Sample Processing

Physical and chemical pretreatments vary depending on the type of material and the depositional environment from which it came. There are two steps:

Physical pretreatment often involves removing rootlets with tweezers, scraping the outer layer of bones or wood, and a reduction in sample size using mortar and pestle.

Chemical pretreatment varies depending on the material being dated. The most common method is an acid-alkali-acid (AAA) wash, which uses HCl to remove carbonates, NaOH to remove humic acid, and HCl again to neutralize the sample and remove any CO2 intruduced during the alkali wash. Between each stage, the sample is rinsed with highly purified (Milli-Q) water.

Combustion

Once the sample has undergone physical and chemical pretreatment it is freeze dried and ready for combustion, which produces CO2 for graphitization. Depending on the age, carbon content, and client requirements, combustion takes place either in evacuated quartz tubes in a muffle oven (4h at 900°C with cupric oxide and silver wire) or on an elemental analyzer.

Samples combusted in a muffle oven are then cryogenically purified on a gas cleanup line and sealed in pyrex breakseals.


Graphitization

Elemental carbon is produced from CO2 at 550–570°C in the presence of a preconditioned (reduced) Fe powder catalyst following the Bosch hydrogen-reduction reaction:

CO2 + 2H2 → C + 2H20

The 14C preparation lab is equipped with newly designed, largely automated 10-port graphitization lines (below). The vacuum line, cooling assembly, ovens, and touch screen controls are supported on a single frame. The stainless steel vacuum lines were orbitally welded to ensure smoother interior joints and to minimize CO2 adherence and cross contamination. Sample CO2 is pre-measured on a separate gas cleaning line so the iron powder is weighed accordingly to avoid splitting the gas sample. The cooling assembly was designed to provide equal, uniform cooling for each reaction module to optimize water extraction during graphitization. Each cooling cup is filled with Syltherm® to maximize heat transfer from the water trap and is cooled to < -45°C by Syltherm® circulating through a chiller connected to a copper coil in the cup. All operations on the line are controlled with a touch screen monitor using a Labview program. Oven and water trap temperatures and pressures are continuously monitored and the ovens and valves are all operated under program control, thus facilitating automation. Safety controls are programmed into the software to prevent human error, to avoid sample loss or vacuum accidents. These automation and safety features contribute to a streamlined design, making graphitization easy, safe, and operator-friendly.

Team