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Samples which are submitted to the AMS lab for radiocarbon dating undergo sample prep process. This includes cleaning sample material of potential contaminates, combustion or dissolution of sample in order to achieve pure CO2 and preparation of graphite.
Read more about sample preparation and graphitization:
J.S. Vogel, J.R. Southon, D.E. Nelson, T.A. Brown, Nuclear Instruments and Methods in Physics Research, B5 (1984) 289
I. Hajdas (1993) PhD thesis.pdf
This page shows some 'key steps' in the sample processing at our laboratory
soxhlet--removes possible contamination with materials for conservation
AAA--(Acid Alkali Acid) removes carbonates and humic acid
2. combustion or dissolution of carbonate--goal is to achieve pure CO2
3. graphitization-- purified CO2 is reduced to graphite
4. Targets
soxhlet--removes possible contamination with materials for conservation
In our procedure all samples (textile, wood, paper and other samples of art objects) that could possibly be contaminated during conservation undergo treatment in the soxhlet apparatus followed by the standard AAA treatment. The soxhlet step involves soaking sample material in a hot bath of solvents that are circulated in the soxhlet apparatus *. Our standard procedure includes baths in hexane, acetone and ethanol.
The modified cleaning procedure of the Acid-Alkali-Acid (AAA) is applied to all samples apart from carbonates and bones. The duration of various steps is varied according to the type of material. Each step is followed by rinsing in distilled water. After rinsing to neutral pH material is left to dry.
2. combustion
Dried material is weighed in a small quartz glass tube (see figure below) and then placed in a larger, 9 mm diameter and 15 cm long quartz glass tube. The larger tube contains wire-formed CuO , which provides oxygen for the burning of organic matter in vacuum.
Tubes containing samples are sealed under the vacuum
Picture below shows combusted tubes--one can see pure cupper after release of O2 for oxidation of material
If enough material is available (more than 20 mg of CaCO3), shell and coral material is leached before hydrolyzing in concentrated phosphoric acid.
Carbonate sample is placed in the small (thumb like) chamber of the tube, acid is poured into the main chamber and the tube is evacuated. After the valve is closed acid and sample material are carefully mixed and left to react.
The method of graphitization applied in our lab follows that described by Vogel et. al.,(1984). Cobalt powder is used as a catalyst for the reaction of CO2 and H2 in which graphite is produced.
Tubes containing samples are broken and the carbon dioxide is transferred to one of the reactors. Hydrogen is added and the water trap placed on the cold finger of the reactor to freeze out water produced during the reaction. The oven is placed over the reactor and the gas is heated. The reaction is completed when the pressure change is minimal, typically after 3 to 4 hrs.
Graphite+cobalt powder is pressed into the pre-roughed surface of copper target and is ready for AMS measurement
