Small-mass graphite preparation by sealed tube zinc reduction method for AMS 14C measurements

Previous work has demonstrated that the sealed tube Zn reduction method for converting CO2 to graphite for AMS 14C measurements produces targets that can be measured with high precision and low background for samples of about 1 mg C down to approximately 0.1 mg C at the Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS). Now a modified method has been developed to prepare small-mass samples ranging from 0.015 to 0.1 mg C. In this modified method, the volume of the sealed reactor tube is reduced to ∼1.9 cm3, and the amounts of Zn and TiH2 reagents are reduced proportionally. The amount of Fe catalyst used remains the same to ensure a long lasting current in the AMS. Small-mass samples prepared by this method generally yield 12C+1 currents of about 0.5 μA per 1 μg C. An in situ simultaneous AMS δ13C measurement allows for correction of both graphitization and machine-induced isotopic fractionation, and is a prerequisite for high precision and accurate measurements using the Zn reducing method. Corrections for modern-carbon and dead-carbon background components are applied to samples based on small-mass samples of a 14C free material and of a modern standard covering the sample size range. It was discovered during additional investigation into lowering the modern-carbon background component that baking assembled reactor tubes at 300 °C for 1 h prior to use resulted in significantly lower modern-carbon background values. The accuracy and precision of small-mass samples prepared by this method are size dependent, but is usually ±10–15‰ for the smallest samples (0.015–0.02 mg C), based on duplicate measurements of primary and secondary standards.