Abstract
The sealed tube zinc reduction method for converting CO2 to graphite for AMS 14C measurements was originally developed for rapid production of graphite in biomedical tracer experiments. The method was usually thought to have low precision and a high background. We have modified the zinc reduction method originally outlined in Vogel [J.S. Vogel, Radiocarbon 34 (3) (1992) 344] by carefully controlling the amounts of reagents (zinc, titanium hydride and Co or Fe catalyst) and now routinely obtain a precision of 2–3‰ and a relatively low background of ∼50,000 14C years when analyzing for 14C at the Keck Carbon Cycle AMS facility at UC Irvine. Fractionation of carbon isotopes does occur during graphitization and depends on the graphitization yield, which can be affected by the amounts of reagents used and other conditions. The δ13C of our zinc-reduced graphite is usually lighter by 2–3‰ than the CO2 from which it is made, but this is corrected for in our system by simultaneous measurement of 13C/12C along with 14C/12C by the spectrometer. This method is suitable for 14C enriched samples, as well as natural abundance 14C samples, especially those with modern 14C contents. With improved precision and background, we believe that many disciplines can benefit from this technique because of its low cost and rapid production of graphite.
Highlights
The production of uniform and high quality graphite targets is essential for the optimal performance of any AMS system
With improved precision and background, we believe that many disciplines can benefit from this technique because of its low cost and rapid production of graphite
Based on the experiments described above, we set the amount of reagents and catalysts we currently use for a 1 mg C sample to be: 30– 35 mg Zn, 10–15 mg TiH2 and 3–5 mg Fe
Summary
The production of uniform and high quality graphite targets is essential for the optimal performance of any AMS system. Graphite production for AMS 14C measurements is most commonly achieved by hydrogen reduction of CO2 with iron or cobalt catalyst [2,3]. Background for 14C measurement can come from contamination of sample, combustion, graphitization and AMS machine background. The background of our measurements has improved over the three years. We routinely obtain a satisfactory background of about $50,000 14C years for 1 mg C sample, without pre-distillation of Zn or pretreatment of other reagents. We have found that pre-distillation of Zn can further improve background (Shuhui Zheng, unpublished data), as well as reducing the amount of Zn used. Machine background improvement [12], use of an ABA (acid–base–acid) pre-cleaned USGS coal 14C analyses of OXII over time – indicating high precision of the measurements
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