Conversion of the LLNL/CAMS 1 MV biological AMS system to a semi-automated natural abundance C spectrometer: system optimization and performance evaluation.

The Lawrence Livermore National Laboratory - Center for Accelerator Mass Spectrometry compact 1 MV biomedical accelerator mass spectrometer was repurposed and optimized for the semi-automated radiocarbon measurement of natural abundance environmental samples. Substantial efforts were made to greatly improve instrument precision and develop semi-automation capabilities for unattended operation. Here we present results from 15 months of routine system operation and evaluate the system performance based on 30 sample wheels measured with directly comparable operating conditions over 7 months from August 2019 to March 2020. Unattended operation was enabled through software that tracks specific error conditions and can initiate a complete instrument shutdown when specific criteria were met. The average measurement precision was found to be 2.7 ± 0.7 ‰ based on repeated measurements of OX I standards. Accuracy was assessed with measurements of standard materials with known C-content, spanning 0.5 to 1.5 modern, and by comparison to split samples measured with the 10 MV FN AMS system. We also assessed sample size and age limitations using C-free materials, finding that we can routinely analyze samples as small as 300 μg C and less than 33000 years without the need for size-specific correction protocols.