Testing sealed-tube Graphitization at the NERC Radiocarbon facility, east Kilbride

Abstract

ABSTRACTGraphitization of 0.5–1.5 mg C, and of smaller samples to a lesser extent, is routinely done at our Facility by reduction over zinc. The method yields low background, good accuracy but offers a limited throughput, requires dedicated equipment and considerable operator time. Sealed-tube graphitization is faster, easier and cost-efficient producing as many graphites as CO2 can be purified in one day with low background, good accuracy and precision, provided precise measurements of δ13C values can be attained by accelerator mass spectrometry (AMS) to correct for isotope fractionation (Xu et al. 2007). We tested sealed-tube graphitization on 0.1 to 1.0 mg C samples and found that while we were able to obtain low backgrounds of >57,000 ±1000 yr BP for ∼1.7 mg C and 41,230 ± 430 yr BP for ∼0.09 mg C (0.0008 ± 0.0001 and 0.0059 ± 0.0003 Fraction Modern, respectively), results were variable for sample sizes <0.5 mg C. Measurements of FIRI Belfast Cellulose and TIRI Barleymash showed 0.3–0.6% precision and 1% accuracy for most sample sizes. We found better results in our laboratory by introducing the following modifications: (1) shorter inner tube (2 cm long), (2) short flame-seal length (∼7–8 cm) and (3) keeping the inner tube with iron separate from the outer tube containing zinc and titanium hydride during cleaning.