Measuring Nitrogen-15 Content of Gases: Comparison of Direct and ARC-Redistribution Methods
- R. J. Buresh and
- E. R. Austin
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.
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