Modifying a sealed tube zinc reduction method for preparation of AMS graphite targets: Reducing background and attaining high precision

The sealed tube zinc reduction method for converting CO2 to graphite for AMS C-14 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 similar to 50,000 C-14 years when analyzing for C-14 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 delta C-13 of our zinc-reduced graphite is usually lighter by 2-37 parts per thousand than the CO2 from which it is made, but this is corrected for in our system by simultaneous measurement of C-13/C-12 along with C-14/C-12 by the spectrometer. This method is suitable for C-14 enriched samples, as well as natural abundance C-14 samples, especially those with modern C-14 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. (c) 2007 Elsevier B.V. All rights reserved.