2.3.3. Measurements of 13C/12C of Methane
Partitioning atmospheric methane into its biotic and abiotic sources is possible through global measurements of its carbon isotopic ratio. Biotic sources such as wetlands have a characteristic isotopic "signature" of roughly -60, whereas abiotic sources such as natural gas have a "signature" near to -40. We have designed an automated system for the analysis of d13C in atmospheric methane. Currently the system analyzes pairs of flasks on a weekly basis from six sites of the CMDL cooperative air sampling network (Figure 2.15). The analysis technique used is gas chromatography followed by isotope ratio mass spectrometry (GC-IRMS). First, the methane is separated from over 99.99% of the air on a cryogenic pre-column (Haysep-D at -120°C). It is then cryo-focused at the head of the analytical column (Molecular Sieve 5A capillary column) where, after warming, the methane is separated from the remaining air and other trace species. The methane is then combusted to produce CO2 and admitted to the mass spectrometer, via an open split capillary tube, for determination of its isotopic ratio. As little as 100 mL of air is used to achieve a precision of better than 0.06 in less than 15 minutes. The measured precision is comparable to that of systems where at least 100 times as much air is used and where analysis and separation times are more than 1 hour.
Preliminary measurements indicate a latitudinal difference of approximately -0.5 as measured between Pt. Barrow, Alaska, and the South Pole (Figure 2.15) during the boreal winter. The sign of this gradient is consistent with the isotopic enrichment expected in the southern hemisphere at this time of year due to enhanced destruction of methane by hydroxyl radical. (The hydroxyl radical reacts preferentially with 12CH4). Our working air standards have not yet been linked to an international scale, therefore, we can only report our results relative to one another at this time. Our standards will be calibrated externally in the near future. An automated flask manifold is also being built so that a greater number of the network sites can be analyzed for d13C of methane.
Fig. 2.15. Latitudinal gradient of d13C of methane during boreal winter.
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