Automated Determination of Nitrogen‐15‐Labeled Dinitrogen and Nitrous Oxide by Mass Spectrometry

Abstract The 15 N content of N 2 and N 2 O emitted from 15 N‐enriched soils conveys information about their sources; such information can only be obtained by mass spectrometry. Conventional dual‐inlet isotoperatio mass spectrometry (IRMS) involve complex gas sampling and inlet systems, which limit the use of the technique. Continuous‐flow IRMS was evaluated for the automated analysis of 15 N‐labeled N 2 or N 2 O. A gas sample is flushed from a vial by a He carrier stream. Carbon dioxide, NO, and water are chemically removed prior to chromatographic separation of N 2 from N 2 O. For N 2 analysis, the carrier stream is passed through a Cu reduction column to remove O 2 . A triple‐collector isotope‐ratio mass spectrometer analyzes a portion of the carrier stream for N 2 at m/z 28, 29, and 30. Accurate values for ratio (29/28 and 30/28) differences in normal and enriched atmospheres were obtained from enrichments up to 2 atom % 15 N with coefficients of variation (CVs) ranging from 0.07 to 1.37%. The reduction column is bypassed for the analysis of N 2 O at m/z 44, 45, and 46. From the 45/44 ratios, the 15 N content of the N 2 O could be determined accurately with enrichments up to 40 atom %. Precision depended on concentration and enrichment, with CVs ranging from 10% for 15 µL N 2 O L −1 at natural abundance to <1% for 100 µL N 2 O L −1 at 10 atom % 15 N. The concentration of N 2 O could be quantified from the ion currents at m/z 44, 45, and 46 with a detection limit of 2.1 µL N 2 O L −1 . A batch of 132 samples can be analyzed automatically for N 2 or N 2 O at a rate of six determinations per hour. Automation of 15 N determination in N 2 and N 2 O should aid the study of the processes resulting in evolution of these gases from soil.