Graphitisation & measurement of microgram radiocarbon samples at ANSTO
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Date
2021-11-17
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Australian Nuclear Science and Technology Organisation
Abstract
The Centre for Accelerator Science (CAS) at ANSTO has been providing radiocarbon analyses for the user
community and internal projects for two and a half decades. Early on, there was a need to develop the
measurement capability for samples containing just tens of micrograms of carbon [1, 2]. We have continued
to develop this capability since.
At first effort was directed at optimising our ‘conventional’ graphitisation furnaces [3]. These have a
minimum reaction volume of ~ 2.5 mL and reduce CO₂ to graphite over an Fe catalyst at 600 °C in an
excess of hydrogen. CAS operates a bank of 24 conventional furnaces which provide for the bulk of our
sample graphitisation for samples containing > 5 μg of carbon.
In 2003 we began developing a novel, miniaturised graphitisation furnace which used a focused infrared laser
to heat the Fe catalyst in a quartz crucible, with the temperature measured indirectly by infrared thermometry
[4]. The prototype unit had an internal reaction volume of ~0.5 mL including pressure transducer and the two
subsequent furnaces ~ 0.3 mL. These small volumes allow a higher initial pressure for small amounts of
CO₂ , improving the efficiency of conversion to graphite. Efficient trapping of the water vapour produced
during the reaction and careful selection of the catalyst are also key to optimising graphitisation of small
samples [5, 6]. By localising the heated region within the reaction volume, the addition of extraneous carbon
is minimised in these furnaces and samples containing just 1-2 μg of carbon are routinely prepared. The laser
heated furnaces (LHF) are preferred for processing the very small samples derived from our ¹⁴ C in situ
program [7].
The fabrication approach developed for the LHF was adapted to a new type of miniaturised furnace we call
micro-conventional furnaces (MCF) [8]. These furnaces have a minimum reaction volume of ~ 0.9 mL with a
small tube furnace to heat the catalyst. Variable temperature cold traps have been developed to optimise
sample processing with samples as small as 5 μg of carbon routinely prepared. The MCF are used extensively
in conjunction with ¹⁴ C measurements of CO, CO₂ and CH₄ derived from ice core and firn air samples.
We present an overview of micro-sample graphitisation and measurement at CAS.
Description
Keywords
Graphitization, Carbon 14, ANSTO, Accelerators, Measuring instruments, Furnaces, Water vapor
Citation
Smith, A., Hua, Q., Varley, S., Williams, A., & Yang, B. (2021). Graphitisation & measurement of microgram radiocarbon samples at ANSTO. Poster plus presentation to the 15th International Conference on Accelerator Mass Spectrometry. ANSTO Sydney, Australia. November 15th – 19th, 2021. (pp. 248). Retrieved from: https://ams15sydney.com/wp-content/uploads/2021/11/AMS-15-Full-Program-and-Abstract-Book-R-1.pdf