Towards the limits: Analysis of microscale C-14 samples using EA-AMS

The coupling of an elemental analyzer (EA) to the gas ion source of an accelerator mass spectrometer (AMS) allows to perform online radiocarbon (C-14) analysis of small combustible samples. EA-AMS measurements of small and ultra-small samples containing carbon in the range of 5-100 mu g are performed on a routine basis at the MICADAS at ETH Zurich. Samples are encapsulated into vessels prior to combustion in the EA, which introduces extraneous C. The mass and fraction modern ((FC)-C-14, i.e. the activity ratio of the sample relative to a modern reference material) of this constant contamination was characterized for various vessel materials and shapes. Different vessel cleaning procedures and storage effects were investigated. A minimum contamination mass of 0.10 +/- 0.02 mu g C per vessel was found for baked Al boats stored in GC-vials. Analysis of standard materials was then performed in order to assess contamination mass and fraction modern of the coupled setup. By applying the model of constant contamination, a mass of 0.44 +/- 0.13 mu g C and a (FC)-C-14 of 0.48 +/- 0.14 were derived. Based on these findings, the mass-dependent detection limit (MDL) was calculated indicating that for a sample containing 5 mu g C the (FC)-C-14 has to be superior to 0.03 to make it distinguishable from the background level. Finally, ultra small samples of three certified reference materials (IAEA C5, C6, C7) were analyzed and corrected according to the model of constant contamination. For all three materials the corrected data was in agreement with the nominal values within the uncertainties.