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

Abstract

The 15N content of N2 and N2O emitted from 15N‐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 15N‐labeled N2 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 % 15N 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 15N 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 µL N2O L−1 at natural abundance to <1% for 100 µL N2O L−1 at 10 atom % 15N. 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 µL 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 15N determination in N2 and N2O should aid the study of the processes resulting in evolution of these gases from soil.

Using stable isotopes to follow excreta N dynamics and N2O emissions in animal production systems

“…For a given molecular species, the ion currents are used to produce ratios (R). Therefore, for N 2 , the ratios (Stevens et al, 1993 (Mulvaney, 1993). Further consideration must also be given to comparing these ratios against standards and determining instrument factors, so that only the true ratio differences between normal and enriched atmospheres are used when deriving gaseous N fluxes (Stevens et al, 1993;Stevens and Laughlin, 1998 (Mulvaney and Boast, 1986;Boast et al, 1988;Mosier and Schimel, 1993;Mulvaney, 1993;Stevens et al, 1993;Stevens and Laughlin, 1998;Bergsma et al, 2001).…”

“…Further consideration must also be given to comparing these ratios against standards and determining instrument factors, so that only the true ratio differences between normal and enriched atmospheres are used when deriving gaseous N fluxes (Stevens et al, 1993;Stevens and Laughlin, 1998 (Mulvaney and Boast, 1986;Boast et al, 1988;Mosier and Schimel, 1993;Mulvaney, 1993;Stevens et al, 1993;Stevens and Laughlin, 1998;Bergsma et al, 2001). The ion currents at m/z 44, 45 and 46 are used to calculate the concentration of N 2 O, in conjunction with a reference gas, whereas its 15 N enrichment is calculated from ratios 45 R or 46 R (Stevens et al, 1993;Stevens and Laughlin, 1998).…”

“…If the 15 N tracer is to be followed in multiple pools (e.g. soil, plant and gases) over time, dilution of the N enrichment to use is the sensitivity of the mass spectrometer (Stevens et al, 1993). The more sensitive the mass spectrometer is, the lower the required 15 N enrichment, all things being equal.…”

“…When measuring gas fluxes from soils with headspace chambers, the sensitivity increases with high gas fluxes and smaller headspaces (large surface area to volume ratios). Stevens et al (1993) provide an excellent example of how to determine sensitivity for a mass spectrometer. As 15 N enrichment and gas fluxes decrease, the coefficient of variation will increase and data quality will suffer (Stevens et al, 1993 (Fry, 2006).…”

“…Stevens et al (1993) provide an excellent example of how to determine sensitivity for a mass spectrometer. As 15 N enrichment and gas fluxes decrease, the coefficient of variation will increase and data quality will suffer (Stevens et al, 1993 (Fry, 2006). The result is that products have a lighter isotopic composition owing to the fractionation process.…”