T04_P28

C-14 AMS data quality assessment: How it’s done at the Rafter Radiocarbon Laboratory

Zondervan A¹, Turnbull J¹, Ginnane C¹, Norris M, Dahl J¹, Lewis C¹

¹GNS Science, Lower Hutt, New Zealand

The Rafter Radiocarbon Laboratory, in operation since 1951, transitioned from decay counting to AMS in the 2nd half of the 1980s. In the decades since, numerous improvements were made to sample preparation lines and protocols and to AMS system hard- and software. The most recent of these were the development of high-precision measurement of ¹⁴C in atmospheric CO₂, high throughput capacity for modern tree ring samples via accelerated solvent extraction, selective combustion of organics through ramped pyrolysis, and establishment of a custom protocol for tiny macrofossil samples. Presently, a new graphitisation system is under construction for reliable preparation for 0.05—0.15 mg C samples. It has been our longstanding practice to aim to report each ¹⁴C analysis with an error value that captures all sources of uncertainty, not just the Poisson counting error.

We tune XCAMS, our present AMS system, on a graphitisation blank and IAEA-C6 sucrose. Oxalic-acid-I (Ox-I) remains our primary standard and every batch measurement on ≤ 40 cathodes contains at least 6 of those. Raw AMS data are normalized to Ox-I per batch, while the blank correction for the samples in that batch is evaluated from blanks during the most recent 6—12 months. Lastly, repeated analysis of a few key radiocarbon inter-comparison materials allows us to estimate the residual, i.e. non-Poisson uncertainty, specific to each sample type and size (range). We will present results for all control materials, to highlight dependencies on date of preparation, preparation method, and sample size.