Once material has been purified for radiocarbon dating, the samples are combusted and their stable isotopes analysed. This is an important part of the quality control. The experience of the Unit in stable isotope work enables us to use both the stable isotope ratios (δ13C and δ15N) and the ratios of carbon to nitrogen to look for signs of impurities.
This data can also provide useful information on food webs. In the case of human bone, this can be important for the dating itself since some aquatic diets can affect radiocarbon dates significantly.
The unit has four stable isotope mass spectrometers. Two Europa Scientific instruments are used in conjunction with automated gas collection systems for radiocarbon samples. Another two (one from Finingan and one from Europa Scientific) that are used for stable isotope research. All are usually operated in continuous flow mode (where the sample is transported in a stream of inert He gas). They are set up to measure the carbon and nitrogen compositions and stable isotope ratios. Precisions are usually better 0.1 to 0.2 per mil for δ13C and 0.3 per mil for δ15N.
At this stage, we reject a significant minority of samples as being unsuitable for dating. Because a wrong radiocarbon date is usually more misleading than no date at all, we never date material if we are unsure of its chemical origins. In borderline cases we will measure the radiocarbon but inform the submitter of the possible uncertainties.
We have two purpose-built gas-collection systems, designed at ORAU, which semi-automate the collection of samples from continuous-flow combustion, and mix the samples with the correct quantities of hydrogen for conversion to graphite.
