O01_07

Studying LiBO₂ fluxes and low-temperature combustion systematics with a fully automated in situ cosmogenic ¹⁴C processing system at PRIME Lab

Lifton N¹, Koester A¹

¹Purdue University, West Lafayette, United States

Extraction procedures for in situ cosmogenic ¹⁴C (in situ ¹⁴C) from quartz require quantitative isotopic yields while maintaining scrupulous isolation from ubiquitous 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 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 first compared single-use high-purity Al₂O₃ vs. reusable Pt/10%Rh sample combustion boats. The Pt/10%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 the fluxes from our original manufacturer. Switching to a new manufacturer returned our blanks to consistently low levels.

We also analyzed the CRONUS-A intercomparison material to investigate sensitivity of extracted ¹⁴C concentrations to the temperature and duration of the combustion step. Results indicate that 1-hr combustion steps at either 500 or 600°C yield results consistent with the original intercomparison value of Jull et al. (2015), while 2 hr at 600°C results in loss of ca. 10% of the high-temperature ¹⁴C inventory.