Mass spectrometric method for the absolute calibration of the intramolecular nitrogen isotope distribution in nitrous oxide

Abstract

A mass spectrometric method to determine the absolute intramolecular (position-dependent) nitrogen isotope ratios of nitrous oxide (N2O) has been developed. It is based on the addition of different amounts of doubly labeled 15N2O to an N2O sample of the isotope ratio mass spectrometer reference gas, and subsequent measurement of the relative ion current ratios of species with mass 30, 31, 44, 45, and 46. All relevant quantities are measured by isotope ratio mass spectrometers, which means that the machines' inherent high precision of the order of 10(-5) can be fully exploited. External determination of dilution factors with generally lower precision is avoided. The method itself can be implemented within a day, but a calibration of the oxygen and average nitrogen isotope ratios relative to a primary isotopic reference material of known absolute isotopic composition has to be performed separately. The underlying theoretical framework is explored in depth. The effect of interferences due to 14N15N16O and 15N14N16O in the 15N2O sample and due to 15N2+ formation are fully accounted for in the calculation of the final position-dependent nitrogen isotope ratios. Considering all known statistical uncertainties of measured quantities and absolute isotope ratios of primary isotopic reference materials, we achieve an overall uncertainty of 0.9 per thousand (1 sigma). Using tropospheric N2O as common reference point for intercomparison purposes, we find a substantially higher relative enrichment of 15N at the central nitrogen atom over 15N at the terminal nitrogen atom than measured previously for tropospheric N2O based on a chemical conversion method: 46.3 +/- 1.4 per thousand as opposed to 18.7 +/- 2.2 per thousand. However, our method depends critically on the absolute isotope ratios of the primary isotopic reference materials air-N2 and VSMOW. If they are systematically wrong, our estimates will also necessarily be incorrect.

High‐precision determination of the changing isotopic composition of atmospheric N2O from 1990 to 2002

“…The calibration of the intramolecular distribution in N 2 O is not straightforward. Results of a mass spectrometric calibration method using mixtures of isotopically labeled N 2 O isotopologues [ Kaiser et al , 2003c] yield a strong difference of the 15 N content at both positions in the molecule. The values of 1 δ 15 N = −23.6‰ and 2 δ 15 N = 36.9‰ for our pure N 2 O running gas translate to 1 δ 15 N = −16.2‰ and 2 δ 15 N = 29.6‰ for our reference air sample containing clean tropospheric air.…”

Measurements of N2O isotopologues in the stratosphere: Influence of transport on the apparent enrichment factors and the isotopologue fluxes to the troposphere

“…Once the scrambling factor is determined, there is additional difficulty in placing the measurements that were made relative to the laboratory working standard onto the international air‐N 2 isotope scale [ Toyoda and Yoshida , 1999]. Details of a purely mass spectrometric technique devised at MPI‐Mainz and UC Berkeley to convert site‐specific isotope measurements to the air‐N 2 scale appear elsewhere [ Kaiser et al , 2003b]. However, discrepancies between the Mainz, UC Berkeley, and Tokyo Institute of Technology groups have yet to be completely resolved.…”

Probing stratospheric transport and chemistry with new balloon and aircraft observations of the meridional and vertical N<sub>2</sub>O isotope distribution

“…Thus, the individual isotope signatures are more easily intercomparable. Moreover, assigning a specific position-dependent nitrogen isotope scale is problematic at the present time because two significantly different absolute isotope calibrations exist and the discrepancies have not been resolved yet (Kaiser et al, 2004a;Toyoda and Yoshida, 1999). To compare the isotope profiles for individual years directly, we apply a further normalization by correcting for the temporal trend in the isotope ratio of tropospheric N 2 O and the age of stratospheric air (Sect.…”