Earlier this month, Dr. Carla Hadden, a Research Scientist at the University of Georgia’s Center for Applied Isotope Studies, gave us a primer in how samples are pretreated prior to dating. In this post we’ll discuss how samples are prepared for taking radiocarbon measurements. Our next post will discuss the actual steps involved in radiocarbon dating.
Pretreatment is an essential step in the dating process. The main purpose of pretreatment is to remove contaminants from the material to be dated. In the case of bone samples, pretreatment includes extracting collagen, the material that is ultimately dated. Archaeological materials almost always include contaminants introduced by the materials that they were buried in or with, such as humic or fulvic acids in soil. Other sources of contamination can be introduced during the collection, conservation, or packaging of samples. These extraneous sources of carbon need to be removed in order to get an accurate measurement of the carbon absorbed by an organism during its lifetime.
First, let’s discuss the steps involved in the pretreatment of bone. This sample is a portion of a deer long bone from the bone from the Orion site in Ontario.
First, a subsample of the bone is cut away using a Dremel tool. That subsample is then cleaned using a scalpel and wire brushes in order to remove any surface contamination such as dirt and root fragments. It’s then gently broken down into pieces approximately 3-5 mm in size. The sample is then placed in a solution of hydrogen chloride (HCl). The HCl works on the material to demineralize it and extract collagen — which is what ultimately gets dated. The HCl and bone solution is kept cold in order to control the pace of the chemical reaction.
The acid is then decanted and the demineralized bone fragments are rinsed multiple times in ultrapure water. The bone fragments are then treated with Sodium Hydroxide (NaOH) to dissolve and remove contaminants such as humic acids. It’s then washed again in ultrapure water three times. The demineralized bone fragments were then rinsed again in HCl to eliminate atmospheric carbon dioxide that might be absorbed during pretreatment. They are then rinsed again in ultrapure water to and placed in a slightly acidic solution and heated at 80ºC for 8 hours to reduce the solution.
The resulting solution is then filtered through a glass fiber filter to isolate the collagen that has been extracted. The collagen is then freeze-dried. At the end of the process, this collagen looks fluffy and crystalline, somewhat like cotton candy.
The majority of samples we have dated thus far during the Dating Iroquoia project have been maize. Plant material goes through a different set of steps during the pretreatment process.
As with bone, the sample is cleaned under a microscope to remove dirt particles and extraneous material.
The sample is then carefully split with a scalpel. The remainder of the sample is retained by the lab in case we need or want to run additional measurements on the material in the future. Sometimes it is very useful to have multiple dates from the same sample. Only a very small amount of material is needed for AMS dating and Dr. Hadden tries to take less than half so that a robust amount remains for future analysis.
Plant material goes through an acid/alkali/acid pretreatment. The plant material is then treated with HCl to remove carbonates and acids that might be present due to contamination. Since these materials can be of a different age than the sample itself it’s important to remove them before dating. The samples are then rinsed, treated with NaOH at room temperature to remove humic substances, rinsed again and then treated with HCl a second time, rinsed repeatedly with ultrapure water, and dried.
That’s it for the pretreatment process! In our next post we will talk about the next steps that happen in the lab and the actual process of radiocarbon dating.