To meet the increasing demand for 14C-analysis of small environmental samples by AMS, we optimised sample processing and explored different approaches for 14C-AMS measurement. Before exploring the possibility of running very small samples as CO2 on the gas ion source or adding carrier before graphitising, we first determined the minimum sample size which could reliably be analysed as graphite. Although some measurements down to 30µgC have been successful, currently only samples larger than 100µgC are routinely processed in the NERC radiocarbon laboratory and analysed as genuine graphite targets on the SUERC AMS (5MV pelletron). This minimal sample size ensures a sufficient target lifetime during measurement and satisfactory process blanks. System blanks of 0.15pMC±0.06 (>100µgC; n=14) and 0.28pMC±0.1 (<100µgC, n=8) have been obtained. There have already been reports on the dependence of the isotopic ratio on the sample size. Results obtained by other facilities may not necessarily reflect the characteristics specific to our facilities as the graphitisation and the AMS measurement can both contribute to the mass dependent fractionation. The effect of the graphitisation was investigated first: Graphites obtained from bulk oxalic acid 2 were recombusted and analysed by IR-MS. Below a target size of 200µgC a depletion of Delta13C=0.8‰±0.1 from the bulk gas value of -17.6‰ was observed. Studies to elucidate the influence of the AMS measurement on the ratio of the different carbon isotopes have been carried out and will be presented in detail. A range of graphite targets of primary and secondary standards (oxalic acid 2, bulk barley mash) have been analysed at low currents making a simultaneous Delta13C measurement possible. The raw data of a set of standards shows no additional fractionation of the 13/12 ratio within a measurement error of 1‰. However, the 14/12 and 14/13 ratio show a significant depletion below a sample size of 100µgC. To recorrect for the fractionation introduced, it is necessary to normalise small secondary standards and unknown samples to equally sized primary standards. To elucidate the nature of the contamination which is likely to contribute to the fractionation observed, analysis of materials of known ages is currently undertaken.
See more of Poster Session II
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)