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Measuring Nitrogen-15 Content of Gases: Comparison of Direct and ARC-Redistribution Methods

ICRAF, P.O. Box 30677, Nairobi, Kenya

E. R. Austin

IFDC, P.O. Box 2040, Muscle Shoals, AL 35662

*Corresponding author.

ABSTRACT

When ¹⁵N-enriched N₂ gas evolves from a soil fertilized with a ¹⁵N-labeled source and mixes with atmospheric N₂, the mixture will have a nonrandom N isotopic distribution. The ¹⁵N enrichment of N₂ in air samples can be determined by measuring the ion signals for mass to charge (m/e) 28, 29, and 30 (direct method). An alternative is to randomize the N isotope distribution with a high voltage arc and then determine the ¹⁵N enrichment by measuring the ion signals for m/e 28 and 29 (arc method). The ¹⁵N enrichments obtained by the direct method and the are method agreed within 1% for (i) ¹⁵N-labeled N₂ mixed with air and (ii) gases evolved from urea-¹⁵N applied to flooded soil. A slight loss of precision in the are method may result from the correction of ¹⁵N values for the N blank associated with processing of the samples before isotope ratio analyses. Storage of ¹⁵N-enriched air samples in evacuated serological tubes resulted in dilution of their ¹⁵N content by the residual air initially present in the tubes; consequently, the ¹⁵N values must be corrected for the N content of the residual air.

Research conducted as part of International Fertilizer Development Center (IFDC) and International Rice Research Inst. (IRRI) Co-operative Project, P.O. Box 933, 1099 Manila, Philippines. Joint contribution from IFDC and IRRI.

Received for publication November 11, 1991.