Porous iron pellets for AMS 14C analysis of small samples down to ultra-microscale size (10–25 μgC)

We developed the use of a porous iron pellet as a catalyst for AMS 14C analysis of small samples down to ultra-microscale size (10–25 μgC). It resulted in increased and more stable beam currents through our HVEE 4130 14C AMS system, which depend smoothly on the sample size. We find that both the expected decrease of oxalic acid standards and increase of backgrounds with decreasing sample size, due to increasing influence of contamination, are reproducible. Using a mass-dependent background correction for dead (1.0 ± 0.4 μgC) and modern (0.25 ± 0.10 μgC) contamination, we obtain reliable results for small samples down to 10 μgC and possibly smaller. Due to our low graphitization yield for ultra-small samples (increases from 40% to 80% on average with sample size), we measured graphite standards as small as 3 μgC. The standard deviation of the corrected activity is about 5% for a 10-μgC HOxII standard.Here we report the iron pellet technique, which is new to the best of our knowledge. It is generally applicable for AMS 14C laboratories that want to measure small samples down to ultra-microscale size. As an illustrative test-case, we analyze 14C data for IAEA-C5, C7 and C8 samples with masses ranging from 15 to 300 μgC.