Once a carbon bearing sample has been completely oxidised or hydrolysed to CO2, the CO2 is introduced into the vacuum line, either via septum sealed vials or heat-sealed glass tubes. First, any water is removed through contact with an ethanol bath at -90oC. The sample is then sized and frozen down using liquid nitrogen into a reactor containing pre-conditioned (hydrogen-reduced) iron powder and water-absorbing magnesium perchlorate. Pure hydrogen is then introduced into the reactor, which is then heated to 550-650oC for up to 7 hours (usually 2 hours is sufficient). This causes the CO2 to reduce to graphite, which grows in filaments on the iron catalyst, while water that is produced as a reaction by-product is absorbed by the magnesium perchlorate (Santos et al., 2004).
The graphite/iron mixture produced in this way is then pressed into stainless steel cathodes (backed by aluminium wire), which are loaded into wheels for subsequent AMS analysis.
Sample unknowns are typically graphitised in parallel with a set of oxalic acid (Ox-II) primary standards (evacuated and combusted separately), a set of secondary standards (e.g. IAEA standards) and a set of ‘radiocarbon blanks’ for assessing background levels (Freeman et al., 2016). Samples and standards of various sizes are used in order to correct for size-dependent modern and dead carbon contamination (Santos et al., 2007).
Freeman, E. et al., 2016, Radiocarbon 57(1), 1-10.
Santos, G.M. et al., 2004, Radiocarbon, v.46 (1), p.165-173.
Santos, G.M. et al., 2007, Nucl. Instr. and Meth in Phys. Res. B, v.259, p.292-302.