A new method for the determination of the nitrogen isotopomers (intramolecular distribution of the nitrogen isotopes) of nitrous oxide has been developed. The method makes use of mass analyses of the molecular (N2O+) and fragment (NO+) ions of N2O on an isotope-ratio mass spectrometer equipped with a special ion collector system. The fragmentation of N2O in the electron impact ion source is fairly stable, and the precision of isotope ratio measurements of the fragment ion relative to the reference gas is better than 0.1‰ for pure N2O samples introduced from a conventional dual-inlet system. Although it is found that the observed isotope ratio of the fragment ion is affected by rearrangement reactions in the ion source, a correction can be applied using an experimentally determined rearrangement fraction. Calibration of the standard N2O for isotopomer measurements is performed by two procedures:  (1) preparation of an N2O standard by thermal decomposition of NH4NO3, (2) relative measurements with pure NO.

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Nitrous oxide (N2O) is an important trace gas in the atmosphere. It is an active greenhouse gas in the troposphere and it also controls ozone concentration in the stratosphere through nitric oxide production. One way to trace the geochemical cycle of N2O is by measuring the natural abundance of stable isotopes, namely 15N and 18O (refs 2-15). Here we report the intramolecular distribution of 15N within the linear NNO molecule, determined by measuring molecular and fragment ions of N2O on a modified mass spectrometer. This revealed a preference for 15N at the central N position, or alpha-site, within N2O isotopomers (isotope-containing molecules). Moreover, this preference varied significantly throughout the atmosphere. In the troposphere, low alpha-site preference indicates local emission of N2O from soils and fossil-fuel combustion, each with distinct isotopomer signatures, which then mixes with background N2O. In the stratosphere, on the other hand, loss of N2O is observed as enhanced alpha-site preference for 15N, due to fractionation during ultraviolet photolysis of N2O. We have constructed an atmospheric mass balance of N2O, incorporating isotopomer abundance, which shows that the intramolecular distribution of 15N is a parameter that has the potential to increase significantly the resolution with which sources and sinks of N2O can be identified and quantified in the atmosphere.