ULTRA SMALL-MASS GRAPHITIZATION BY SEALED TUBE ZINC REDUCTION METHOD FOR AMS 14C MEASUREMENTS

A modified sealed tube Zn reduction method based on Khosh et al. (2010) has been developed to graphitize ultra small-mass samples ranging from 4-15 mu g carbon (C) for accelerator mass spectrometry (AMS) radiocarbon measurements. In this method, the reagent TiH2 is removed from the previous method while the amounts of Zn and Fe powder remain the same. The volume of the sealed reactor is further reduced by similar to 40% to similar to 0.75 cm(3) and the graphitization temperature is lowered to 450 degrees C. Graphite targets produced by this method generally yield C-12(+1) currents of about 0.5 mu A per 1 mu g C, similar to the small mass (15-100 mu g C) sealed tube Zn reduction method previously reported by Khosh et al. (2010) when measured on the same AMS system at KCCAMS, University of California, Irvine. Change of Fe powder to Sigma-Aldrich (400-mesh) has yielded further improved backgrounds over Fe powder of Alfa Aesar (325-mesh). Modern C background from combustion and graphitization is estimated to be 0.2-0.8 mu g C, and dead-C background to be 0.1-0.4 mu g C. The accuracy and precision of ultra small-mass samples prepared by this method are size and C-14 content dependent, but is usually +/- 4-5% for the smallest sample size of similar to 4-5 mu g C with modern C-14 content. AMS on-line. delta C-13 measurement that allows for correction of both graphitization and machine-induced isotopic fractionation is the key for applying the sealed tube Zn reduction method to ultra small-mass sample graphitization.