Once the sample has been processed to either carbon dioxide or elemental carbon (in the form of graphite). The proportion of radiocarbon atoms in the sample is determined by AMS.
All measurements on the AMS are made in batches and are relative to the Oxalic Acid II standard. A typical batch comprises between four and six oxalic acid standards, two known-age samples, two anthracite backgrounds and a selection of other backgrounds relevant to combustion and other aspects of pre-treatment in addition to about thirty-five unknown samples.
All qraphite wheels are made up of either 'normal graphites' (typically 1.7mgC) or 'small graphites' (typically 1mgC, down to 0.5mgC). This is to ensure consistency between standards and unknown samples. Samples smaller than 0.5mgC are included in 'very small graphite' wheels.
The stable isotope ratio used for fractionation correction is measured on the AMS instrument itself allowing us to correct for any variability in the mass-dependent fractionation taking place during graphitisation and in the AMS system itself and also to check for large fractionation effects (>0.5%) which might indicate other problems. Precisions are high because of the efficiency of the AMS system and its overall stability, and because direct measurement of the stable isotope ratio in the AMS allows fractionation corrections to include the final stages in the whole process.
After measurement, all the results are analysed to look at the reproducibility of the standards and background materials. The relationship between stable isotope values measured on the AMS is compared to that measured on the stable isotope mass spectrometer. We never release results unless the corresponding data meets our quality control standards.
