Graphitisation & measurement of microgram radiocarbon samples at ANSTO.
Dr Andrew Smith1, Dr Quan Hua1, Mr Simon Varley1, Mr Alan Williams2, Dr Bin Yang1
1ANSTO, Lucas Heights, Australia, 2Deceased, 9th February 2021, ,
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 . 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 . 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 ¹⁴C in situ program .
The fabrication approach developed for the LHF was adapted to a new type of miniaturised furnace we call micro-conventional furnaces (MCF) . 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 ¹⁴C 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.
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2. V.A. Levchenko, et al., Nucl. Instr. and Meth. B 123 (1997) 290 .
3. Q. Hua, et al., Nucl. Instr. and Meth. B 223-224 (2004) 284 .
4. A.M. Smith, et al., Nucl. Instr. and Meth. B 268 (2010) 919 .
5. A.M. Smith, et al., Radiocarbon 49 (2007) 245 .
6. B. Yang, A.M. Smith, S. Long, Nucl. Instr. and Meth. B 361 (2015) 363.
7. R.-H. Fülöp, et al., Nucl. Instr. and Meth. B 438 (2019) 207.
8. B. Yang, A.M. Smith, Radiocarbon 59 (2017) 859.
Andrew is a Physicist with 32 years’ experience in Accelerator Mass Spectrometry [AMS] and 45 years’ experience with particle accelerator systems. Andrew leads research activity in the Environment theme and Centre for Accelerator Science platform at ANSTO, with interests in the applications of 7Be, 10Be and 14C to environmental and climate change studies. Andrew has made six scientific expeditions to Antarctica for sample collection, supported by the Australian Antarctic Division, one to Summit, Greenland, and another to Taylor Glacier, Antarctica; the latter two were supported by the National Science Foundation (USA).