Technical note: Studying Li-metaborate fluxes and low-temperature combustion/high-temperature extraction systematics with a new, fully automated in situ cosmogenic ¹⁴C processing system at PRIME Lab

Abstract. Extraction procedures for in situ cosmogenic ¹⁴C (in situ ¹⁴C) from quartz require quantitative isotopic yields while maintaining scrupulous isolation from atmospheric/organic ¹⁴C. These time- and labor-intensive procedures are ripe for automation; unfortunately, our original automated in situ ¹⁴C extraction and purification systems, reconfigured and retrofitted from our original systems at the University of Arizona, proved less reliable than hoped. We therefore installed a fully automated stainless-steel system (except for specific glass or fused-quartz components) incorporating more reliable valves and improved actuator designs, along with a more robust liquid nitrogen distribution system. As with earlier versions, the new system uses a degassed Li-metaborate (LiBO₂) flux to dissolve the quartz sample in an ultra-high-purity oxygen atmosphere, after a lower-temperature combustion step to remove atmospheric/organic ¹⁴C.

We compared single-use high-purity Al₂O₃ vs. reusable 90 %Pt/10 %Rh (Pt/Rh) sample combustion boats. The Pt/Rh boats heat more evenly than the Al₂O₃, reducing procedural blank levels and variability for a given LiBO₂ flux. This lower blank variability also allowed us to trace progressively increasing blanks to specific batches of fluxes from our original manufacturer. Switching to a new manufacturer returned our blanks to consistently low levels on the order of (3.4 ± 0.9) x 10^{4 14}C atoms.

We also analyzed the CRONUS-A intercomparison material to investigate sensitivity of extracted ¹⁴C concentrations to the temperature and duration of the combustion and extraction steps. Results indicate that 1-hr combustion steps at either 500 or 600 °C yield results consistent with the consensus value of Jull et al. (2015), while 2 hr at 600 °C results in loss of ca. 9 % of the high-temperature ¹⁴C inventory. Results for 3 hr extractions at temperatures ranging from 1050 °C to 1120 °C and 4.5 hr at 1000 °C yielded similar results that agreed with the nominal value as well as with published results from most laboratories. On the other hand, an extraction for 3 hr at 1000 °C was judged to be incomplete due to a significantly lower measured concentration. Based on these results, our preferred technique is now combustion for 1 hr at 500 °C followed by a 3 hr extraction at 1050 °C. Initial analyses of the CoQtz-N intercomparison material at our lab yielded concentrations ca. 60 % less than those of CRONUS-A, but more analyses of this material from this and other labs are clearly needed to establish a consensus value.