Accelerator Mass Spectrometry (AMS) analysis requires conversion of biological samples to a form that retains the isotopic ratio from the original sample, while also providing chemical and physical equivalence for all carbon atoms. Solid sample analysis is a well-established, accurate technique, and historically, liquid samples were converted to graphite in order to be analyzed by an AMS instrument. Graphite conversion continues to be an option in biomedical research, although with newer tools, other options may be easier and faster, while still retaining the accuracy of solid-sample AMS analysis.
Solid samples are combusted to produce carbon dioxide, and then a catalytic reduction is used to form graphite. The graphite is then pressed into aluminum cathodes and installed in the ion source of the bioAMS instrument, which counts individual rare isotope atoms using an ion beam.
How it works
Preparing solid samples for AMS analysis is a multi-step, labor-intensive process. The bioAMS facility has a dedicated graphite laboratory, where radiocarbon-tagged biological samples are converted to filamentous graphite. This process includes the following steps:
- The sample is sealed in an evacuated quartz vial with added oxidant.
- The vial is heated to 900ºC and the sample material is oxidized to produce a mixture of gaseous oxides.
- The carbon dioxide and water in the gaseous mixture are then cryogenically separated out and trapped in another vial that contains zinc and a metal catalyst.
- This second vial is then heated to 500ºC, and the reactions produce graphite.
This multi-step process improves measurement accuracy because there is no need to add a carrier carbon-bulking agent, which is often utilized to enable small-sample graphite AMS measurement. However, it can take up to three days to convert solid samples into graphite. The graphite is then pressed into the depression of an aluminum cathode, which is placed in a wheel-shaped magazine for installation in the bioAMS instrument’s ion source.
Once inside the AMS instrument, magnetic and electrostatic fields separate the different carbon isotopes according to their nuclear charge and mass. Interfering molecular isobars of carbon-14 are destroyed. The amount of carbon-14 is measured relative to a more abundant isotope (e.g., carbon-13 or carbon-12). The AMS instrument counts individual rare isotope atoms using a solid-state particle detector and measures a stable carbon isotope current with a Faraday cup. Absolute quantification comes from comparing the sample’s measured isotope ratio to that of measured standards of known ratios that are traceable through the National Institute of Standards and Technology (NIST).
