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|Title: ||Early results from the ANSTO/NIWA 14C of atmospheric methane program|
|Authors: ||Smith, A|
|Issue Date: ||24-Mar-2011|
|Publisher: ||12th International Conference on Accelerator Mass Spectrometry (AMS-12)|
|Citation: ||Smith, A., Brailsford, G., Yang, B., Bromley, T., Martin, R. (2011). Early results from the ANSTO/NIWA 14C of atmospheric methane program. 12th International Conference on Accelerator Mass Spectrometry (AMS 12), 20th - 25th March 2011. Museum of New Zealand: Te Papa Tongarewa, Wellington, New Zealand.|
|Abstract: ||development and proving of the laser heated microfurnace we have used it to prepare 45
samples of ~ 16 μg of carbon. These comprised CO2, derived from atmospheric methane, frozen
back into glass breakseals following measurement for δ13C at NIWA. There were three sample sets:
15 from Baring Head, NZ (BHD), collected each ~ 15 days between March and September 2009, 9
from Arrival Heights, Antarctica (SCT), collected each ~ 41 days between February 2008 and
January 2009, plus 21 samples taken along a Pacific Ocean voyage from Nelson (NZ) – Osaka
(Japan) in December 2005 (FTW). All samples were measured to better than 1% precision,
sufficient to reveal a 14CH4 signal. The BHD set shows significant temporal variation in 14C for
baseline air passing over the Southern Ocean, whereas the SCT set shows a lesser variation for
Antarctic air. The FTW set covers a S-N transect across the Pacific Ocean, showing the influence of
the ITCZ (5°-10° N) and different meteorological conditions on the concentration, δ13C and Δ14C of
CH4 and demonstrates that CH4 is not well mixed.
Graphitisation reactions averaged 32 min with 0.7 mg of Fe, reduced from Fe2O3, as the catalyst.
The samples, blanks and standards were measured in two 10 minute blocks; some were measured
again to improve statistics. Average 13C4+ currents per microgram of carbon were 13, 4 and 2
nA/μg for each 10 min block. Similarly-sized targets prepared in the conventional furnace with
Fe2O3 gave 8, 5 and 2 nA/μg for each 10 min block. Graphitisation efficiencies were typically 90-
100% for microfurnace samples, compared with 37-84% for conventional furnace samples.
Subsequent examination by microscope showed that the cesium beam was well-centred on the 1
mm diameter recess and that effectively all C/Fe was sputtered, leading to a (minimum) estimation
of ~4% overall AMS measurement efficiency. Copyright (c) 2011 AMS12|
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