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IGERT program in Archeological Science

Principles of AMS

Radiocarbon dating

Theory

Pretreatment

Measurement

Correction

Age Calculation

Calibration

Cosmogenic Radioisotopes

Publication

 
 
 

I. Sample Pretreatment

Contaminants can have a considerable effect on ages of older materials, but for less than about 1,000 years of age, the amount of contaminants required to produce a significant age effect are large. For example, a 10% contaminating of an 8-year old sample with modern material would produce an 80-year shift in its apparent

age. Therefore, properly removing the contaminants is an important step in the 14C age measurement. There are several different pretreatment methods:

  • Most radiocarbon laboratories adopt a minimum "standard" pretreatment, consisting of soaking the sample sequentially in dilute hydrochloric acid, distilled water, diluted sodium hydroxide, distilled water, acid again, and then distilled water until the washing water is neutral.
    The acid step removes carbonates, such as from wind-blown dust, hard water or soil, the base step removes many soluble organic materials such as fatty acids.


  • For many art works, additional chemical cleaning steps are often employed, such as successive solvent extractions using a range of organic solvents in a soxhlet device. In the case of textiles, we use a series of solvents employing hexane, ethanol and methanol, followed by a final rinse in distilled water.

  • In other cases such as the removal of protein-based glues from canvas, soaking the canvas sample in hydrochloric acid followed by 1% sodium hydroxide at 50°C for 1-2 days is often required.

  • For bones, we modify Longin's technique [1971, Nature 230(5291)241] to remove the apatite fraction in acid, and then hydrolyze the bone collagen which remains as the desired sample.

  • For some carbonates such as corals, we use the selective dissolution method of Burr et al. [1992 Radiocarbon 34(3)61] to remove 20%-50% of the outer layer.

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