AUTOMATED-DETERMINATION OF NITROGEN-15-LABELED DINITROGEN AND NITROUS-OXIDE BY MASS-SPECTROMETRY

The N-15 content of N2 and N2O emitted from N-15-enriched soils conveys information about their sources; such information can only be obtained by mass spectrometry. Conventional dual-inlet isotope-ratio 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 N-15-labeled N, or N2O. 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 N2 from N2O. For N2 analysis, the carrier stream is passed through a Cu reduction column to remove O2. A triple-collector isotope-ratio mass spectrometer analyzes a portion of the carrier stream for N2 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 % N-15 with coefficients of variation (CVs) ranging from 0.07 to 1.37 %. The reduction column is bypassed for the analysis of N2O at m/z 44, 45, and 46. From the 45/44 ratios, the N-15 content of the N2O could be determined accurately with enrichments up to 40 atom %. Precision depended on concentration and enrichment, with CVs ranging from 10% for 15 muL N2O L-1 at natural abundance to < 1% for 100 muL N2O L-1 at 10 atom % N-15. The concentration of N2O could be quantified from the ion currents at m/z 44, 45, and 46 with a detection limit of 2.1 muL N2O L-1. A batch of 132 samples can be analyzed automatically for N2 or N2O at a rate of six determinations per hour. Automation of N-15 determination in N2 and N2O should aid the study of the processes resulting in evolution of these gases from soil.