Development of low-background vacuum extraction and graphitization systems for 14C dating of old (40–60 ka) samples

At the University of Arizona's Desert Laboratory, we recently constructed new low-background vacuum extraction and graphitization systems that are dedicated to preparing old (40–60 ka) samples for 14C dating. These systems are designed to minimize the amount of contaminant carbon, specifically atmospheric carbon, that is introduced to a sample during laboratory processing. Excluding contaminants is particularly important for 14C dating of old samples because the impact of contamination increases with sample age. In this study, we processed 20 pretreated and 4 untreated aliquots of Ceylon graphite (a naturally-occurring geological graphite) to determine the total procedural background level, and hence the practical limit, of our systems. Samples were heated under vacuum at 240 °C for 1 h to drive off water vapor and other atmospheric gases, and then combusted in ultra-high-purity (UHP) O2 at 500 and 850 °C to monitor the removal of contaminants and to ensure complete combustion. After SOX, NOX, and halide species were removed, sample CO2 was converted to graphite via catalytic reduction of CO. Fe and Zn powders used in the graphitization process were oxidized, “scrubbed”, and reduced with UHP O2, He, and H2, respectively, to remove sorbed atmospheric C species. Graphite targets were stored in UHP Ar until measurement by accelerator mass spectrometry (AMS) to avoid potential interaction with atmospheric gases. Based on the AMS results, the background level of our system is characterized by a nonlinear inverse relationship with sample mass (adjusted R2=0.75; n=24). For a 1 mg graphite target, the total procedural blank, including chemical pretreatment, combustion, cleanup, graphitization, storage, and AMS measurement, is 0.05±0.01 pMC (2σ), equivalent to a 14C “age” of 61.1±1.8 ka. This should not be taken as the upper limit of our system, however, because if the 14C activity of a sample is statistically indistinguishable from the appropriate mass-dependent blank value at the 95% confidence level (2σ), then its age is considered to be “infinite”. Thus, for a 1 mg target, the practical limit of our system is actually ∼55 ka; for a 0.5 mg target, the practical limit is ∼50 ka. Although our extraction system can accommodate inorganic samples (e.g., calcite, aragonite), the above limits are only applicable to geological graphite, charcoal, and organic samples that are processed via combustion. Future work will be directed toward determining the appropriate background levels for inorganic materials.