Technical note: Studying lithium metaborate fluxes and extraction protocols with a new, fully automated in situ cosmogenic ¹⁴C processing system at PRIME Lab

Extraction procedures for in situ cosmogenic C-14 (in situ C-14) from quartz require quantitative isotopic yields while maintaining scrupulous isolation from atmospheric and organic C-14. These time- and labor-intensive procedures are ripe for automation; unfortunately, our original automated in situ C-14 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 borosilicate glass or fused-silica 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 lithium metaborate (LiBO2) flux to dissolve the quartz sample in an ultra-high-purity oxygen atmosphere, after a lower-temperature combustion step to remove atmospheric and organic C-14. We compared single-use high-purity Al2O3 against reusable 90 %Pt 10 %Rh () sample combustion boats. The 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) C-14 atoms. We also analyzed the CRONUS-A intercomparison material to investigate sensitivity of extracted C-14 concentrations to the temperature and duration of the combustion and extraction steps. Results indicate that 1 h combustion steps at either 500 or 600 C-degrees yield results consistent with the consensus value of Jull et al. (2015), while 2 h at 600 C-degrees results in loss of ca. 9 % of the high-temperature C-14 inventory. Results for 3 h extractions at temperatures ranging from 1050 to 1120 C-degrees and 4.5 h at 1000 C-degrees yielded similar results that agreed with the nominal value and published results from most laboratories. On the other hand, an extraction for 3 h at 1000 C-degrees was judged to be incomplete due to a significantly lower measured concentration. Based on these results, our preferred technique is now combustion for 1 h at 500 C-degrees followed by a 3 h extraction at 1050 C-degrees. Initial analyses of the CoQtz-N intercomparison material at our lab yielded concentrations ca. 60 % lower than those of CRONUS-A, but more analyses of this material from this and other labs are clearly needed to establish a consensus value.