⁵N values for errors associated with diffusion.

Joint contribution of the USDA-ARS Plant Science Research Unit, U.S. Dairy Forage Research Center (Minnesota Cluster), and Minnesota Agric. Exp. Stn. Paper no. 20552 of the scientific journal series.

Received for publication June 7, 1993.

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